Medical Microbiology

238 4 BacteriaasHumanPathogens

Extracellulartoxinsandenzymes.Themostimportantinthecontextof

pathogenicityare:

&StreptolysinO,streptolysinS.Destroythemembranesoferythrocytes

andothercells.StreptolysinOactsasanantigen.Pastinfectionscanbede-

tectedbymeasuringtheantibodiestothistoxin(antistreptolysintiter).

&Pyrogenicstreptococcalexotoxins(PSE)A,B,C.Responsibleforfever,

scarletfeverexanthemandenanthem,sepsis,andsepticshock.Thepyro-

genicexotoxinsaresuperantigensandthereforeinduceproductionoflarge

amountsofcytokines(p. 7 7).

&Streptokinase.Dissolvesfibrin;facilitatesspreadofstreptococciintis-

sues.

&Hyaluronidase.Breaksdownasubstancethatcementstissuestogether.

&DNases.BreakdownofDNA,producingrunnypus.

Pathogenesisandclinicalpictures.Streptococcaldiseasescanbeclassified

aseitheracute,invasiveinfectionsorsequelaetothem.

&Invasiveinfections.Thepathogensenterthroughtraumasormicrotrau-

masintheskinormucosaandcauseinvasivelocalorgeneralizedinfections

(Fig.4. 4 ).Therarecasesofseveresepticinfectionandnecrotizingfasciitis

occurinpersonswithahigh-riskMHCIIallotype.Inthesepatients,the

PSEsuperantigens(especiallyPSEA)inducelargeamountsofcytokineby

bindingatthesametimetotheMHCIIcomplexandthebchainofthe

Tcellreceptor.Theexcesscytokinesthusproducedarethecauseofthesymp-

toms.

&Sequelae.Glomerulonephritisisanimmunecomplexdisease(p. 11 3)and

acuterheumaticfevermaybeatypeIIimmunedisease(p. 1 09).

Diagnosis.Whatisinvolvedindiagnosisisdetectionofthepathogenby

meansofmicroscopyandculturing.GroupAantigencanbedetectedusing

particlescoatedwithantibodiesthatprecipitateagglutination(latexagglu-

tination,coagglutination).Usingthesemethods,directdetectionofAstrep-

tococciintonsillitisisfeasibleinthemedicalpractice.However,thisdirect

detectionmethodisnotassensitiveastheculture.DifferentiationofAstrep-

tococcifromotherb-hemolyticstreptococcicanberealizedinthelaboratory

withthebacitracindisktest,becauseAstreptococciaremoresensitivetobac-

itracinthantheothertypes.

Therapy.TheagentsofchoicearepenicillinGorV.Resistanceisunknown.

Alternativesareoralcephalosporinsormacrolideantibiotics,althoughresis-

tancetothelattercanbeexpected.Intreatmentofsepticshock,apolyvalent

immunoglobulinisusedtoinactivatethePSE.

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Kayser, Medical Microbiology © 2005 Thieme