Legionella spp.

Family: Legionellaceae. 52 species and >70 serogroups

Clinical disease = Legionnaires’ disease

<aside> 💡 Legionnaires' Disease:

• 90% caused by Legionella pneumophila.

• 84% of cases caused by Legionella pneumophila serogroup 1 (sg1) </aside>

• List of Legionella spp. reported to cause human disease

History

• Outbreak of pneumonia at the annual convention of the American Legion, Philadelphia 1976
  • 29 of 182 affected died
• Organism first identified 1977 by culture
  • Named Legionella pneumophila due to this.
  • Source identified as air conditioning system - patients infected by aerosolised infected water
• Earlier outbreak of mild respiratory symptoms in Pontiac, Michigan 1968, termed Pontiac fever. Retrospectively identified as being caused by L. pneumophila

| Site | Intracellular organism

In environment - found naturally in freshwater 20-42C - parasitise free-living amoeba (e.g. Acanthamoeba and Naegleria) and protozoa Can colonise human-made water systems Able to form biofilms therefore resist disinfectants

Once aerosolised - cause human respiratory tract disease | | --- | --- | | Risk Factors | Age>50 Smokers Chronic lung disease Immunosuppressed host Occupational, e.g. construction Foreign travel

Specific risks: gardeners - longbeachae | | Pathogenic mechanisms | - Enter alveolar macrophages and epithelial cells via phagocytosis.

• Dot/Icm Type IV Secretion System (T4SS) delivers effector proteins into host cells that inhibit phagosome-lysosome fusion and allow bacterial proliferation.
• Once nutrients are exhausted by the replication process, the bacterial flagellae release caspase-1, host cells undergo apoptosis, and legionellae are released to infect other cells. | | Clinical syndromes | Severe atypical community acquired pneumonia
• Incubation period 2-10 days
• Initial symptoms fatigue, fever, myalgia, headache, followed by cough and dyspnoea
• Nausea, vomiting and confusion common.
• Typically bilateral patchy or diffuse infiltrates on chest XR

Investigations:

• Hyponatraemia
• Transaminitis

Case fatality rate 10% |

Lab diagnostics

Species ID (Identify to genus lab in diagnostic labs. To species level in reference lab)

<aside> 💡 Images of Gram and Agar available here: https://microbe-canvas.com/Bacteria/gram-negative-rods/obligate-aerobic-3/oxidase-positive-2/legionella-pneumophila.html

</aside>

• Microscopy:
  • Gram negative, pleomorphic
  • Coccobacilli in tissue/secretions 3-5µm. Filamentous in culture 10-25µm
  • Small size makes visualisation difficult with Gram. Immunofluorescent microscopy more sensitive (rarely used)
• Culture (increasingly rarely performed)
  • Growth conditions:
    • Aerobic, nutritionally fastidious (will not grow on BA/McConkey)
    • Agars:
      • Buffered-charcoal-yeast extract (BCYE) agar base supplemented with ACES (N-2- acetamido-2-aminoethanesulphuronic acid) buffer with L-cysteine incubated for up to 10 days in a moist atmosphere at 35-37°C
      • Selective agar; Buffered cefamandole, polymyxin, anisomycin, α-ketoglutarate medium (BMPAα) incubated for up to 10 days in a moist atmosphere at 35-37°C.
    • L-Cysteine + soluble iron supports growth
    • 5% CO2 improves growth of some species
    • Grows at 72hrs incubation
  • Colony appearance:
    • Flat 0.5-1mm. Become smooth convex iridescent and opal-like with prolonged incubation. Thick string may form when a loop is inserted into the colony
  • Biochemical tests:
    • Catalase positive, oxidase variable
  • Pros: Can identify organism for AST and typing
  • Cons: Growth takes 48-72hrs, requires specialist agar and laboratory expertise, requires respiratory samples