Biological Properties
Legionella species belong to the family Legionellaceae, which comprises a single genus, Legionella. Of more than 50 species, 22 are pathogenic to humans. The primary causative agent is Legionella pneumophila, accounting for approximately 95% of legionellosis cases.
Morphology: Small Gram-negative rods (0.5–0.7 × 2–5 µm), although they can form long filaments (up to 20 µm). They are motile by means of flagella but lack spores and capsules.
Cultivation and Biochemistry:
- Obligate aerobes.
- Grow slowly (3–6 days) at 35 °C in an enhanced carbon dioxide environment (2.5–3%).
- Require specialized culture media (such as buffered charcoal-yeast extract [BCYE] agar). Obligate growth factors include L-cysteine, iron pyrophosphate, and alpha-ketoglutarate.
- Biochemical activity is very weak: they do not ferment carbohydrates, do not reduce nitrates, and do not produce urease. However, they possess catalase and can occasionally liquefy gelatin.
Epidemiology
In nature, Legionella species inhabit freshwater bodies, parasitizing protozoa (such as amoebae) or coexisting with blue-green algae. In urban environments, they have successfully adapted to man-made water systems, air conditioning cooling towers, and medical equipment.
A key factor in their survival and resistance to disinfectants is the formation of biofilms on rubber and synthetic surfaces.
Infection occurs exclusively via the aerogenic (aspiration) route by inhaling fine aqueous aerosols. Crucially, the infection is not transmitted from person to person. It occurs worldwide, most frequently in the summer months, and predominantly affects immunocompromised individuals.
Pathogenesis and Clinical Manifestations
Legionella species are facultative intracellular pathogens. Upon reaching the lungs, they selectively target and invade alveolar macrophages, where they actively replicate.
The bacteria utilize several virulence factors:
- Cytotoxin and superoxide dismutase protect the bacterium from the macrophage respiratory burst.
- Metalloprotease (cytolysin) prevents the fusion of the phagosome with lysosomes and damages host tissues.
- Endotoxin is released upon bacterial cell lysis, contributing to systemic toxicity.
After destroying the macrophage, the bacteria escape to infect neighboring cells, triggering severe inflammation.
Two main clinical forms of legionellosis:
- Legionnaires' disease: A severe form of pneumonia with an incubation period of 2–10 days. Characterized by high fever, chest pain, and dyspnea. Frequently complicated by respiratory and renal failure. Mortality can reach 25% (and higher in immunocompromised patients).
- Pontiac fever: An acute, self-limiting respiratory illness without pneumonia. It has a short incubation period (36–48 hours) and presents with milder flu-like symptoms, including dry cough and fever. There are no fatalities, but the attack rate in exposed populations can reach 100%.
Diagnosis and Treatment
Diagnostic samples include sputum, pleural fluid, and bronchoalveolar lavage. Host defense relies primarily on cell-mediated immunity (antibodies play little to no protective role).
Diagnostic Methods:
- Rapid diagnostics: Detection of soluble Legionella antigen in urine via ELISA or immunochromatography (days 2–10). Direct fluorescent antibody (DFA) staining and PCR are also used for respiratory specimens.
- Serology (retrospective): Demonstration of a significant rise in antibody titer between acute and convalescent sera.
- Culture: Isolation on specialized selective media.
Treatment requires antibiotics capable of penetrating human cells to reach the intracellular pathogen. The primary therapeutic agents are macrolides and fluoroquinolones. There is no specific vaccine available; prevention relies entirely on rigorous sanitary maintenance and disinfection of water and ventilation systems.