Medical Microbiology

262 4 BacteriaasHumanPathogens

Mycobacterium....................................................

&Mycobacteriaareslenderrodbacteriathatarestainedwithspecialdiffer-

entialstains(Ziehl-Neelsen).Oncethestaininghastaken,theycannotbede-

stainedwithdiluteacids,hencethedesignationacid-fast.Intermsofhuman

disease,themostimportantmycobacteriaarethetuberculosisbacteria(TB)

M.tuberculosisandM.bovisandtheleprosypathogen(LB)M.leprae.

TBcanbegrownonlipid-richculturemediums.Theirgenerationtimeis

12 – 18 hours.Initialdropletinfectionresultsinprimarytuberculosis,loca-

lizedmainlyintheapicesofthelungs.Theprimarydiseasedevelopswiththe

Ghonfocus(Ghon’scomplex),wherebythehilarlymphnodesareinvolvedas

well.Ninetypercentofprimaryinfectionfociremainclinicallysilent.In 10 %

ofpersonsinfected,primarytuberculosisprogressestothesecondarystage

(reactivationororgantuberculosis)afterafewmonthsorevenyears,whichis

characterizedbyextensivetissuenecrosis,forexamplepulmonarycaverns.

Thespecificimmunityandallergythatdevelopinthecourseofaninfection

reflectTlymphocytefunctions.Theallergyismeasuredintermsofthetu-

berculinreactiontocheckforclinicallyinapparentinfectionswithTB.Diag-

nosisoftuberculosisrequiresidentificationofthepathogenbymeansofmi-

croscopyandculturing.Modernmolecularmethodsarenowcomingtothe

foreinTBdetection.Manifesttuberculosisistreatedwithtwotofouranti-

tuberculechemotherapeuticsineitherashortregimenlastingsixmonthsor

astandardregimenlastingninemonths.

IncontrasttoTB,theLBpathogensdonotlendthemselvestoculturingon

artificialnutrientmediums.Leprosyismanifestedmainlyinskin,mucosa,

andnerves.Inclinicalterms,thereisa(malignant)lepromatoustypeleprosy

anda(benign)tuberculoidtype.Nondifferentialformsarealsofrequent.Hu-

mansarethesoleinfectionreservoir.Transmissionofthediseaseisbyclose

contactwithskinormucosa. &

ThegenusMycobacteriumbelongstotheMycobacteriaceaefamily.Thisgenus

includessaprophyticspeciesthatarewidespreadinnatureaswellasthecau-

sativepathogensofthemajorhumandiseasecomplexestuberculosisand

leprosy.MycobacteriaareGram-positive,althoughtheydonottakegram

stainingwell.Theexplanationforthisisacellwallstructurerichinlipids

thatdoesnotallowthealkalinestainstopenetratewell.Atanyrate,once

mycobacteriahavebeenstained(usingradicalmethods),theyresistdestain-

ing,evenwithHCl-alcohol.Thispropertyisknownasacidfastness.

4

Kayser, Medical Microbiology © 2005 Thieme