Medical Microbiology

240 4 BacteriaasHumanPathogens

Epidemiologyandprophylaxis.Infectionfrequencyvariesaccordingtogeo-

graphicalarea,season,andage.Humansaretheonlypathogenreservoirfor

S.pyogenes.Transmissionisbydirectcontact(smearinfection)ordroplets.

Theincubationperiodisonetothreedays.Theincidenceofcarriersamong

childrenis 10 – 20 %,butcanbemuchhigherdependingontheepidemio-

logicalsituation.Carriersandinfectedpersonsarenolongercontagious

24 hoursafterthestartofantibiotictherapy.Microbiologicalfollow-up

checksofpatientsandfirst-degreecontactsarenotnecessary(exception:

rheumatichistory).

Inpersonswithrecurringinfectionsorwithacuterheumaticfeverintheir

medicalhistories,continuouspenicillinprophylaxiswithalong-termpeni-

cillinisappropriate(e.g., 1 .2millionIUbenzathinepenicillinpermonth).

Streptococcuspneumoniae(Pneumococci)...........

Morphologyandculturing.PneumococciareGram-positive,ovaltolancet-

shapedcoccithatusuallyoccurinpairsorshortchains(Fig.4.5a).Thecellsare

surroundedbyathickcapsule.

Whenculturedonbloodagar,S.pneumoniaedevelopa-hemolyticcolo-

nieswithamucoid(smooth,shiny)appearance(hence“S”form,Fig.4.5b).

Mutantswithoutcapsulesproducecolonieswitharoughsurface(“R”form).

Antigenstructure.Pneumococciareclassifiedin 90 differentserovarsbased

onthefinechemicalstructureofthecapsulepolysaccharidesactingasanti-

gens.Thiscapsuleantigencanbeidentifiedusingspecificantiseraina

reactionknownascapsularswelling.

Pathogenesisandclinicalpictures.Thecapsuleprotectsthepathogensfrom

phagocytosisandisthemostimportantdeterminantofpneumococcalviru-

lence.Unencapsulatedvariantsarenotcapableofcausingdisease.Otherpo-

tentialvirulencefactorsincludepneumolysinwithitseffectsonmembranes

andanIgA 1 protease.

Thenaturalhabitatofpneumococciisprovidedbythemucosaofthe

upperrespiratorytract.About 40 – 70 %ofhealthyadultsarecarriers.Pneu-

mococcalinfectionsusuallyarisefromthisnormalflora(endogenousinfec-

tions).Predisposingfactorsincludeprimarycardiopulmonarydiseases,pre-

viousinfections(e.g.,influenza),andextirpationofthespleenorcomplement

systemdefects.

Themostimportantpneumococcalinfectionsarelobarpneumoniaand

bronchopneumonia.Otherinfectionsincludeacuteexacerbationofchronic

bronchitis,otitismedia,sinusitis,meningitis,andcornealulcer.Severepneu-

mococcalinfectionsfrequentlyinvolvesepsis.

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Kayser, Medical Microbiology © 2005 Thieme