Legionella

1
Legionella
Medical Microbiology

• BIOL 533
• Lecture 14

2
History

• American Legion Convention in 1976 in
  Philadelphia hotel
• Respiratory distress and fever
• 200 affected
• 34 died

3
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)

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

5
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)

6
Microbial Physiology and Structure

• Species can be differentiated by
• DNA homology
• Cell wall fatty acids
• Biochemical testing
• Immunological serotyping

7
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

8
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

9
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

10
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

11
Pathogenesis

• Different from pneumococcal infection
• Organisms located inside of macrophage
• Inhibit lysosomal fusion and acidification of
  phagocyte

12
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

13
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

14
Disease Syndromes

• In general, healthy people rarely get disease
• Immunosuppressed patients and heavy smokers with
  a history of respiratory problems

15
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

16
Disease Syndromes

• Legionaires disease
• Much more severe, with high mortality unless
  promptly treated
• Overall mortality is 15-20
• Symptoms reviewed in Pathogenesis section

17
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

18
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

19
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

20
Laboratory Diagnosis

• Culture buffered charcoal yeast extract (BCYE)
• Grow after 3-5 days
• Appear as small colonies with ground glass
  appearance

21
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

22
Laboratory Diagnosis

• Serology indirect fluorescent antibody test
• Four-fold or greater increase in antibody titer
• Response can be delayed as long as eight weeks

23
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

24
Treatment

• Erythromycin is antibiotic of choice
• Can penetrate white blood cells
• Most strains possess ?-lactamases

25
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

26
Lecture 14

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