Medical Microbiology

234 4 BacteriaasHumanPathogens

Coagulase-NegativeStaphylococci(CNS).............

CNSareanelementinthenormalfloraofhumanskinandmucosa.Theyare

classicopportuniststhatonlycauseinfectionsgivenacertainhostdisposi-

tion.

&S.epidermidis.ThisisthepathogenmostfrequentlyencounteredinCNS

infections(7 0 – 80 %ofcases).CNScausemainlyforeignbodyinfections.Ex-

amplesoftheforeignbodiesinvolvedareintravasalcatheters,continuous

ambulantperitonealdialysis(CAPD)catheters,endoprostheses,metalplates

andscrewsinosteosynthesis,cardiacpacemakers,artificialheartvalves,and

shuntvalves.Theseinfectionsfrequentlydevelopwhenforeignbodiesinthe

macroorganismarecoveredbymatrixproteins(e.g.,fibrinogen,fibronectin)

towhichthestaphylococcicanbindusingspecificcellwallproteins.They

thenproliferateonthesurfaceandproduceapolymericsubstance—thebasis

ofthedevelopingbiofilm.Thestaphylococciwithinthebiofilmareprotected

fromantibioticsandtheimmunesystemtoagreatextent.Suchbiofilmscan

becomeinfectionfocifromwhichtheCNSenterthebloodstreamandcause

sepsislikeillnesses.Removaloftheforeignbodyisoftennecessary.

&S.saprophyticusisresponsiblefor 10 – 20 %ofacuteurinarytractinfec-

tions,inparticulardysuriainyoungwomen,andforasmallproportionof

casesofnonspecificurethritisinsexuallyactivemen.

AntibiotictreatmentofCNSinfectionsisoftenproblematicduetothemulti-

pleresistanceoftenencounteredinthesestaphylococci,especiallyS.hemo-

lyticus.

StreptococcusandEnterococcus...................................

&StreptococciareGram-positive,nonmotile,catalase-negative,faculta-

tivelyanaerobiccoccithatoccurinchainsorpairs.Theyareclassifiedbased

ontheirhemolyticcapacity(a-,b-,c-hemolysis)andtheantigenicityofa

carbohydrateoccurringintheircellwalls(Lancefieldantigen).

b-hemolyticgroupAstreptococci(S.pyogenes)causeinfectionsofthe

upperrespiratorytractandinvasiveinfectionsoftheskinandsubcutaneous

connectivetissue.Dependingonthestatusoftheimmunedefensesandthe

geneticdisposition,thismayleadtoscarletfeverandsevereinfectionssuch

asnecrotizingfasciitis,sepsis,orsepticshock.Sequelaesuchasacuterheu-

maticfeverandglomerulonephritishaveanautoimmunepathogenesis.The

a-hemolyticpneumococci(S.pneumoniae)causeinfectionsoftherespiratory

tract.Penicillinsaretheantibioticsofchoice.Resistancetopenicillinsis

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Kayser, Medical Microbiology © 2005 Thieme