Medical Microbiology

Gram-NegativeAnaerobes 317

Gram-NegativeAnaerobes

&Theobligateanaerobic,Gram-negative,pleomorphicrodsarecompo-

nentsofthenormalmucosalfloraoftherespiratory,intestinal,andgenital

tracts.Amongthemanygenera,Bacteroides,Prevotella,Porphyromonas,and

Fusobacterium,eachofwhichcomprisesnumerousspecies,areofmedical

significance.Theycauseendogenousnecroticinfectionswithsubacuteto

chroniccoursesintheCNS,head,lungs,abdomen,andfemalegenitals.Atyp-

icalcharacteristicofsuchinfectionsisthatamixedfloraincludinganaerobes

aswellasaerobesisalmostalwaysfoundtobecausative.Laboratorydiag-

nosticproceduresseektoidentifythepathogens.Specialtransportvessels

arerequiredtotransportspecimenstothelaboratory.Identificationisbased

onmorphologicalandphysiologicalcharacteristics.AspecialtechniqueisGC

organicacidassay.Potentiallyeffectiveantibioticsincludecertainpenicillins

andcephalosporins,clindamycin,andmetronidazole. &

Occurrence.Thesebacteriaincludealargeandheterogeneousgroupof

Gram-negative,nonsporing,obligateanaeroberods,manyofwhicharecom-

ponentsofthenormalhumanmucosalflora.

Theirnumbersareparticularlylargeintheintestinaltract,wheretheyare

found 1000 !asfrequentlyasEnterobacteriaceae.Theyalsooccurregularlyin

theoralcavity,upperrespiratorytract,andfemalegenitals.

Classification.Thetaxonomyandnomenclaturehaschangedconsiderablyin

recentyears.ThefamiliesBacteroidaceae,Prevotellaceae(nov.fam.),Porphyr-

omonadaceae(nov.fam.),andFusobacteriaceae(nov.fam.)includesignificant

humanpathogens(Table4. 11 ).

Morphologyandculture.TheGram-negativeanaerobesshowapronounced

pleomorphy;theyarestraightorcurved,inmostcasesnonmotile,Gram-

negativerods.Fusobacteriaoftentakeongramstainingirregularlyandfre-

quentlyfeaturepointedpoles(Fig.4. 22 ).

Culturegrowthisonlyachievedunderstringentanaerobicconditions.

Somespeciesaresosensitivetooxygenthattheentireculturingprocedure

mustbecarriedoutinananaerobicchamber(controlledatmosphereglove

box).Anaerobesproliferatemoreslowlythanaerobes,sotheculturesmustbe

incubatedfortwodaysormore.

Pathogenesisandclinicalpictures.InfectionswithGram-negativeanae-

robesparticipationarealmostexclusivelyendogenousinfections.Theorgan-

ismsshowlowlevelsofpathogenicity.Theythereforearenotfoundtofeature

anyspectacularpathogenicityfactorslikeclostridialtoxins.Somehaveacap-

suletoprotectthemfromphagocytosis.Someproducevariousenzymesthat

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Kayser, Medical Microbiology © 2005 Thieme