Medical Microbiology

StreptococcusandEnterococcus 243

S.mutans,S.sanguis,andS.mitisare,besidesActinomycesviscosusandA.

naeslundii,responsiblefordentalcaries(Fig.4. 6 ).Thesestreptococcicanat-

tachtotheproteinscoveringthetoothenamel,wheretheythenconvertsu-

croseintoextracellularpolysaccharides(mutan,dextran,levan).Thesesticky

substances,inwhichtheoriginalbacteriallayeralongwithsecondarybac-

terialcolonizersareembedded,formdentalplaque.Thefinalmetabolites

ofthenumerousplaquebacteriaareorganicacidsthatbreachtheenamel,

allowingthedifferentcariesbacteriatobegindestroyingthedentin.

Enterococcus(Enterococci)............................

Enterococciareawidespreadbacterialgenus(p.220)normallyfoundinthe

intestinesofhumansandotheranimals.Theyarenonmotile,catalase-nega-

tive,andcharacterizedbygroupantigenD.Theyareabletoproliferateat

458 C,inthepresenceof6.5%NaClandatpH9,qualitiesthatdifferentiate

themfromstreptococci.Asclassicopportunists,enterococcishowonly

lowlevelsofpathogenicity.However,theyarefrequentlyisolatedascompo-

nentsofamixedflorainnosocomialinfections(p.343).Ninetypercentof

suchisolatesareidentifiedasE.faecalis, 5 – 10 %asE.faecium.Amongthe

mostdangerousenterococcalinfectionsisendocarditis,whichmustbetreat-

edwithacombinationofanaminopenicillinandstreptomycinorgentamicin.

Therapeuticsuccessdependsonthebactericidalefficacyofthecombination

used.Theefficacylevelwillbeinsufficientinthepresenceofhighlevelsof

resistancetoeitherstreptomycin(MIC> 1000 mg/l)orgentamicin(MIC

> 500 mg/l)orresistancetotheaminopenicillin.Enterococcifrequently

developresistancetoantibiotics.Strainsmanifestingmultipleresistance

arefoundmainlyinhospitals,inkeepingwiththeclassicopportunistic

PronouncedDentalCaries

Fig.4. 6 Certainoralstrep-

tococci(S.mutans)arethe

mainculpritsintoothdecay.

4

Kayser, Medical Microbiology © 2005 Thieme