Fastidiosos

Legionella pneumophila

Thin Gram-negative bacilli that stain poorly by Gram

Identification & Growth

Identification

Grows only on BCYE with L-cysteine; failure to grow on blood agar confirms the requirement
Urinary antigen detects serogroup 1, the most frequent

Growth Conditions

BCYE, 35 °C, humid atmosphere, 3–5 days; ground-glass colonies

Clinical Significance

Severe pneumonia with hyponatraemia and diarrhoea; outbreaks from water systems and cooling towers

Intrinsic Resistance

Beta-lactams and aminoglycosides are ineffective in vivo (intracellular pathogen), even when active in vitro

Bench Alerts

No routine susceptibility testing: treat with a macrolide or a fluoroquinolone
Notifiable disease — inform public health surveillance

Clinical Notes

Clinical presentations

Legionnaires' disease: severe atypical pneumonia, sometimes with multisystem involvement (confusion, diarrhea, hyponatremia, elevated transaminases).
Pontiac fever: self-limited febrile illness without pneumonia, resolving spontaneously within a few days.
Relevant pathogen in severe community-acquired pneumonia, especially in smokers, immunosuppressed patients and the elderly.

Specimens and collection

Urinary antigen test detects Legionella pneumophila serogroup 1, rapid and widely available, but with limited sensitivity for other serogroups and species.
Culture of respiratory secretions (sputum, bronchoalveolar lavage) on selective BCYE (buffered charcoal yeast extract) agar, essential for typing and outbreak investigation.
PCR on respiratory specimens extends detection beyond serogroup 1 and other Legionella species, useful when urinary antigen is negative but clinical suspicion remains high.

Epidemiology and at-risk populations

Environmental organism of freshwater and man-made water systems (cooling towers, showers, hot-water systems), transmitted via aerosols.
No person-to-person transmission; outbreaks linked to hotels, hospitals and cruise ships require environmental investigation and mandatory reporting.
Higher risk in smokers, heavy alcohol users, older adults, immunosuppressed patients and those with chronic lung disease.

Resistance and therapeutic implications

Intracellular pathogen; beta-lactams are clinically ineffective despite occasional in vitro activity, as they do not penetrate the intracellular compartment adequately.
Respiratory fluoroquinolones (levofloxacin) and macrolides (azithromycin) are the classes of choice, with adequate intracellular activity.
In vitro susceptibility testing correlates poorly with clinical outcome and is not routinely performed; therapy is guided by clinical practice guidelines rather than antibiograms.

Bench and reporting notes

Faintly staining Gram-negative rod requiring BCYE supplemented with cysteine and iron; it does not grow on routine bacteriologic media.
Legionellosis is a notifiable disease in many countries; confirmed cases trigger environmental source investigation.
A negative urinary antigen result does not exclude the diagnosis; pair with culture or PCR when clinical suspicion is high.

Sources

Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.

FAQ: Frequently Asked Questions

How to identify Legionella pneumophila in the lab?

Legionella pneumophila is identified through grows only on bcye with l-cysteine; failure to grow on blood agar confirms the requirement, urinary antigen detects serogroup 1, the most frequent.

What are the intrinsic resistances of Legionella pneumophila?

This organism is naturally resistant to beta-lactams and aminoglycosides are ineffective in vivo (intracellular pathogen), even when active in vitro. These drugs should not be reported as susceptible.

Where is Legionella pneumophila commonly found?

It is typically associated with severe pneumonia with hyponatraemia and diarrhoea; outbreaks from water systems and cooling towers.

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