Medical Microbiology

302 4 BacteriaasHumanPathogens

H.influenzaeisafacultativeanaeroberequiringgrowthfactorsXandVin

itsculturemedium.TheXfactorishemin,requiredbythebacteriatosynthe-

sizeenzymescontainingheme(cytochromes,catalase,oxidases).TheXfactor

requirementisgreatlyreducedinanaerobicculturing.TheVfactorwasiden-

tifiedasNADorNADP.Astandardbloodagarplatedoesnotcontainsufficient

freeVfactor.Somebacteria,inparticularStaphylococcusaureus,produceex-

cessNADandevensecretethiscoenzymeintothemedium.ThatiswhyH.

influenzaecanproliferateintheimmediatevicinityofS.aureuscolonies.This

isknownasthesatellitephenomenon(Fig.4. 21 b).Themediumnormally

usedtocultureH.influenzaeischocolateagarcontainingsufficientamounts

oftheXandVfactors.

Pathogenesisandclinicalpictures.H.influenzaeisamucosalparasiteofthe

upperrespiratorytractpresentin 30 – 50 %ofhealthypersons.Thestrains

usuallyfoundarenonencapsulatedandthereforehardlyvirulent.Thecapsule

protectsthecellsfromphagocytosisandisthustheprimarydeterminantof

pathogenicity.OthersincludetheaffinityofH.influenzaetorespiratorytract

mucosaandmeningesandproductionofanIgA 1 protease(seep. 1 5).

H.influenzaeinfectionsareseenfrequentlyinchildrenagedfromsix

monthstofouryearsofageduetothelowlevelsofanticapsuleantibodies

inthisagegroup.Maternalantibodiesstillprotectchildrenduringthefirst

monthsoflife.Thebodyhasbuiltupasufficientstoreofantibodiesbytheage

offour.Anylistofpotentialclinicaldevelopmentsmustbeginwithmenin-

gitis,followedbyepiglottitis,pneumonia,empyema,septicarthritis,osteo-

myelitis,pericarditis,cellulitis,otitismedia,andsinusitis.Haemophilusinfec-

tionsinadultsareusuallysecondarycomplicationsofsevereprimaryill-

nessesortheresultofcompromisedimmunedefenses.Themostfrequent

complicationisanacuteexacerbationofchronicbronchitis.Pneumonias

causedbyH.influenzaearealsoobserved,oftenassuperinfectionsfollowing

viralinfluenza.Inimmunocompromisedadults,eventhenonencapsulated

strainscancauseinfectionsoftheupperandlowerrespiratorytract.

Diagnosis.Themethodofchoiceisidentificationofthepathogenincere-

brospinalfluid,blood,pus,orpurulentsputumusingmicroscopyandculture

assays.SatellitingonbloodagarisanindicationofaVfactorrequirement.An

Xfactorrequirementisconfirmedmostreadilybytheporphyrintest,witha

negativeresultinthepresenceofH.influenzae.

Therapy.Inviewoftheincreasingnumberofbetalactamase-producingH.

influenzaestrainsobservedinrecentyears,penicillinase-stablebetalactam

antibioticsshouldbeusedtotreattheseinfections.Thelikelihoodthata

strainproducesbetalactamaseis 5 – 30 %inmostcountries.4-quinolones

areanalternativetobetalactamsthatshouldnot,however,beusedinchil-

dren.Theagentofchoiceinmeningitisisceftriaxone.

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Kayser, Medical Microbiology © 2005 Thieme