Medical Microbiology

Pathogens 5

kingdoms.Pathogenicmicroorganismsarefoundinthedomainsbacteriaand

eucarya.

Bacteria,Archaea,Eucarya

Bacteria.Thisdomainincludesthekingdomoftheheterotrophiceubacteriaand

includesallhumanpathogenbacteria.Theotherkingdoms,forinstancethatofthe

photosyntheticcyanobacteria,arenotpathogenic.Itisestimatedthatbacterialspe-

ciesonEarthnumberinthehundredsofthousands,ofwhichonlyabout 5500 have

beendiscoveredanddescribedindetail.

Archaea.Thisdomainincludesformsthatliveunderextremeenvironmentalcon-

ditions,includingthermophilic,hyperthermophilic,halophilic,andmethanogenic

microorganisms.Theearliertermforthearchaeawasarchaebacteria(ancientbac-

teria),andtheyareindeedakindoflivingfossil.Thermophilicarchaeathrivemainly

inwarm,moistbiotopessuchasthehotspringsatthetopofgeothermalvents.The

hyperthermophilicarchaea,amorerecentdiscovery,liveneardeep-seavolcanic

plumesattemperaturesexceeding 1008 C.

Eucarya.Thisdomainincludesalllifeformswithcellspossessingagenuinenucleus.

Theplantandanimalkingdoms(animalesandplantales)arealleukaryoticlife

forms.Pathogeniceukaryoticmicroorganismsincludefungalandprotozoanspe-

cies.

Table 1. 2 liststhemaindifferencesbetweenprokaryotic(bacteriaandar-

chaea)andeukaryoticpathogens.

Bacteria.....................................................

&Classicbacteria.Theseorganismsreproduceasexuallybybinarytrans-

versefission.Theydonotpossessthenucleustypicalofeucarya.Thecell

wallsoftheseorganismsarerigid(withsomeexceptions,e.g.,themycoplas-

ma).

&Chlamydiae.Theseorganismsareobligateintracellularparasitesthatare

abletoreproduceincertainhumancellsonlyandarefoundintwostages:the

infectious,nonreproductiveparticlescalledelementarybodies(0.3lm)and

thenoninfectious,intracytoplasmic,reproductiveformsknownasinitial(or

reticulate)bodies(1lm).

&Rickettsiae.Theseorganismsareobligateintracellularparasites,rod-

shapedtococcoid,thatreproducebybinarytransversefission.Thediameter

oftheindividualcellisfrom0.3– 1 lm.

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Kayser, Medical Microbiology © 2005 Thieme