Legionella

Legionella stains poorly with Gram stain because of its branched-chain fatty acid cell wall and requires buffered charcoal yeast extract agar with cysteine and iron for growth; silver staining demonstrates it in tissue. It replicates within alveolar macrophages, which is why cell-mediated immunity matters and why smokers, the elderly, and the immunosuppressed are at risk. Sources are aerosolizing water systems — cooling towers, hot tubs, decorative fountains, showers, humidifiers — and there is no person-to-person spread. The clinical picture that distinguishes it from other atypical pneumonias is the combination of high fever, relative bradycardia, gastrointestinal symptoms especially watery diarrhoea, confusion, raised transaminases, and hyponatraemia from SIADH. Diagnosis is usually by urinary antigen, which detects only serogroup 1 and remains positive for weeks but is unaffected by prior antibiotics. Treatment is a respiratory fluoroquinolone such as levofloxacin, or a macrolide such as azithromycin; beta-lactams are ineffective because they do not penetrate cells. Pontiac fever is the self-limited flu-like form without pneumonia.