Legionella pneumophila
One-Sentence Definition
Legionella pneumophila is an aquatic Gram-negative intracellular pathogen that infects amoebae and human alveolar macrophages, causing Legionnaires’ disease (severe pneumonia) and Pontiac fever after inhalation of contaminated aerosols.
Taxonomy & Morphology
- Gram-negative rod (stains poorly on routine Gram of sputum)
- Fastidious: buffered charcoal yeast extract (BCYE) with L-cysteine
- Serogroup 1 causes most diagnosed disease; many other species exist
Virulence Highlights
- Dot/Icm type IV secretion — remodels phagosome into replicative niche
- Intracellular lifestyle in macrophages mirrors amoebal infection
- Biofilms in water systems (Biofilm) protect from disinfectants
Clinical Syndromes
- Legionnaires’ disease: atypical pneumonia, hyponatremia, GI/neuro clues
- Pontiac fever: flu-like, self-limited
- Outbreaks from cooling towers, showers, decorative fountains
Diagnosis
- Urine antigen (mainly serogroup 1) — rapid but incomplete coverage
- Culture on BCYE (gold standard for epi); PCR on lower respiratory samples
- Pair with water system investigation in clusters
Treatment Notes (conceptual)
- Legionella is intracellular — use agents with intracellular activity (macrolides, fluoroquinolones; historically also doxycycline/rifampin in combinations)
- β-lactams alone inadequate despite in-vitro quirks
- Severity-guided duration
AMR
- Clinical resistance uncommon vs other respiratory pathogens; monitor MIC in reference labs for treatment failures
- Environmental biocide tolerance ≠ antibiotic resistance
Genomics & Computational Notes
- Sequence-based typing (SBT) / cgMLST for outbreak linkage to water sources
- Comparative genomics of Dot/Icm and effector repertoires
- See Phylogenomics and Outbreak Typing · Infection Prevention and Control
Related Notes
- Community-Acquired Pneumonia · Mycoplasma pneumoniae · Biofilm · One Health
- MOCs: MOC - Bacteriology · MOC - Public Health & Epidemiology · MOC - Diseases by System
Active Recall
- Why might sputum Gram stain miss Legionella?
- Limitation of urine antigen testing?
- Why are penicillins a poor monotherapy choice?