Lecture 14 BIOL 533 1
Legionella
BIOL 533
Lecture 14
Medical Microbiology
Lecture 14 BIOL 533 2
History
? American Legion Convention in 1976 in
Philadelphia hotel
– Respiratory distress and fever
– 200 affected
– 34 died
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History
? Six months after outbreak,CDC isolated
bacteria from post-mortum lung tissue
– Inoculated lung tissue into peritoneal cavity of
guinea pigs
– After animal became ill,removed its spleen
and injected tissue into fertilized chicken eggs
(rickettsial methodology)
Lecture 14 BIOL 533 4
History
? Organism was thought to be unique,given
new genus and species name,
Legionella pneumophila
– Epidemiological search revealed outbreak in
same hotel two years earlier
? Culture problems because organisms,
– Do not grow on common laboratory media
– Do not readily stain
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Microbial Physiology
and Structure
? Family Legionellaceae
? One genus,Legionella
– 25 species and 42 serotypes
– Legionella pneumophila responsible for 85%
of infections (serotype 1 most common)
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Microbial Physiology
and Structure
? Species can be differentiated by,
– DNA homology
– Cell wall fatty acids
– Biochemical testing
– Immunological serotyping
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Microbial Physiology
and Structure
? Morphology
– Gram— rods (pleomorphic on artificial media)
? Do not stain well except with special silver stain
– Culture,do not grow in ordinary lab media,
even though they are aerobic organisms
? Require high concentration of cysteine and are
inhibited by sodium ions and aromatic compounds
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Microbial Physiology
and Structure
? Media developed have,
– Charcoal to absorb aromatic compounds
– Non-sodium ion buffer
– Antibiotics to suppress other organisms
– Dyes to make colonies visible on solid media
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Pathogenesis
? Encounter,aquatic; found in lakes and
streams
– Large numbers in polluted water around power
stations
– Can live in chlorinated drinking water
– Normally found in hot water tanks of buildings
? Get into pipes and multiply in sediment and
accumulate over period of years
– Do not spread from person to person
– Sediment provides shelter as well as nutrition for other
bacteria that can supply cysteine
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Pathogenesis
? Entry,spread,and multiplication
– Entry,inhalation of organisms
? Incubation period,2-10 days
– Initial host response,acute inflammatory
response of aveoli and then bronchioles
(similar to pneumococcal infection)
? Neutrophils accumulate followed by macrophage
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Pathogenesis
– Different from pneumococcal infection
? Organisms located inside of macrophage
? Inhibit lysosomal fusion and acidification of
phagocyte
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Damage
? Organism possesses several exotoxins
– Proteases,hemolysins,and other cytotoxins
? One inhibits oxidative killing of neutrophils
? May also damage tissue directly
– Spread rapidly in lung tissue
? Symptoms resulting from inflammatory disease in
lung
– Cough,chest pains,abnormal breathing sounds,and
fever
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Damage
? Other target sites,heart,kidney,lymph
nodes,spleen,liver,and brain
? Neurological symptoms and diffuse
metabolic abnormalities result from
bloodstream invasion,confusion,delirium
? Some patients have gastrointestinal
symptoms,including diarrhea
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Disease Syndromes
? In general,healthy people rarely get
disease
– Immunosuppressed patients and heavy
smokers with a history of respiratory
problems
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Disease Syndromes
? Pontiac fever (flu-like illness)
– Pontiac,Michigan in 1968
– Symptoms,fever,chills,muscle aches,
malaise,headache
– Develops over 12-hour period,persists for 2-5
days,then spontaneously resolves
– Minimal morbidity and no mortality
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Disease Syndromes
? Legionaire’s disease
– Much more severe,with high mortality unless
promptly treated
? Overall mortality is 15-20%
? Symptoms reviewed in Pathogenesis section
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Epidemiology
? Incidence is poorly understood because
disease documentation is difficult
– Pneumonias due to organism,less than 1% to
greater than 30%
? Estimated that 25,000 to 50,000 cases occur
annually
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Epidemiology
– Time of year
? Sporadic infections throughout year
? Epidemic infections occur in late summer and fall
? Elderly with decreased pulmonary function and
cellular immunity are at increased risk
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Laboratory Diagnosis
? Microscopy
– Difficult because of
? lack of staining
? intracellular nature
? Require large number of organisms to detect
– Best test,direct fluorescent antibody
? Test is very specific
? False positives observed rarely with Pseudomonas,
Bacteriodes,and other organisms
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Laboratory Diagnosis
? Culture,buffered charcoal yeast extract
(BCYE)
– Grow after 3-5 days
– Appear as small colonies with ground glass
appearance
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Laboratory Diagnosis
? Detection of Ag in respiratory secretions
or urine
– Can be detected immunologically or by
hybridization
– Excretion in urine can occur for as long as a
year
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Laboratory Diagnosis
? Serology,indirect fluorescent antibody
test
– Four-fold or greater increase in antibody titer
– Response can be delayed as long as eight
weeks
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Laboratory Diagnosis
? Identification
– Demonstration of typical morphology and
growth requirements
? Gram— bacteria (pleomorphic and weakly
straining)
? Growth on BCYE with cysteine but no BCYE
? Confirmation by fluorescent antibody
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Treatment
? Erythromycin is antibiotic of choice
– Can penetrate white blood cells
? Most strains possess ?-lactamases
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Prevention
? Complete elimination of organism from
water supplies is almost impossible
? Decrease in numbers normally effective
– Normally,healthy people not at risk
? Hyperchlorination and heating of water
frequently employed
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Lecture 14
? Questions?
? Comments?
? Assignments..,