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23.4B: Protiste as menslike patogene - Biologie

23.4B: Protiste as menslike patogene - Biologie


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Baie protiste bestaan ​​as parasiete wat infekteer en siektes in hul gashere veroorsaak.

Leerdoelwitte

  • Identifiseer die uitwerking op mense van protistiese patogene

Kern punte

  • Die protistiese parasiet Plasmodium moet beide 'n muskiet en 'n gewerwelde dier koloniseer; P. falciparum, wat verantwoordelik is vir 50 persent van malariagevalle, word deur die Afrika-malariamuskiet aan mense oorgedra, Anopheles gambiae.
  • Wanneer P. falciparum rooibloedselle besmet en vernietig, dit bars, en parasitiese afval lek in die bloedstroom, wat deliruim, koors en bloedarmoede by pasiënte veroorsaak.
  • Trypanosoma brucei is verantwoordelik vir Afrika-slaapsiekte waarteen die menslike immuunstelsel nie kan waak nie, aangesien dit duisende moontlike antigene het en oppervlakglikoproteïene met elke aansteeklike siklus verander.
  • Nog een Trypanosoma spesies, T. cruzi, bewoon die hart- en spysverteringstelselweefsels, wat wanvoeding en hartversaking veroorsaak.

Sleutel terme

  • Trypanosoma: besmet 'n verskeidenheid gashere en veroorsaak verskeie siektes, insluitend die noodlottige Afrika-slaapsiekte by mense
  • plasmodium: parasitiese protosoë wat 'n muskiet en 'n gewerwelde dier moet koloniseer om sy lewensiklus te voltooi
  • patogeen: enige organisme of stof, veral 'n mikro-organisme, wat siektes kan veroorsaak, soos bakterieë, virusse, protosoë of swamme

Menslike patogene

'n Patogeen is enigiets wat siekte veroorsaak. Parasiete leef in of op 'n organisme en benadeel daardie organisme. 'n Beduidende aantal protiste is patogene parasiete wat ander organismes moet besmet om te oorleef en voort te plant. Protistiese parasiete sluit in die veroorsakende middels van malaria, Afrika-slaapsiekte en watergedraagde gastro-enteritis by mense.

Plasmodium spesies

Lede van die genus Plasmodium moet beide 'n muskiet en 'n gewerwelde dier koloniseer om hul lewensiklus te voltooi. In gewerwelde diere ontwikkel die parasiet in lewerselle en gaan voort om rooibloedselle te besmet, wat uitbars en die bloedselle vernietig met elke ongeslagtelike replikasiesiklus. Van die vier Plasmodium spesies wat bekend is om mense te besmet, P. falciparum verantwoordelik vir 50 persent van alle malariagevalle en is die primêre oorsaak van siekteverwante sterftes in tropiese streke van die wêreld. In 2010 is beraam dat malaria tussen een en 'n half miljoen sterftes veroorsaak het, meestal in Afrika-kinders. Gedurende die verloop van malaria, P. falciparum kan meer as die helfte van 'n mens se sirkulerende bloedselle besmet en vernietig, wat lei tot ernstige bloedarmoede. In reaksie op afvalprodukte wat vrygestel word wanneer die parasiete uit besmette bloedselle bars, begin die gasheer se immuunstelsel 'n massiewe inflammatoriese reaksie met episodes van delirium-induserende koors, aangesien parasiete rooibloedselle lyseer en parasitiese afval in die bloedstroom mors. P. falciparum word deur die Afrika-malariamuskiet aan mense oorgedra, Anopheles gambiae. Tegnieke om blootstelling aan hierdie hoogs-aggressiewe muskietspesie dood te maak, te steriliseer of te vermy, is noodsaaklik vir malariabeheer.

Plasmodium

Daar word getoon dat rooibloedselle besmet is met P. falciparum, die veroorsakende middel van malaria. In hierdie ligmikroskopiese beeld geneem met 'n 100× olie-onderdompelingslens, die ringvormig P. falciparumvlekke pers.

Trypanosome

Trypanosoma brucei, die parasiet wat verantwoordelik is vir Afrika-slaapsiekte, verwar die menslike immuunstelsel deur sy dik laag oppervlakglikoproteïene met elke aansteeklike siklus te verander. Die glikoproteïene word deur die immuunstelsel as vreemde antigene geïdentifiseer en 'n spesifieke teenliggaam verdediging word teen die parasiet gemonteer. Maar T. brucei het duisende moontlike antigene; met elke daaropvolgende generasie skakel die protiste oor na 'n glikoproteïenbedekking van 'n ander molekulêre struktuur. Op hierdie manier, T. brucei is in staat om voortdurend te repliseer sonder dat die immuunstelsel ooit daarin slaag om die parasiet skoon te maak. Sonder behandeling, T. brucei val rooibloedselle aan, wat veroorsaak dat die pasiënt in 'n koma verval en uiteindelik sterf. Gedurende epidemiese periodes kan sterftes as gevolg van die siekte hoog wees. Groter toesig- en beheermaatreëls lei tot 'n vermindering in aangemelde gevalle; van die laagste getalle wat in 50 jaar aangemeld is (minder as 10 000 gevalle in die hele Afrika suid van die Sahara) het sedert 2009 plaasgevind.

In Latyns-Amerika, 'n ander spesie, T. cruzi, is verantwoordelik vir Chagas-siekte. T. cruzi infeksies word hoofsaaklik deur 'n bloedsuiende gogga veroorsaak. Die parasiet bewoon hart- en spysverteringstelselweefsels in die chroniese fase van infeksie, wat lei tot wanvoeding en hartversaking as gevolg van abnormale hartritmes. Na raming is 10 miljoen mense met Chagas-siekte besmet; dit het 10 000 sterftes in 2008 veroorsaak.


Ontduiking van fagotrofiese predasie deur protistiese gashere en aangebore immuniteit van metazoïese gashere deur Legionella pneumophila

Legionella pneumophila is 'n alomteenwoordige omgewingsbakterie wat ontwikkel het om binne amoebes en ander protiste te besmet en te vermeerder. Daar word gemeen dat die toevallige inaseming van besmette waterdeeltjies deur mense hierdie patogeen in staat gestel het om binne alveolêre makrofage te prolifereer en longontsteking te veroorsaak. Die hoogs ontwikkelde makrofage is egter toegerus met meer gesofistikeerde ingebore verdedigingsmeganismes as protiste, soos die evolusie van fagotrofiese voeding in fagositose met meer ontwikkelde aangebore verdedigingsprosesse. Nie verrassend nie, die meerderheid van proteïene betrokke by fagosoom biogenese (

80%) het oorsprong in die fagotrofiestadium van evolusie. Daar is 'n oorvloed van hoogs ontwikkelde sellulêre en ingebore metazoïese prosesse, wat nie in protistiese biologie verteenwoordig word nie, wat deur L. pneumophila gemoduleer word, insluitend TLR2-sein, NF-KB, apoptotiese en inflammatoriese prosesse, histoonmodifikasie, caspases en die NLRC-Naip5 inflammasome. Dit is belangrik dat L. pneumophila hemosiete van die ongewerwelde Galleria mellonella infekteer, G. mellonella-larwes doodmaak en in Drosophila volwasse vlieë en Caenorhabditis elegans prolifereer en doodmaak. Alhoewel ko-evolusie met protiste-gashere 'n aansienlike bloudruk vir L. pneumophila verskaf het om makrofage te infekteer, bespreek ons ​​die verdere evolusionêre aspekte van ko-evolusie van L. pneumophila en die aanpassing daarvan om verskeie hoogs ontwikkelde ingebore metazoïese prosesse te moduleer voordat dit 'n menslike patogeen word.

Sleutelwoorde: Legionella ekologie immunologie infeksie meganisme van werking mikrobiese-sel interaksie.

© 2018 John Wiley & Sons Bpk.

Syfers

Manipulasie van evolusionêr bewaarde en...

Manipulasie van evolusionêr bewaarde en metazoan-spesifieke ingebore verdedigingsprosesse deur L. pneumophila .…


Wêreldwye opname van miRNA's en tRNA-afgeleide klein RNA's van die menslike parasitiese protis Trichomonas vaginalis

Agtergrond: Klein nie-koderende RNA's speel kritieke regulerende rolle in post-transkripsie. Hulle kenmerke in Trichomonas vaginalis, die veroorsakende middel van menslike seksueel oordraagbare trichomoniase, moet egter nog bepaal word.

Metodes: Klein RNA-transkriptome van Trichomonas-trofosoïete is diep georden deur gebruik te maak van die Illumina NextSeq 500-stelsel en omvattend ontleed om Trichomonas mikroRNA's (miRNA's) te identifiseer en RNA (tRNA)-afgeleide klein RNA's (tsRNA's) oor te dra. Die tsRNA kandidate is bevestig deur stam-lus kwantitatiewe omgekeerde transkripsie-PCR, en motiewe om die splitsing van tsRNAs te lei is voorspel deur gebruik te maak van die GLAM2 algoritme.

Resultate: Daar is gevind dat die miRNA's teenwoordig is in T. vaginalis, maar teen 'n uiters lae oorvloed (0,0046%). Drie kategorieë van endogene Trichomonas tsRNAs is geïdentifiseer, naamlik 5'tritsRNAs, mid-tritsRNAs en 3'tritsRNAs, met die 5'tritsRNAs wat die dominante kategorie (67.63%) van tsRNAs uitmaak. Interessant genoeg het die splitsingsplekanalise beide konvensionele klasse van tRNA-afgeleide fragmente (tRF's) en tRNA-helftes in tritsRNA's geverifieer, wat die uitdrukking van tRNA-helftes in die nie-stres toestand aandui. 'n Totaal van 25 tritsRNA's is eksperimenteel bevestig, wat verantwoordelik is vir 78.1% van alle getoetsde kandidate. Drie motiewe is voorspel om die produksie van tritsRNA's te lei. Die resultate bewys die uitdrukking van tRFs en tRNA-helftes in die T. vaginalis transkriptoom.

Gevolgtrekkings: Dit is die eerste verslag van genoomwye ondersoek van klein RNA's, veral tsRNA's en miRNA's, van Trichomonas-parasiete. Ons bevindinge demonstreer die uitdrukkingsprofiel van tsRNA's in T. vaginalis, terwyl miRNA skaars opgespoor is. Hierdie resultate kan verdere navorsing bevorder wat daarop gemik is om 'n beter begrip te verkry van die evolusie van klein nie-koderende RNA in T. vaginalis en hul funksies in die patogenese van trichomoniase.

Sleutelwoorde: Oordrag RNA Trichomonas vaginalis Trichomoniasis tRFs tRNA-afgeleide klein RNAs tRNA-helftes.


Vrylewende amoebes en plakkers in die natuur: ekologiese en molekulêre kenmerke

Vrylewende amoebes is protiste wat gereeld in water en grond voorkom. Hulle voed op ander mikroörganismes, hoofsaaklik bakterieë, en verteer hulle deur fagositose. Dit word aanvaar dat hierdie amoebes 'n belangrike rol speel in die mikrobiese ekologie van hierdie omgewings. Daar is 'n hernieude belangstelling vir die vrylewende amoebes sedert die ontdekking van patogeniese bakterieë wat fagositose kan weerstaan ​​en van reuse-virusse, wat onderliggend is dat amoebes 'n rol kan speel in die evolusie van ander mikroörganismes, insluitend verskeie menslike patogene. Onlangse vooruitgang, met behulp van molekulêre metodes, laat toe om nuwe inligting oor vrylewende amoebes bymekaar te bring. Hierdie oorsig het ten doel om 'n omvattende oorsig te verskaf van die nuut versamelde insigte oor (1) die vrylewende amoebe-diversiteit, geassesseer met molekulêre gereedskap, (2) die geenfunksies wat beskryf word om die biologie van die amoebes te ontsyfer en (3) hul interaksies met ander mikroörganismes in die omgewing.

Sleutelwoorde: omgewing geen funksie mikrobiese diversiteit protist simbiose.


Mikrobes bekend as protiste word onderbestudeer, maar hul impak op ekosisteme kan groot wees

Onder die groot groep mikrobioomspelers val bakterieë al lank die kollig. Maar die eensellige organismes bekend as protiste kry uiteindelik die hoofrol wat hulle verdien.

'n Groep wetenskaplikes wat die interaksies tussen plante en mikrobes bestudeer, het 'n nuwe studie vrygestel wat die dinamiese verhoudings tussen grondbewonende protiste en ontwikkelende plante uiteensit, wat demonstreer dat grondprotiste baie soos bakterieë op plantseine reageer.

’n Enorme verskeidenheid en diversiteit van mikrobes leef in grond, en die bestudering van hoe hierdie organismes met mekaar en met plantwortels in wisselwerking is, is ’n warm onderwerp in biologie, aangesien dit toepassings het vir landbou, grondbeheer en weerbaarheidstegnologieë vir klimaatsverandering.

"Protiste verteenwoordig 'n nuwe grens in die studie van grondmikrobiese ekologie," sê hoofskrywer Javier A. Ceja Navarro, 'n navorsingswetenskaplike by Lawrence Berkeley National Laboratory (Berkeley Lab). "Hier wys ons dat hierdie groep organismes werklik ingesluit moet word in mikrobiese studies wat daarop gemik is om te verstaan ​​hoe mikrobes in wisselwerking met plante werk."

Protiste is nie 'n duidelike geslag van organismes nie, maar eerder 'n kategorie wat aan enige eensellige eukariotiese organisme ('n organisme wie se selle 'n kern bevat) wat nie 'n plant, swamme of dier is nie, toegeken word. Hierdie diverse groep van 200 000+ spesies (nuwes word voortdurend ontdek) sluit amoebes, diatome, dinoflagellate, slymskimmels en selfs verskeie parasiete in -- soos die malaria-veroorsakende Plasmodium en die gelyknamige Giardia-veroorsakende genus van protosoë.

Protiste word oor die hele planeet in 'n verskeidenheid ekosisteme aangetref. Sommige spesies, soos sekere mariene plankton-protiste en menslike siekteveroorsakende protiste, is noukeurig bestudeer. Maar vir die meeste spesies begin wetenskaplikes net die oppervlak krap van wat die organisme doen en hoe hulle op die omgewing reageer. Dit is die geval vir grondprotiste.

Volgens Navarro is dit bekend dat protiste grondmikrobiese dinamika en voedingstofkringloop beheer deur op ander mikrobes te voed. Alhoewel daar 'n goeie hoeveelheid kennis is oor hul interaksies met ander lede van die grondmikrobioom, is min bekend oor hoe protiste op veranderinge in hul omgewing reageer.

"Al is protiste belangrik en die relevansie daarvan al dekades lank bekend, is ons studie die eerste een wat 'n assosiasie van protiste met plante in 'n grootskaalse veldeksperiment toon," het projekleier Mary Firestone, 'n fakulteitswetenskaplike in Berkeley Lab's Earth opgemerk. en Omgewingswetenskappe Area en 'n professor aan UC Berkeley. Die projek was 'n samewerking tussen wetenskaplikes van Berkeley Lab, UC Berkeley, Lawrence Livermore National Laboratory (LLNL), die Noble Research Institute en die Universiteit van Oklahoma.

Die span het skakelgras - 'n gewas wat vir grootskaalse biobrandstofproduksie voorgestel is - van saailinge op twee grootskaalse veldpersele gekweek en monsters geneem van die grond rondom die wortels van plante in verskillende stadiums van groei. Hulle het die volgende generasie genoomvolgorde gebruik om die tipe protiste wat in elke monster teenwoordig is en die oorvloed van elke spesie te identifiseer.

"Soos plante groei, stel die selle in hul wortels metaboliete vry wat seine uitstuur na die omliggende grondomgewing," het Jennifer Pett-Ridge, 'n senior personeelwetenskaplike van LLNL, bygevoeg. "Ons het gesien dat protistegemeenskappe verskuif en verander in reaksie op die plant se effekte - op 'n manier wat soortgelyk is aan wat ons vir bakteriese gemeenskappe waargeneem het."

"Toekomstige studies wat daarop fokus om die meganismes van plantvestiging in grond te verstaan, sal protiste as 'n sleuteldeel van die plantmikrobioom moet oorweeg," het Navarro bygevoeg, wat deel is van Berkeley Lab se Biowetenskappe-area. "Om protiste in terrestriële ekologiese studies te ignoreer, sal 'n groot kennisgaping tot gevolg hê wat ons begrip van die omgewingsmikrobioom onvolledig sal maak."


23.4 Ekologie van Protiste

Aan die einde van hierdie afdeling sal jy die volgende kan doen:

  • Beskryf die rol wat protiste in die ekosisteem speel
  • Beskryf belangrike patogene spesies van protiste

Protiste funksioneer in verskeie ekologiese nisse. Terwyl sommige protiste spesies noodsaaklike komponente van die voedselketting en genereerders van biomassa is, funksioneer ander in die ontbinding van organiese materiale. Nog ander protiste is gevaarlike menslike patogene of veroorsakende middels van vernietigende plantsiektes.

Primêre Produsente/Voedselbronne

Protiste is noodsaaklike bronne van voedsel en verskaf voeding vir baie ander organismes. In sommige gevalle, soos met soöplankton, word protiste direk verteer. Alternatiewelik dien fotosintetiese protiste as produsente van voeding vir ander organismes. Paramecium bursaria en verskeie ander spesies siliate is mengotrofies as gevolg van 'n simbiotiese verhouding met groen alge. Dit is 'n tydelike weergawe van die sekondêr endosimbiotiese chloroplast wat in Euglena. Maar hierdie simbiotiese assosiasies is nie beperk tot protiste nie. Byvoorbeeld, fotosintetiese dinoflagellate genoem soöxanthellae voorsien voedingstowwe vir die koraalpoliepe (Figuur 23.32) wat hulle huisves, wat korale 'n hupstoot gee van energie om hul kalsiumkarbonaatskelet af te skei. Op hul beurt voorsien die korale die protiste met 'n beskermde omgewing en die verbindings wat nodig is vir fotosintese. Hierdie tipe simbiotiese verhouding is belangrik in voedingstofarm omgewings. Sonder dinoflagellaat-simbiote verloor korale algepigmente in 'n proses wat genoem word koraal bleik, en hulle sterf uiteindelik. Dit verklaar waarom rifbou-korale gewoonlik nie in waters dieper as 20 meter woon nie: onvoldoende lig bereik daardie dieptes vir dinoflagellate om te fotosinteer.

Die protiste en hul produkte van fotosintese is noodsaaklik – direk of indirek – vir die oorlewing van organismes wat wissel van bakterieë tot soogdiere (Figuur 23.33). As primêre produsente voed protiste 'n groot deel van die wêreld se waterspesies. (Op land dien landplante as primêre produsente.) Trouens, ongeveer 25 persent van die wêreld se fotosintese word uitgevoer deur fotosintetiese protiste, veral dinoflagellate, diatome en meersellige alge.

Protiste skep nie voedselbronne net vir seebewonende organismes nie. Onthou dat sekere anaërobiese parabasalied spesies bestaan ​​in die spysverteringskanale van termiete en houtetende kakkerlakke, waar hulle 'n noodsaaklike stap bydra in die vertering van sellulose wat deur hierdie insekte ingeneem word terwyl hulle hout verteer.

Menslike patogene

Soos ons gesien het, is 'n patogeen enigiets wat siektes veroorsaak. Parasitiese organismes leef in of op 'n gasheerorganisme en benadeel die organisme. ’n Klein aantal protiste is ernstige patogene parasiete wat ander organismes moet besmet om te oorleef en voort te plant. Protistiese parasiete sluit byvoorbeeld die veroorsakende middels van malaria, Afrika-slaapsiekte, amoebiese enkefalitis en watergedraagde gastro-enteritis by mense in. Ander protistiese patogene prooi op plante, wat massiewe vernietiging van voedselgewasse bewerkstellig.

Plasmodium Spesies

In 2015 het die WGO meer as 200 miljoen gevalle van malaria aangemeld, meestal in Afrika, Suid-Amerika en Suider-Asië. Dit is egter nie bekend dat malaria ook 'n algemene en aftakelende siekte was in die Noord-Sentraal-streek van die Verenigde State, veral Michigan, met sy duisende mere en talle moerasse nie. Voor die burgeroorlog, en die dreinering van baie moerasse, het feitlik almal wat na Michigan geïmmigreer het malaria opgetel (ague soos dit in die laat 1800's genoem is), en die bleek, vaal, opgeblase gesigte van daardie tydperk was die reël. Die enigste gesonde gesigte is gedra deur die immigrante wat pas aangekom het. Trouens, daar was meer sterftes as gevolg van malaria in Michigan as dié van die Burgeroorlog.

Ons weet nou dat malaria veroorsaak word deur verskeie spesies van die apicomplexan protist genus Plasmodium. Lede van Plasmodium moet opeenvolgend beide 'n muskiet en 'n gewerwelde dier benodig om hul lewensiklus te voltooi. By gewerwelde diere ontwikkel die parasiet in lewerselle (die ekso-eritrositiese stadium) en gaan voort om rooibloedselle (die eritrositiese stadium) te besmet, en bars uit en vernietig die bloedselle met elke ongeslagtelike replikasiesiklus (Figuur 23.34). Van die vier Plasmodium spesies wat bekend is om mense te besmet, P. falciparum verantwoordelik vir 50 persent van alle malariagevalle en is die primêre (en dodelikste) oorsaak van siekteverwante sterftes in tropiese streke van die wêreld. In 2015 is beraam dat malaria meer as 400 000 sterftes veroorsaak het, meestal in Afrika-kinders. Gedurende die verloop van malaria, P. falciparum kan meer as die helfte van 'n mens se sirkulerende bloedselle besmet en vernietig, wat lei tot ernstige bloedarmoede. In reaksie op afvalprodukte wat vrygestel word soos die parasiete uit besmette bloedselle bars, begin die gasheer se immuunstelsel 'n massiewe inflammatoriese reaksie met episodes van delirium-induserende koors (paroksismes) soos parasiete rooibloedselle lyseer, wat parasietafval in die bloedstroom mors. P. falciparum word deur die Afrika-muskiet aan mense oorgedra, Anopheles gambiae. Tegnieke om blootstelling aan hierdie hoogs aggressiewe muskietspesie dood te maak, te steriliseer of te vermy, is noodsaaklik vir malariabeheer. Ironies genoeg het 'n tipe genetiese beheer ontstaan ​​in dele van die wêreld waar malaria endemies is. Die besit van een kopie van die HbS beta-globien-alleel lei tot malariaweerstand. Ongelukkig het hierdie alleel ook 'n ongelukkige tweede effek wanneer dit homosigoties is, veroorsaak dit sekelselsiekte.

Skakel na Leer

Hierdie fliek beeld die patogenese van Plasmodium falciparum, die veroorsakende middel van malaria.

Trypanosome

Trypanosoma brucei (Figuur 23.35), oorgedra deur tsetsevlieë (Glossina spp) in Afrika, en verwante vlieë in Suid-Amerika, is 'n gevlagde endoparasiet wat verantwoordelik is vir die dodelike siekte nagana by beeste en perde, en vir Afrika-slaapsiekte by mense. Hierdie tripanosoom verwar die menslike immuunstelsel deur sy dik laag oppervlakglikoproteïene met elke aansteeklike siklus te verander. (Die glikoproteïene word deur die immuunstelsel as vreemde antigene geïdentifiseer, en 'n spesifieke teenliggaamverdediging word teen die parasiet gemonteer.) T. brucei het duisende moontlike antigene, en met elke daaropvolgende generasie skakel die protiste oor na 'n glikoproteïenbedekking met 'n ander molekulêre struktuur. Op hierdie manier, T. brucei is in staat om voortdurend te repliseer sonder dat die immuunstelsel ooit daarin slaag om die parasiet skoon te maak. Sonder behandeling, T. brucei val rooibloedselle aan, wat veroorsaak dat die pasiënt in 'n koma verval en uiteindelik sterf. Gedurende epidemiese periodes kan sterftes as gevolg van die siekte hoog wees. Groter toesig- en beheermaatreëls lei tot 'n vermindering in aangemelde gevalle, sommige van die laagste getalle wat in 50 jaar aangemeld is (minder as 10 000 gevalle in die hele Afrika suid van die Sahara) het sedert 2009 plaasgevind.

Skakel na Leer

Hierdie fliek bespreek die patogenese van Trypanosoma brucei, die veroorsakende middel van Afrika-slaapsiekte.

In Latyns-Amerika, 'n ander spesie van tripanosoom, T. cruzi, is verantwoordelik vir Chagas-siekte. T. cruzi infeksies word hoofsaaklik veroorsaak deur 'n bloedsuiende "soengogga" in die genus Triatoma. Hierdie "ware goggas" byt die gasheer gedurende die nag en ontlas dan op die wond, wat die tripanosoom na die slagoffer oordra. Die slagoffer krap die wond en ent die plek verder met trypanosome by die plek van die byt. Na ongeveer 10 weke betree individue die chroniese fase, maar die meeste ontwikkel nooit verdere simptome nie. In ongeveer 30 persent van gevalle veroorsaak die tripanosoom egter verdere skade, veral aan die hart- en spysverteringstelselweefsels in die chroniese fase van infeksie, wat lei tot wanvoeding en hartversaking as gevolg van abnormale hartritmes. Na raming is 10 miljoen mense met Chagas-siekte besmet, en dit het in 2008 10 000 sterftes veroorsaak.

Plant Parasiete

Protistiese parasiete van landplante sluit middels in wat voedselgewasse vernietig. Die oomyceet Plasmopara viticola parasiteer druiwe plante, wat veroorsaak dat 'n siekte genoem donsskimmel (Figuur 23.36). Druiweplante wat besmet is met P. viticola lyk stamp en het verkleurde, verdorde blare. Die verspreiding van donsskimmel het die Franse wynbedryf in die negentiende eeu amper in duie gestort.

Phytophthora infestans is 'n oomyceet wat verantwoordelik is vir aartappellaatroes, wat veroorsaak dat aartappelstingels en -stingels in swart slym verval (Figuur 23.37). Wydverspreide aartappelroes veroorsaak deur P. infestans die welbekende Ierse aartappelhongersnood in die negentiende eeu tot gevolg gehad het wat die lewens van ongeveer 1 miljoen mense geëis het en gelei het tot die emigrasie van minstens 1 miljoen meer uit Ierland. Laatroes gaan voort om aartappelgewasse in sekere dele van die Verenigde State en Rusland te pla, en vee soveel as 70 persent van oeste uit wanneer geen plaagdoders toegedien word nie.

Protistiese Ontbinders

Die swamagtige protist saprobes is gespesialiseerd om voedingstowwe te absorbeer uit nie-lewende organiese materiaal, soos dooie organismes of hul afval. Byvoorbeeld, baie soorte oomycete groei op dooie diere of alge. Saprobiese protiste het die noodsaaklike funksie om anorganiese voedingstowwe na die grond en water terug te keer. Hierdie proses maak voorsiening vir nuwe plantgroei, wat op sy beurt voedsel vir ander organismes langs die voedselketting genereer. Inderdaad, sonder saprobe-spesies, soos protiste, swamme en bakterieë, sou lewe ophou bestaan ​​aangesien alle organiese koolstof in dooie organismes "vasgebind" geraak het.

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    • Skrywers: Mary Ann Clark, Matthew Douglas, Jung Choi
    • Uitgewer/webwerf: OpenStax
    • Boektitel: Biologie 2e
    • Publikasiedatum: 28 Maart 2018
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    • Afdeling URL: https://openstax.org/books/biology-2e/pages/23-4-ecology-of-protists

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    Inhoud

    Excavata word as primitiewe eukariote beskou. Hulle word gekenmerk deur 'n voedingsgroef met 'n posterior geleë flagella, wat hulle in staat stel om 'n stroom te skep wat klein voedseldeeltjies vasvang. [4] Die sitostoom is die gespesialiseerde struktuur wat die protiste hierdie funksie toelaat. Hierdie supergroep Excavata sluit die subgroepe in Diplomonades (Fornicata), Parabasaliede, en Euglenozoans. [5]

    Diplomonads wysig

    Dipolomonades is vroeër gedefinieer as Fornicata, maar hul eienskappe bly dieselfde ten spyte van hul hernoeming. Hulle is mikroaerofiele protiste. Diplomonades is voorheen gedefinieer deur die gebrek aan 'n mitochondria, maar onlangse studies het bevind dat hulle 'n nie-funksionele, mitochondriale oorblyfselorganel het wat 'n mitosoom genoem word. Die meeste is skadeloos behalwe vir Giardia, Hexamita salmonis, en Histomonas meleagridis. Giardia veroorsaak diarree, Hexamita salmonis is 'n vis parasiet, en Histomonas meleagridis is 'n kalkoenpatogeen.

    Giardia intestinalis is 'n menslike patogeen, wat deur sistebesoedelde water oorgedra word. Dit veroorsaak epidemiese diarree van besmette water. Jy kan sien dat jy dalk besmet is deur die waarneming van siste of trofosoïete in stoelgang en ELISA (ensiem-gekoppelde immunosorbent-toets) toets. Om kontaminasie te voorkom, vermy enige moontlik besmette water, en as besmette water die enigste ding is om te drink, moet 'n stadige sandfilter gebruik word. 'n Studie het bevind dat die chlorering van water en voedingsintervensie geen effek op giardia-infeksie in die kinderjare gehad het nie. Slegs handewas en higiëniese sanitasie-intervensies het infeksiekoerse by kinders verminder. [6]

    Hexamita salmonis is 'n algemene gevlaggeleerde vispatogeen. Besmette visse is swak, uitgeteer en swem gewoonlik op hul sy. [7]

    Histomonas meleagridis is 'n algemene voëlpatogeen wat histomoniase veroorsaak. Tekens van histomoniase sluit in verminderde eetlus, hangende vlerke, onversorgde vere en geel fekale mis. [8]

    Parabasilia wysig

    Die meeste Parabasalia is gevlageerde endosimbiote van diere. Hulle het nie 'n duidelike sitostoom nie, wat beteken dat hulle fagositose moet gebruik om voedsel te verswelg. Daar is twee subgroepe: Trichonympha en Trichomonadida. Trichonympha is verpligte mutualiste van houtvretende insekte soos termiete. Hulle skei sellulase af, wat gebruik word om hout te verteer. Die volgende subgroep, Trichomonadida, benodig nie suurstof nie en beskik oor hidrogenosome. Hulle plant slegs voort deur ongeslagtelike voortplanting en sommige stamme is menslike patogene. Daar is drie tipes patogene parabasalia: Trichomonas foetus, Dientamoeba fragilis, en Trichomonas vaginalis. Trichomonas fetus veroorsaak spontane aborsie by beeste, Dientamoeba fragilis veroorsaak diarree by mense, en Trichomonas vaginalis is 'n seksueel oordraagbare siekte. [1]

    Trichomonas fetus is 'n parasiet wat in die urogenitale kanaal van beeste woon en bees trichomoniase veroorsaak. Trichomoniasis is 'n seksueel oordraagbare siekte wat onvrugbaarheid by verse veroorsaak. Die meeste onvrugbaarheid word veroorsaak deur skielike embrioniese dood. [9] Verskeie imidazole is gebruik om besmette bulle te behandel, maar nie een is veilig en doeltreffend nie. Ipronidasool is waarskynlik die doeltreffendste, maar dit veroorsaak gereeld steriele absesse by inspuitplekke. [10]

    Dientamoeba fragilis is 'n parasiet wat in die dikderm van mense woon. Niemand weet hoe nie D. fragilis word versprei een moontlikheid is om besmette water of kos in te sluk. Baie mense wat met hierdie parasiet besmet is, toon geen tekens dat hulle besmet is nie. Soms kan die infeksie waargeneem word, die mees algemene simptome sluit in diarree, maagpyn, verlies aan eetlus, naarheid en moegheid. [11]

    Trichomonas vaginalis is 'n seksueel oordraagbare siekte. Mans wat besmet is toon selde enige simptome (asimptomaties). Vroue wat besmet is, toon gewoonlik tekens van seerheid, ontsteking en rooiheid rondom die vagina en 'n moontlike verandering in vaginale afskeiding. Trichomonas vaginalis kan behandel word met 'n kursus antibiotika. [12]

    Euglenozoa wysig

    Die meeste Euglenozoa is foto-outotrofies, maar sommige is chemo-organotrofe (saprofities). Hulle word algemeen in varswater aangetref. Die lede van die filum Euglenozoa het 'n pellikel vir ondersteuning, 'n rooi oogvlek wat 'n stigma genoem word om die sel na lig te oriënteer, chlorofil a en b om te help met die proses van fotosintese, kontraktiele vakuole en flagella.

    Een belangrike patogeen van die filum Euglenozoa is Leishmania. Leishmania leishmaniasis veroorsaak. Die simptome van leishmaniasis sluit sistemiese en vel/membraanskade in. Leishmania-parasiete versprei deur flebotomien-sandvlieë in die trope, subtrope en suidelike Europa. [13] Hulle kan kutane manifesteer (kutane leishmaniasis) as velsere met as skurfte 'n paar weke na die byt of inwendig (viscerale leishmaniasis), wat die organe aantas, wat lewensgevaarlik kan wees. Kutane leishmaniasis kan na die slymvliese versprei en selfs jare na die aanvanklike infeksie mukosale leishmaniasis veroorsaak. [14] Kutane leishmaniasis genees vanself en laat slegte letsels. [15] Slegs FDA goedgekeur vir viscerale leishmaniasis is amfoterisien B en orale miltefosien vir kutane en mukosale leishmaniasis diagnose - weefselmonster, beenmurg, bloedtoetse bespeur teenliggaampies teen parasiet vir viscerale leishmaniasis. [16] [17]

    Die tweede patogeen uit hierdie filum is Trypanosoma cruzi. Trypanosoma cruzi veroorsaak Chagas-siekte en word oorgedra deur die reduviid-gogga, ook bekend as die "soengogga". Chagas-siekte word gediagnoseer met 'n fisiese ondersoek en bloedtoets. [18] Die enigste behandeling sluit slegs antiparasitiese middels van die CDC in, wat nie deur die FDA goedgekeur is nie. [19] Akute Chagas-siekte het 'n vinnige aanvang, die tripanosome gaan die bloedstroom binne, hulle word amastigote en repliseer. Akute Chagas-siekte kan behandel word met benznidasool of nifurtimox. Chroniese chagas-siekte is asimptomaties en veroorsaak dat hart- en gastro-intestinale selle aangetas word. Tans is daar slegs ondersoekbehandelings vir hierdie siekte. Ongelukkig is entstowwe nie effektief met Chagas-siekte nie as gevolg van antigeenvariasie. Hierdie patogeen veroorsaak skade aan die senuweestelsel.

    Afrikaanse Slaapsiekte word veroorsaak deur Trypanosoma brucei rhodensiense en Trypanosoma brucei gambiense, en word deur die tsetsevlieg oorgedra. Dit word gediagnoseer deur 'n fisiese ondersoek en bloedtoets. Afrika-slaapsiekte veroorsaak binne een tot drie jaar interstisiële ontsteking, lusteloosheid, breinswelling en dood. Geneesmiddelterapie, met behulp van Eflornithine en Melarsoprol Pentamidine vir T. gambiense en Suramin (Antrypol) vir óf Trypanosoma brucei rhodensiense en Trypanosoma brucei gambiense, of kombinasies van hierdie medikasie, kan help om hierdie siekte te behandel, maar entstowwe kan nie gebruik word nie as gevolg van antigeniese variasie.

    Amoebozoa word gekenmerk deur die gebruik van pseudopodia vir beweging en voeding. Hierdie protiste reproduseer deur binêre of meervoudige splitsing.

    Entamoebida Edit

    Entamoebida het nie mitochondria nie en besit mitosome. Entamoeba histolytica is 'n patogene parasiet wat bekend is om amoebiase te veroorsaak, wat die derde grootste oorsaak van parasitiese sterftes is. [20] Dit word gediagnoseer deur die assessering van stoelgangmonsters. [21] Amoebiase word veroorsaak deur die inname van voedsel of water wat besmet is met ontlasting of ander liggaamlike afval van 'n besmette persoon, wat siste bevat, die dormante vorm van die mikrobe. Hierdie siste gee by die bereiking van die terminale ileum-gebied van die spysverteringskanaal aanleiding tot 'n massa prolifererende selle, die trofosoïetvorm van die parasiet, deur die proses van eksistasie. [22] Simptome van hierdie infeksie sluit diarree met bloed en slym in, en kan wissel tussen hardlywigheid en remissie, buikpyn en koors. Simptome kan vorder tot ameboma, fulminante kolitis, giftige megakolon, kolonulkusse, wat lei tot perforasie en absesse in lewensbelangrike organe soos lewer, long en brein. Amoebiasis kan behandel word met die toediening van anti-amoebiese verbindings, dit sluit dikwels die gebruik van Metronidasool, Ornidasool, Chlorokien, Secnidasool, Nitazoxanide en Tinidasool in. Tinidasool kan effektief wees om kinders te genees. [23] Die gebruik van konvensionele terapeutika om amoebiase te behandel indien dit dikwels met aansienlike newe-effekte verband hou, 'n bedreiging vir die doeltreffendheid van hierdie terapeutika, wat verder vererger word deur die ontwikkeling van geneesmiddelweerstand in die parasiet. [20] Amoebiese meningoencefalitis en keratitis is 'n breinvretende amoeba wat veroorsaak word deur vrylewende Naeglaria en Acanthomoeba. Een manier waarop hierdie patogeen verkry kan word, is deur kontaklense in water in plaas van kontakoplossing te week. Dit sal lei tot progressiewe ulserasie van die kornea. [24] Hierdie patogeen kan gediagnoseer word deur demonstrasie van amoebes in kliniese monsters. Daar is tans geen geneesmiddelterapie beskikbaar vir amoebiese meningoencefalitis en keratitis nie.

    Die supergroep SAR sluit Rhizaria, Alveolata en Stramenopiles in en word onderskei deur fyn pseudopodia wat vertakt, eenvoudig of verbind kan wees.

    Stramenopila wysig

    Sommige lede van Stramenopila is bruin alge, diatome en waterskimmels. 'n Voorbeeld van Stramenopila is Peronosporomycetes. Die bekendste voorbeeld van Peronosporomycetes is Phytophthora infestans. Hierdie organisme het die Groot Hongersnood van Ierland in die 1850's veroorsaak. [25]

    Alveolata wysig

    Alveolata is a large group, which includes Dinoflagellata, Ciliophora, and Apicomplexa. [26]

    Balantidium Coli (Balantidiasis) is an example of a member of the phylum Ciliophora. Balantidiasis is the only ciliate known to be capable of infecting humans, and swine are the primary reservoir host. [27] Balantidiasis is opportunistic and rare in Western countries. [28] Apicomplexans are parasites of animals and contain an arrangement of organelles called the apical complex. One example of an apicomplexan is Malaria. Five species of plasmodium cause malaria in animals. Malaria is transmitted by the bite of an infected female mosquito. Symptoms of malaria include: periodic chills and fever, anemia, and hypertrophy of the liver and spleen. Cerebral malaria can occur in children. In order to diagnose Malaria, doctors will look for parasites in Wright-or-Giemsa-stained red blood cells and serological tests. Treatment includes antimalarial drugs, however, resistance has been observed. New vaccines are being discovered to this day. Preventative measures that can be taken include sleeping with netting and using insecticide to prevent mosquitoes. Eimeria is another example of an apicomplexan pathogen. This pathogen causes cecal coccidiosis in chickens. Coccidiosis is a parasitic disease of the intestinal tract. [29] This disease is treated by placing anticoccidials in the chickens’ feed. It also causes malabsorption, diarrhea, and sometimes bloody diarrhea in animals. Theileria parva & T. annulata are tick-borne parasites which cause fatal East Coast fever in cattle. East Coast fever is transmitted by the bite of the three-host tick Phipicephalus appendiculatus and results in respiratory failure and death in African cattle. Most hosts of P. appendiculatus succumb to pulmonary edema and die within three weeks of infection. The severity of the infection can be lessened by treatment with antiprotozoal drugs like buparvaquone. Toxoplasma causes toxoplasmosis and can be acquired from undercooked meat or cat feces containing Toxoplasma gondii. The majority of the 60 million Americans infected with T. gondii are asymptomatic. The group most vulnerable to this pathogen are the fetuses of mothers who have been infected with the parasite for the first time during pregnancy. This can result in damage to the fetus’s brain, eyes, and other organs. Treatment is available for pregnant women and the immunosuppressed. [30] Cryptosporidiosis can be contracted through contact with water, food, soil, or surfaces contaminated with feces containing the Cryptosporidium. Immunocompromised people are the most susceptible. Cryptosporidiosis causes watery diarrhea and can resolve itself without medical intervention. It is diagnosed by examining stool samples, and diarrhea can be treated using Nitazoxanide. [31]

    Rhizaria Edit

    Plasmodiophorids and Halosporidians are two examples of parasitic Rhizaria. Plasmodiophorids cause infections in crops such as Spongospora subterranea. They cause powdery scabs and galls and disrupt growth. Halosporidians cause infections in marine invertebrates such as Mikrocytos mackini in Pacific oysters. Mikrocytos mackini are abscesses or green pustules on palps and mantles of certain molluscs. [32]

    Die supergroep Archaeplastida includes red algae, green algae and land plants. Each of these three groups have multicellular species and the green and red algae have many single-celled species. The land plants are not considered protists. [33]

    Red algae are primarily multicellular, lack flagella, and range in size from microscopic, unicellular to large, multicellular forms. Sommige spesies rooi alge bevat phycoerythrins, fotosintetiese bykomstige pigmente wat rooi van kleur is en die groen tint van chlorofil oortref, wat hierdie spesies as verskillende skakerings van rooi laat voorkom. This group doesn’t include many pathogens. [34]

    Green algae exhibit similar features to the land plants, particularly in terms of chloroplast structure. The green algae are subdivided into the chlorophytes and charophytes. It is very rare for green algae to become parasitic.

    Prototheca moriformis belongs to the subdivision Chloroplastida. P. moriformis is a green algae that lacks chlorophyll and has turned to parasitism. It is found in sewage and the soil. P. moriformis causes a disease called protothecosis. This disease mainly infects cattle and dogs. Cattle can be affected by prototheca enteritis and mastitis. [35] Protothecosis is commonly seen in dogs it enters the body through the mouth or nose and causes infection in the intestines. Treatment with amphotericin B has been reported. [36]

    Scientists have been researching new ways to fight protozoan infections, including targeting channels and transporters involved in the diseases [37] and finding the link between a persons microbiome and their ability to resist a protozoan infection [38]


    Inhoud

    Viral Edit

    This type of pathogen is not cellular, and is instead composed of either RNA (Ribonucleic acid), or DNA (Deoxyribonucleic acid). Pathogenic viruses infiltrate host cells and manipulate the organelles within the cell such as the Ribosomes, Golgi Apparatus, and Endoplasmic Reticulum to reproduce a multitude of times which commonly results in the death of the host cell via cellular decay. All the virus's that were contained within the lipid bilayer of the cell membrane are then released into the intercellular matrix to infect neighboring cells to continue the cycle.

    The white blood cells are responsible for swallowing up the virus using a mechanism known as endocytosis within the extracellular matrix to reduce and fight the infection. The components within the white blood cell are responsible for destroying the virus and recycling it's components for the body to use.

    Bacterial Edit

    Although the vast majority of bacteria are harmless or beneficial to one's body, a few pathogenic bacteria can cause infectious diseases. The most common bacterial disease is tuberculosis, caused by the bacterium Mycobacterium tuberculosis, which affects about 2 million people mostly in sub-Saharan Africa. Pathogenic bacteria contribute to other globally important diseases, such as pneumonia, which can be caused by bacteria such as Streptokokke en Pseudomonas, and foodborne illnesses, which can be caused by bacteria such as Shigella, Campylobacter, en Salmonella. Pathogenic bacteria also cause infections such as tetanus, typhoid fever, diphtheria, syphilis, and Hansen's disease. They typically range between 1 and 5 micrometers in length.

    Fungal Edit

    Fungi are a eukaryotic kingdom of microbes that are usually saprophytes, but can cause diseases in humans. Life-threatening fungal infections in humans most often occur in immunocompromised patients or vulnerable people with a weakened immune system, although fungi are common problems in the immunocompetent population as the causative agents of skin, nail, or yeast infections. Most antibiotics that function on bacterial pathogens cannot be used to treat fungal infections because fungi and their hosts both have eukaryotic cells. Most clinical fungicides belong to the azole group. The typical fungal spore size is 1-40 micrometers in length.

    Other parasites Edit

    Protozoans are single-celled eukaryotes that feed on microorganisms and organic tissues. Considered as "one-celled animal" as they have animal like behaviors such as motility, predation, and a lack of a cell wall. Many protozoan pathogens are considered human parasites as they cause a variety of diseases such as: malaria, amoebiasis, babesiosis, giardiasis, toxoplasmosis, cryptosporidiosis, trichomoniasis, Chagas disease, leishmaniasis, African trypanosomiasis (sleeping sickness), Acanthamoeba keratitis, and primary amoebic meningoencephalitis (naegleriasis).

    Parasitic worms (Helminths) are macroparasites that can be seen by the naked eye. Worms live and feed in their living host, receiving nourishment and shelter while affecting the host's way of digesting nutrients. They also manipulate the host's immune system by secreting immunomodulatory products [2] which allows them to live in their host for years. Many parasitic worms are more commonly intestinal that are soil-transmitted and infect the digestive tract other parasitic worms are found in the host's blood vessels. Parasitic worms living in the host can cause weakness and even lead to many diseases. Parasitic worms can cause many diseases to both humans and animals. Helminthiasis (worm infection), Ascariasis, and enterobiasis (pinworm infection) are few that are caused by various parasitic worms.

    Prionic Edit

    Prions are misfolded proteins that are transmissible and can influence abnormal folding of normal proteins in the brain. They do not contain any DNA or RNA and cannot replicate other than to convert already existing normal proteins to the misfolded state. These abnormally folded proteins are found characteristically in many neurodegenerative diseases as they aggregate the central nervous system and create plaques that damages the tissue structure. This essentially creates "holes" in the tissue. It has been found that prions transmit three ways: obtained, familial, and sporadic. It has also been found that plants play the role of vector for prions. There are eight different diseases that affect mammals that are caused by prions such as scrapie, bovine spongiform encephalopathy (mad cow disease) and Feline spongiform encephalopathy (FSE). There are also ten diseases that affect humans such as, Creutzfeldt–Jakob disease (CJD). [3] and Fatal familial insomnia (FFI).

    Animal pathogens Edit

    Animal pathogens are disease-causing agents of wild and domestic animal species, at times including humans. [4]

    Virulence (the tendency of a pathogen to cause damage to a host's fitness) evolves when that pathogen can spread from a diseased host, despite that host being very debilitated. An example is the malaria parasite, which can spread from a person near death, by hitching a ride to a healthy person on a mosquito that has bitten the diseased person. This is called horizontal transmission in contrast to vertical transmission, which tends to evolve symbiosis (after a period of high morbidity and mortality in the population) by linking the pathogen's evolutionary success to the evolutionary success of the host organism.

    Transmission of pathogens occurs through many different routes, including airborne, direct or indirect contact, sexual contact, through blood, breast milk, or other body fluids, and through the fecal-oral route. One of the primary pathways by which food or water become contaminated is from the release of untreated sewage into a drinking water supply or onto cropland, with the result that people who eat or drink contaminated sources become infected. In developing countries, most sewage is discharged into the environment or on cropland even in developed countries, periodic system failures result in sanitary sewer overflows.


    Eukaryome and Its Relationships with Microbiome

    Eukaryotic microbes co-evolved with mammals over millions of years and are a normal component of the microbiome from an evolutionary point of view [7,11]. Many are stable, long-term colonists rather than transient invaders [25]. The eukaryome can have strong effects on the composition and dynamics of the microbiome [14], likely with cascading consequences for our health. Although less numerous than bacteria, gut-dwelling eukaryotes are much bigger and they may have a disproportionate influence, similar to large animals in other ecosystems. For example, sharks on tropical reefs and wolves in Yellowstone have a profound effect on the entire ecosystem, and removal of these keystone species has wide consequences. It is worth testing whether targeted removal of eukaryotes—potential keystone components of the gut microbiome—in industrialized countries has contributed disproportionately to the diversity loss observed in the bacterial microbiome [12] and other negative health consequences discussed above. In summary, there are many exciting prospects for investigating potential benefits of the human eukaryome, all while keeping in mind the well-documented detrimental impact of some eukaryotic symbionts, particularly when present in large numbers and in mammalian hosts experiencing food limitation [26].


    22.4 Bacterial Diseases in Humans

    Aan die einde van hierdie afdeling sal jy die volgende kan doen:

    • Identify bacterial diseases that caused historically important plagues and epidemics
    • Describe the link between biofilms and foodborne diseases
    • Explain how overuse of antibiotics may be creating “super bugs”
    • Explain the importance of MRSA with respect to the problems of antibiotic resistance

    To a prokaryote, humans may be just another housing opportunity. Unfortunately, the tenancy of some species can have harmful effects and cause disease. Bacteria or other infectious agents that cause harm to their human hosts are called pathogens . Devastating pathogen-borne diseases and plagues, both viral and bacterial in nature, have affected humans and their ancestors for millions of years. The true cause of these diseases was not understood until modern scientific thought developed, and many people thought that diseases were a “spiritual punishment.” Only within the past several centuries have people understood that staying away from afflicted persons, disposing of the corpses and personal belongings of victims of illness, and sanitation practices reduced their own chances of getting sick.

    Epidemiologists study how diseases are transmitted and how they affect a population. Often, they must following the course of an epidemic —a disease that occurs in an unusually high number of individuals in a population at the same time. In contrast, a pandemic is a widespread, and usually worldwide, epidemic. An endemic disease is a disease that is always present, usually at low incidence, in a population.

    Long History of Bacterial Disease

    There are records about infectious diseases as far back as 3000 B.C. A number of significant pandemics caused by bacteria have been documented over several hundred years. Some of the most memorable pandemics led to the decline of cities and entire nations.

    In the 21 st century, infectious diseases remain among the leading causes of death worldwide, despite advances made in medical research and treatments in recent decades. A disease spreads when the pathogen that causes it is passed from one person to another. For a pathogen to cause disease, it must be able to reproduce in the host’s body and damage the host in some way.

    The Plague of Athens

    In 430 B.C., the Plague of Athens killed one-quarter of the Athenian troops who were fighting in the great Peloponnesian War and weakened Athens’s dominance and power. The plague impacted people living in overcrowded Athens as well as troops aboard ships that had to return to Athens. The source of the plague may have been identified recently when researchers from the University of Athens were able to use DNA from teeth recovered from a mass grave. The scientists identified nucleotide sequences from a pathogenic bacterium, Salmonella enterica serovar Typhi (Figure 22.20), which causes typhoid fever. 3 This disease is commonly seen in overcrowded areas and has caused epidemics throughout recorded history.

    Bubonic Plagues

    From 541 to 750, the Plague of Justinian, an outbreak of what was likely bubonic plague, eliminated one-quarter to one-half of the human population in the eastern Mediterranean region. The population in Europe dropped by 50 percent during this outbreak. Astoundingly, bubonic plague would strike Europe more than once!

    Bubonic plague is caused by the bacterium Yersinia pestis. One of the most devastating pandemics attributed to bubonic plague was the Black Death (1346 to 1361). It is thought to have originated in China and spread along the Silk Road, a network of land and sea trade routes, to the Mediterranean region and Europe, carried by fleas living on black rats that were always present on ships. The Black Death was probably named for the tissue necrosis (Figure 22.21c) that can be one of the symptoms. The "buboes" of bubonic plague were painfully swollen areas of lymphatic tissue. A pneumonic form of the plague, spread by the coughing and sneezing of infected individuals, spreads directly from human to human and can cause death within a week. The pneumonic form was responsible for the rapid spread of the Black Death in Europe. The Black Death reduced the world’s population from an estimated 450 million to about 350 to 375 million. Bubonic plague struck London yet again in the mid-1600s (Figure 22.21). In modern times, approximately 1,000 to 3,000 cases of plague arise globally each year, and a “sylvatic” form of plague, carried by fleas living on rodents such as prairie dogs and black footed ferrets, infects 10 to 20 people annually in the American Southwest. Although contracting bubonic plague before antibiotics meant almost certain death, the bacterium responds to several types of modern antibiotics, and mortality rates from plague are now very low.

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    Watch a video on the modern understanding of the Black Death—bubonic plague in Europe during the 14 th century.

    Migration of Diseases to New Populations

    One of the negative consequences of human exploration was the accidental “biological warfare” that resulted from the transport of a pathogen into a population that had not previously been exposed to it. Over the centuries, Europeans tended to develop genetic immunity to endemic infectious diseases, but when European conquerors reached the western hemisphere, they brought with them disease-causing bacteria and viruses, which triggered epidemics that completely devastated many diverse populations of Native Americans, who had no natural resistance to many European diseases. It has been estimated that up to 90 percent of Native Americans died from infectious diseases after the arrival of Europeans, making conquest of the New World a foregone conclusion.

    Emerging and Re-emerging Diseases

    The distribution of a particular disease is dinamies. Changes in the environment, the pathogen, or the host population can dramatically impact the spread of a disease. According to the World Health Organization (WHO), an emerging disease (Figure 22.22) is one that has appeared in a population for the first time, or that may have existed previously but is rapidly increasing in incidence or geographic range. This definition also includes re-emerging diseases that were previously under control. Approximately 75 percent of recently emerging infectious diseases affecting humans are zoonotic diseases. Zoonoses are diseases that primarily infect animals but can be transmitted to humans some are of viral origin and some are of bacterial origin. Brucellosis is an example of a prokaryotic zoonosis that is re-emerging in some regions, and necrotizing fasciitis (commonly known as flesh-eating bacteria) has been increasing in virulence for the last 80 years for unknown reasons.

    Some of the present emerging diseases are not actually new, but are diseases that were catastrophic in the past (Figure 22.23). They devastated populations and became dormant for a while, just to come back, sometimes more virulent than before, as was the case with bubonic plague. Other diseases, like tuberculosis, were never eradicated but were under control in some regions of the world until coming back, mostly in urban centers with high concentrations of immunocompromised people. WHO has identified certain diseases whose worldwide re-emergence should be monitored. Among these are three viral diseases (dengue fever, yellow fever, and zika), and three bacterial diseases (diphtheria, cholera, and bubonic plague). The war against infectious diseases has no foreseeable end.

    Foodborne Diseases

    Prokaryotes are everywhere: They readily colonize the surface of any type of material, and food is not an exception. Most of the time, prokaryotes colonize food and food-processing equipment in the form of a biofilm, as we have discussed earlier. Outbreaks of bacterial infection related to food consumption are common. A foodborne disease (commonly called “food poisoning”) is an illness resulting from the consumption the pathogenic bacteria, viruses, or other parasites that contaminate food. Although the United States has one of the safest food supplies in the world, the U.S. Centers for Disease Control and Prevention (CDC) has reported that “76 million people get sick, more than 300,000 are hospitalized, and 5,000 Americans die each year from foodborne illness.”

    The characteristics of foodborne illnesses have changed over time. In the past, it was relatively common to hear about sporadic cases of botulism, the potentially fatal disease produced by a toxin from the anaerobic bacterium Clostridium botulinum. Some of the most common sources for this bacterium were non-acidic canned foods, homemade pickles, and processed meat and sausages. The can, jar, or package created a suitable anaerobic environment where Clostridium could grow. Proper sterilization and canning procedures have reduced the incidence of this disease.

    While people may tend to think of foodborne illnesses as associated with animal-based foods, most cases are now linked to produce. There have been serious, produce-related outbreaks associated with raw spinach in the United States and with vegetable sprouts in Germany, and these types of outbreaks have become more common. The raw spinach outbreak in 2006 was produced by the bacterium E coli serotype O157:H7. A serotype is a strain of bacteria that carries a set of similar antigens on its cell surface, and there are often many different serotypes of a bacterial species. Die meeste E coli are not particularly dangerous to humans, but serotype O157:H7 can cause bloody diarrhea and is potentially fatal.

    All types of food can potentially be contaminated with bacteria. Recent outbreaks of Salmonella reported by the CDC occurred in foods as diverse as peanut butter, alfalfa sprouts, and eggs. A deadly outbreak in Germany in 2010 was caused by E coli contamination of vegetable sprouts (Figure 22.24). The strain that caused the outbreak was found to be a new serotype not previously involved in other outbreaks, which indicates that E coli is continuously evolving. Outbreaks of listeriosis, due to contamination of meats, raw cheeses, and frozen or fresh vegetables with Listeria monocytogenes, are becoming more frequent.

    Biofilms and Disease

    Recall that biofilms are microbial communities that are very difficult to destroy. They are responsible for diseases such as Legionnaires’ disease, otitis media (ear infections), and various infections in patients with cystic fibrosis. They produce dental plaque and colonize catheters, prostheses, transcutaneous and orthopedic devices, contact lenses, and internal devices such as pacemakers. They also form in open wounds and burned tissue. In healthcare environments, biofilms grow on hemodialysis machines, mechanical ventilators, shunts, and other medical equipment. In fact, 65 percent of all infections acquired in the hospital (nosocomial infections) are attributed to biofilms. Biofilms are also related to diseases contracted from food because they colonize the surfaces of vegetable leaves and meat, as well as food-processing equipment that isn’t adequately cleaned.

    Biofilm infections develop gradually and may not cause immediate symptoms. They are rarely resolved by host defense mechanisms. Once an infection by a biofilm is established, it is very difficult to eradicate, because biofilms tend to be resistant to most methods used to control microbial growth, including antibiotics. The matrix that attaches the cells to a substrate and to other another protects the cells from antibiotics or drugs. In addition, since biofilms grow slowly, they are less responsive to agents that interfere with cell growth. It has been reported that biofilms can resist up to 1,000 times the antibiotic concentrations used to kill the same bacteria when they are free-living or planktonic. An antibiotic dose that large would harm the patient therefore, scientists are working on new ways to get rid of biofilms.

    Antibiotics: Are We Facing a Crisis?

    Die woord antibiotic kom uit die Grieks anti meaning “against” and bios meaning “life.” An antibiotic is a chemical, produced either by microbes or synthetically, that is hostile to or prevents the growth of other organisms. Today’s media often address concerns about an antibiotic crisis. Are the antibiotics that easily treated bacterial infections in the past becoming obsolete? Are there new “superbugs”—bacteria that have evolved to become more resistant to our arsenal of antibiotics? Is this the beginning of the end of antibiotics? All these questions challenge the healthcare community.

    One of the main causes of antibiotic resistance in bacteria is overexposure to antibiotics. The imprudent and excessive use of antibiotics has resulted in the natural selection of resistant forms of bacteria. The antibiotic kills most of the infecting bacteria, and therefore only the resistant forms remain. These resistant forms reproduce, resulting in an increase in the proportion of resistant forms over non-resistant ones. In addition to transmission of resistance genes to progeny, lateral transfer of resistance genes on plasmids can rapidly spread these genes through a bacterial population. A major misuse of antibiotics is in patients with viral infections like colds or the flu, against which antibiotics are useless. Another problem is the excessive use of antibiotics in livestock. The routine use of antibiotics in animal feed promotes bacterial resistance as well. In the United States, 70 percent of the antibiotics produced are fed to animals. These antibiotics are given to livestock in low doses, which maximize the probability of resistance developing, and these resistant bacteria are readily transferred to humans.

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    Watch a recent news report on the problem of routine antibiotic administration to livestock and antibiotic-resistant bacteria.

    One of the Superbugs: MRSA

    The imprudent use of antibiotics has paved the way for the expansion of resistant bacterial populations. Byvoorbeeld, Staphylococcus aureus, often called “staph,” is a common bacterium that can live in the human body and is usually easily treated with antibiotics. However, a very dangerous strain, methicillin-resistant Staphylococcus aureus (MRSA) has made the news over the past few years (Figure 22.25). This strain is resistant to many commonly used antibiotics, including methicillin, amoxicillin, penicillin, and oxacillin. MRSA can cause infections of the skin, but it can also infect the bloodstream, lungs, urinary tract, or sites of injury. While MRSA infections are common among people in healthcare facilities, they have also appeared in healthy people who haven’t been hospitalized, but who live or work in tight populations (like military personnel and prisoners). Researchers have expressed concern about the way this latter source of MRSA targets a much younger population than those residing in care facilities. The Journal of the American Medical Association reported that, among MRSA-afflicted persons in healthcare facilities, the average age is 68, whereas people with “community-associated MRSA” ( CA-MRSA ) have an average age of 23. 4

    In summary, the medical community is facing an antibiotic crisis. Some scientists believe that after years of being protected from bacterial infections by antibiotics, we may be returning to a time in which a simple bacterial infection could again devastate the human population. Researchers are developing new antibiotics, but it takes many years of research and clinical trials, plus financial investments in the millions of dollars, to generate an effective and approved drug.

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    Epidemiologist

    Epidemiology is the study of the occurrence, distribution, and determinants of health and disease in a population. It is, therefore, part of public health. An epidemiologist studies the frequency and distribution of diseases within human populations and environments.

    Epidemiologists collect data about a particular disease and track its spread to identify the original mode of transmission. They sometimes work in close collaboration with historians to try to understand the way a disease evolved geographically and over time, tracking the natural history of pathogens. They gather information from clinical records, patient interviews, surveillance, and any other available means. That information is used to develop strategies, such as vaccinations (Figure 22.26), and design public health policies to reduce the incidence of a disease or to prevent its spread. Epidemiologists also conduct rapid investigations in case of an outbreak to recommend immediate measures to control it.