Legionella

Legionella

Subjects: Microbiology · Systems: Microbiology · Tags: Microbiology

One-line summary

Legionella pneumophila is a fastidious, facultative intracellular, gram-negative bacillus that lives in aquatic environments (replicates in amoebae and human macrophages). It causes two syndromes: Legionnaires’ disease (severe atypical pneumonia) and Pontiac fever (self-limited, influenza-like illness). ([CDC][1], [NCBI][2])

1) Microbiology & lab culture essentials

• Classification: gram-negative bacillus; many species (L. pneumophila is the most clinically important; serogroup 1 causes most human disease). Cultures require special media — buffered charcoal yeast extract (BCYE) supplemented with L-cysteine and iron (Legionella will not grow on routine blood/sabouraud media). Charcoal also helps neutralize toxic metabolites. ([NCBI][2], [Thermo Fisher][3])
• Microscopy: organisms are poorly visualized on Gram stain (may look faintly gram-negative); better visualized with silver stains (Dieterle, Warthin-Starry) or by immunofluorescent methods. ([PMC][4], [legionella.com][5])
• Physiology: aerobic, non–spore forming, needs cysteine & iron; can resist some disinfectants by surviving in biofilms and inside free-living amoebae. ([Wikipedia][6])

2) Intracellular lifestyle & virulence (why it causes severe pneumonia)

• Facultative intracellular pathogen: in humans Legionella primarily infects and replicates inside alveolar macrophages (and other mononuclear phagocytes). To do so it uses a large type IV secretion system (Dot/Icm) that injects many effector proteins into the host cell to remodel the phagosome into a protective Legionella-containing vacuole (LCV), preventing normal phagosome-lysosome killing and enabling intracellular multiplication. Important virulence factors include Dot/Icm, Mip (macrophage infectivity potentiator), and other secreted effectors. This intracellular lifestyle explains why antibiotics that penetrate and act inside cells (macrolides and fluoroquinolones) work best. ([PMC][7], [Oxford Academic][8])

3) Reservoirs, transmission & incubation

• Natural reservoir: fresh water (lakes, rivers), man-made water systems (plumbing, hot water tanks, cooling towers, hot tubs, decorative fountains). Legionella replicates inside amoebae and persists in biofilms — aerosolized water droplets are the usual transmission route (inhalation of contaminated aerosols). Person-to-person transmission is exceptionally rare. ([CDC][9], [Wikipedia][10])

• Two clinical syndromes:

  • Legionnaires’ disease — pneumonia; incubation usually 2–14 days (commonly ~5–6 days).
  • Pontiac fever — a milder febrile illness without pneumonia; incubation hours to 3 days and high attack rate in exposed groups. ([CDC][11])

4) Clinical features — what to look for

• Presentation: cough (may be nonproductive), high fever, dyspnea; commonly GI symptoms (nausea, diarrhea, abdominal pain), myalgias, and confusion/AMS. Lab clues often include hyponatremia, elevated liver enzymes, and high C-reactive protein/ESR; chest imaging often shows patchy unilobar or multilobar consolidation (may progress to severe/necrotizing pneumonia and ARDS). Risk factors: older age, smoking, chronic lung disease, immunosuppression, recent travel/hotels/cruise ships, hospital water exposure. ([CDC][12], [NCBI][13])

5) Diagnosis — tests, strengths & limitations

• Urinary antigen test (UAT): widely used, rapid. Important limitation — most commercial UATs detect L. pneumophila serogroup 1 only (so a negative UAT does NOT rule out other Legionella species/serogroups). UAT is sensitive for SG1 and remains positive for days–weeks after infection. Always pair UAT with lower respiratory testing when possible. ([CDC][14])
• Lower respiratory tract culture on BCYE (selective BCYE with antibiotics): the gold standard for diagnosis and for public-health typing (required to link clinical isolates to environmental isolates during outbreaks). Requires special lab handling and may take several days. ([CDC][14])
• PCR/molecular tests on respiratory specimens: increasingly used (fast, can detect non-SG1), variable availability; useful adjunct to UAT and culture.
• Direct fluorescent antibody (DFA) and immunohistochemistry can detect organisms in tissue. Serology (paired sera) has historical use but is less helpful for acute management. ([CDC][14], [PMC][4])

6) Treatment — principles and recommended agents

• Rationale: because Legionella replicates inside macrophages, choose antibiotics with good intracellular penetration and activity: macrolides (azithromycin) or respiratory fluoroquinolones (levofloxacin, moxifloxacin) are first-line agents. For moderate–severe Legionella pneumonia, treatment courses are typically 7–10 days in otherwise healthy patients; longer courses (10–21 days) may be needed for immunocompromised patients or complicated infections — follow clinical response. Some guidelines favor fluoroquinolones for severe disease based on clinical outcome data; either macrolide or respiratory fluoroquinolone is acceptable as first-line depending on clinical scenario and local guidance. Start therapy promptly — do not delay antibiotics for testing. ([PMC][15], [CDC][16])

Practical examples (common regimens used in adults — check local protocols/dosing references before prescribing):

• Levofloxacin 750 mg PO/IV once daily (or 500 mg PO/IV twice daily in some regimens) — good lung penetration and bactericidal action.
• Azithromycin 500 mg IV/PO once daily (or 500 mg PO day 1 then 250 mg PO daily in some CAP regimens) — macrolide with high intracellular concentrations. (These are examples used in many centers; durations vary by severity and host factors.) ([IDSA][17], [PMC][15])

7) Outbreaks, public-health, and prevention

• Outbreaks are usually linked to contaminated building water systems (cooling towers, large plumbing systems, spa/hot tub aerosols). Public-health response includes: rapid case reporting, obtaining clinical respiratory specimens (culture + PCR), environmental sampling, and molecular comparison of isolates to identify the source; control measures include temperature control, chlorination/hyperchlorination, superheating/flushes, and engineering fixes to limit aerosolization. Hospitals and large facilities must monitor and manage water systems (risk management programs). ([CDC][18])

8) High-yield exam pearls

• Think “atypical pneumonia + GI symptoms + hyponatremia” → consider Legionella. ([CDC][12], [NCBI][13])
• Urine antigen = rapid & useful but detects mainly L. pneumophila serogroup 1 → negative UAT doesn’t exclude Legionella species/serogroups. Always send lower respiratory specimen for culture/PCR if Legionella is suspected. ([CDC][14])
• Special growth requirement: needs L-cysteine and iron and grows on BCYE — will NOT grow on routine blood agar. (Test trick: “Legionella won’t grow on blood agar — needs BCYE”.) ([Thermo Fisher][3])
• Pontiac fever = self-limited, no pneumonia, short incubation (~hours–3 days); Legionnaires’ disease = pneumonia, incubation 2–14 days (commonly 5–6 days). ([CDC][19])

9) Short diagnostic/treatment algorithm (for exam or ward)

1. Suspect Legionella in community/hospital/cluster pneumonia with GI symptoms, hyponatremia, recent hotel/hospital/cruise exposures, or in immunocompromised host. ([CDC][12])
2. Send urine antigen (rapid) + lower respiratory sample for culture (BCYE) and PCR before starting antibiotics when possible. ([CDC][14])
3. Start empiric therapy that covers Legionella if clinically indicated: azithromycin or levofloxacin (choose IV vs PO and agent per severity); narrow when organism/ susceptibilities known. ([PMC][15], [CDC][16])

10) Quick practice vignette (1-minute)

A 68-year-old smoker returns from a week-long hotel stay and presents with fever, cough, confusion, watery diarrhea, and sodium 125 mmol/L. Chest x-ray shows a right lower-lobe consolidation. Which pathogen is high on your differential and what tests/treatment do you request/start now? Answer: Legionella pneumophila. Send urine Legionella antigen and a lower respiratory specimen for culture/PCR; start empiric intracellular-active therapy (e.g., IV levofloxacin or azithromycin) while awaiting results. ([CDC][12], [PMC][15])

Disclaimer: For education only. Not medical advice; always follow your institution's guidance.