Morphology: Faintly Gram-negative bacillus; intracellular (replicates within alveolar macrophages). Requires charcoal yeast extract (BCYE) agar — does NOT grow on standard media. Dieterle silver stain visualizes in tissue.
Typical drugs
- #1Levofloxacin— Preferred for severe disease — superior intracellular penetration vs macrolides; respiratory FQ as monotherapy in CAP.
- #2Azithromycin— Preferred IV agent if FQ contraindicated; combo with FQ for severe disease occasionally used.
- #3Moxifloxacin
- #4Doxycycline— For mild outpatient disease; less data than FQ or macrolide.
Empiric therapy when resistant
Fluoroquinolone or macrolide remains active. β-lactams INHERENTLY ineffective (intracellular pathogen). For severe / immunocompromised: levofloxacin 750 mg daily ± azithromycin combination; treat 7–14 days (longer for severe / immunocompromised).
Resistance mechanisms
Efflux pump
Efflux pump upregulation rare in clinical isolates
Example: Treatment failure usually reflects under-dosing or wrong drug class.
Resistance notes
Resistance is rare. Treatment failure usually = wrong empiric (β-lactam-only regimen) or under-dosing. β-lactams completely inactive — Legionella replicates inside cells.
Common syndromes
Pearls
Severe CAP with classic extras: GI symptoms (diarrhea), hyponatremia, transaminitis, hypophosphatemia, relative bradycardia, headache, AMS. Always send urine antigen in hospitalized CAP. Source: contaminated water (hot tubs, cooling towers, plumbing in old buildings, cruise ships, hospitals). Higher mortality in immunocompromised, smokers, elderly. Pontiac fever (self-limited febrile illness, no pneumonia) is the milder form. β-lactams useless — if patient on ceftriaxone alone is failing, add atypical coverage.
References
- IDSA/ATS Community-Acquired Pneumonia in Adults (2019)
- CDC Legionella Surveillance