Medical Microbiology

258 4 BacteriaasHumanPathogens

Actinomyces...........................................

ActinomycetesareGram-positivebacteriathattendtogrowintheformof

branchedfilaments.Theresultingmycelialmassesare,however,notob-

servedinoldercultures,whichstronglyresemblethoseofcorynebacteria

intheirmorphology.

Occurrence.Actinomycetesarepartofthenormalmucosalflorainhumans

andanimals.Theycolonizemainlytheoralcavity,andanactinomycosisin-

fectionisthereforealwaysendogenous.Ninetypercentofactinomycetesin-

fectionsinhumansarecausedbyA.israelii,withfarfewercasescausedbyA.

naeslundiiandotherspecies.

Morphologyandculture.ActinomycetesareGram-positive,pleomorphic

rodbacteriathatsometimesalsoshowgenuinebranching(Fig.4. 11 ).Theyel-

lowishsulfurgranules,measuring 1 – 2 mm,canbeobservedmacroscopically

inactinomycetespus.TheseparticlesareconglomeratesofsmallActinomyces

coloniessurroundedbyawallofleukocytes.Mycelialfilamentsextendra-

diallyfromthecolonies(actinium=Greekforraylike).Culturingtheorganism

requiresenrichedmediumsandananaerobicmilieucontaining 5 – 10 %CO 2.

Mycelialmicrocoloniesformonlyduringthefirstdays.Whitishmacrocolo-

nies,oftenwitharoughsurface,begintoappearaftertwoweeks.

Pathogenesisandclinicalpicture.Thepathogensbreachmucosa(perhaps

normaldermisaswell)andareabletoestablishthemselvesintissuein

thepresenceofalowredoxpotential.Thefactorsresponsibleforthesecon-

ditionsincludepoorbloodperfusionand,aboveall,contributingbacterial

Actinomycesisraelii

Fig.4. 11 Gramstainingofapus

preparationincervicofacialactino-

mycosis: mass of Gram-positive,

branchedrods;nexttothemmixed

Gram-negativeflora.Tentativeclin-

icaldiagnosis:actinomycosis.

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Kayser, Medical Microbiology © 2005 Thieme