Medical Microbiology

288 4 BacteriaasHumanPathogens

15 serovars,respectively,basedontheirantigenstructures.Shigellaearenon-

motileandthereforehavenoflagellar(H)antigens.

Pathogenesis.Shigellaeareonlypathogenicinhumans.Thepathogensare

ingestedorally.Onlyafewhundredbacteriasufficeforaninfectivedose.

Shigellaeentertheterminalileumandcolon,wheretheyaretakenupby

theMcellsintheintestinalmucosa,whichinturnareinclosevicinityto

themacrophages.Followingphagocytosisbythemacrophages,theshigellae

lysethephagosomeandactivelyinducemacrophageapoptosis.Theshigellae

releasedfromthedeadmacrophagesarethentakenupbyenterocytesviathe

basolateralsideofthemucosa(i.e.,retrogradetransport).Theinvasionis

facilitatedbyoutermembranepolypeptides,theinvasins,whicharecoded

byinvgeneslocalizedon 180 – 240 kbplasmids.Adjacententerocytesarein-

vadedbymeansoflateraltransferfrominfectedcells.Intheenterocytes,the

shigellaereproduce,finallydestroyingthecells.Shigelladysenteriaeproduces

shigatoxin,theprototypeforthefamilyofshigaliketoxins(orverocytotox-

ins),whichalsooccurinseveralotherEnterobacteriaceae.Thetoxininhibits

proteinsynthesisineukaryoticcellsbysplittingthe23SrRNAatacertain

locus.Shigatoxincontributestothecolonicepithelialdamage,thesmallin-

testinediarrheawithwaterystoolsattheonsetofshigellosisand(lessfre-

quent)thehemolytic-uremicsyndrome(HUS).

Clinicalpicture.Followinganincubationperiodoftwotofivedays,thedis-

easemanifestswithprofusewaterydiarrhea(=smallintestinediarrhea).

Later,stoolsmaycontainmucus,pus,andblood.Intestinalcramps,painful

stoolelimination(tenesmus),andfeverareobservedinthefurthercourse

oftheinfection.Complicationsincludemassiveintestinalbleedingandper-

forationperitonitis.ThesesevereeffectsarecausedmainlybyS.dysenteriae,

whereasS.sonneiinfectionsusuallyinvolveonlydiarrhea.

Diagnosisrequiresidentificationofthepathogeninaculture.Combinedse-

lective/indicatormediumsmustbeusedfortheprimaryculture.Suspected

coloniesareidentifiedbyusingindicatormediatodetectcertainmetabolic

characteristics(p. 21 4).Theserovarisdeterminedwithspecificantiserainthe

slideagglutinationtest.

Therapy.Anti-infectiveagentsarethefirstlineoftreatment(aminopenicillins,

4-quinolones,cephalosporins).Lossesofwaterandelectrolytesmayhavetobe

replaced.

Epidemiology and prevention. Bacterial dysentery occurs worldwide,

althoughitisusuallyseenonlysporadicallyindevelopedcountries.Inde-

velopingcountries,itsoccurrenceismorelikelytobeendemicandevenepi-

demic.Thesourceofinfectionisalwayshumans,inmostcasesinfectedper-

sonswhosestoolscontainpathogensforuptosixweeksafterthediseasehas

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Kayser, Medical Microbiology © 2005 Thieme