Legionella

1. Medical Microbiology Legionella 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 • Legionaire’s 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 • Questions? • Comments? • Assignments...