Medical Microbiology

Legionella(Legionnaire’sDisease) 311

eitherlymphogenouslyorhematogenously,andcausesecondaryabscesses

there.MalleusnolongeroccursinEurope.

Burkholderiapseudomallei.Thisspeciesisthecausativeorganisminme-

lioidosis,adiseaseofanimalsandhumansresemblingmalleus.Thenatural

reservoirsofB.pseudomalleiaresoilandsurfacewater.Thepathogeninvades

thebodythroughinjuriesoftheskinormucosaandcausesmultiplesubcu-

taneousandsubserousabscessesandgranulomas.Startingfromprimaryfoci,

theinfectioncandisseminateandcauseabscessesinanumberofdifferent

organs.ThisdiseaseisobservedmainlyinAsia.

Legionella(Legionnaire’sDisease)

&LegionellaistheonlygenusinthefamilyLegionellaceae.Thespecies

Legionellapneumophilaisresponsibleformostlegionellosesinhumans.

Legionellaearedifficulttostain.TheyareGram-negative,aerobicrodbacter-

ia.Specialmediumsmustbeusedtogrowthemincultures.Infectionswith

Legionellaoccurwhendropletscontainingthepathogensareinhaled.Two

clinicallydistinctformsareonrecord:legionnaire’sdiseaseleadingtoa

multifocalpneumoniaandnonpneumoniclegionellosisorPontiacfever.

Thepersonsmostlikelytocontractlegionnaire’sdiseasearethosewithapri-

marycardiopulmonarydiseaseandgenerallyweakenedimmunedefenses.

Laboratorydiagnosticmethodsincludemicroscopywithdirectimmuno-

fluorescence,culturingonspecialmediumsandantibodyassays.Theantibio-

ticsofchoicearethemacrolides.Thenaturalhabitatoflegionellaeisdamp

biotopes.Thesourcesofinfectionlistedintheliteratureincludehotandcold

watersupplysystems,coolingtowers,moisturizingunitsinairconditioners,

andwhirlpoolbaths.Legionellosescanoccurbothsporadicallyandinepi-

demics. &

Classification.Legionellabacteriawerediscoveredin 197 6,occasionedbyan

epidemicamongthoseattendingaconferenceofAmericanLegionnaires(for-

merprofessionalsoldiers).TheyarenowclassifiedinthefamilyLegionella-

ceae,whichtodatecomprisesonlythegenusLegionella.Thisgenuscontains

numerousspeciesnotlistedhere.MosthumaninfectionsarecausedbyL.

pneumophila,whichspeciesissubdividedinto 12 serogroups.Humaninfec-

tionsarecausedmainlybyserogroup 1.

Morphologyandculture.L.pneumophilaisarodbacterium0.3– 1 lmwide

and 2 – 20 lmlong.ItscellwallstructureisoftheGram-negativetype,but

gramstaininghardly“takes”withthesebacteriaatall.Theycanberendered

visiblebymeansofdirectimmunofluorescence.

4

Kayser, Medical Microbiology © 2005 Thieme