Covers the morphology, growth needs and staining difficulties of Legionella pneumophila, then moves through its virulence factors shown in a table and the stages of intracellular infection in alveolar macrophages. It also sets out laboratory diagnosis in a table listing samples, microscopy, culture, urine antigen, PCR and serology, followed by Legionnaires' disease, Pontiac fever, antibiotic classes, prevention and a closing summary.
Topic 65 of 113 in the MU Sofia syllabus for Microbiology (Medicine, Year 3). The free AceMed note covers it in 9 sections, about 238 words (3 min of reading), with 15 flashcards.
In this note
Family Legionellaceae
General Characteristics
Virulence Factors
Pathogenesis
Laboratory Diagnosis
Diseases Caused
Treatment
Prevention & Control
Summary Points
General Characteristics
Genus:Legionella, most important species: Legionella pneumophila
Morphology:
Gram-negative bacilli (often faintly staining)
Slender rods or coccobacilli
Non-spore forming, non-capsulated
Motility: Motile by flagella
Atmosphere: Strict aerobes
Staining peculiarity: Require silver stain (Dieterle stain) or Gimenez stain; poor visibility in Gram stain
Fastidious: Requires L-cysteine and iron to grow — not seen on blood agar
Culture Medium:
Buffered Charcoal Yeast Extract (BCYE) agar (enriched with cysteine and iron)
Incubated for 3–5 days at 35–37°C
Reservoir: Natural aquatic environments – freshwater, cooling towers, air conditioners, etc.
Pathogenesis
Inhaled via aerosolised water → reaches alveoli
Phagocytosed by alveolar macrophages, but survives due to Type IV secretion system
Forms replicative vacuole inside macrophages → multiplies intracellularly
Eventually kills the macrophage → releases bacteria → spreads to other cells
Induces strong inflammatory response, with tissue necrosis in the lung
Diseases Caused
1. Legionnaires' Disease
Severe pneumonia with:
High fever, dry cough
Confusion, diarrhea, and hyponatremia
Multilobar consolidation on CXR
Mortality rate: ~15% in hospitalised patients
2. Pontiac Fever
Milder illness, flu-like
No pneumonia
Resolves spontaneously in 2–5 days
Incubation: 24–48 hrs
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