Biological properties
Legionellae belong to the family Legionellaceae, which includes a single genus, Legionella. Of more than 50 species, 22 are pathogenic to humans. The main causative agent is Legionella pneumophila, which accounts for about 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, but lack spores and capsules.
Cultivation and biochemistry:
- These are strict aerobes.
- They grow slowly (3–6 days) at 35 °C with an elevated carbon dioxide concentration (2.5–3%).
- They require special nutrient media (e.g., buffered charcoal-yeast extract agar). Obligate growth factors: cysteine, iron pyrophosphate, and alpha-ketoglutaric acid.
- 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 sometimes liquefy gelatin.
Epidemiology
In nature, legionellae inhabit freshwater bodies, parasitizing protozoa (e.g., amoebae) or coexisting with blue-green algae. In urban environments, they have successfully adapted to technogenic conditions: water supply systems, air conditioners, and medical equipment.
The key factor for their survival and resistance to disinfectants is biofilm formation on rubber and synthetic surfaces.
Infection occurs exclusively via the aerogenic (aspiration) route by inhaling fine water aerosols. It is important to remember: the infection is not transmitted from person to person. It occurs universally, most frequently in the summer, especially affecting immunocompromised individuals.
Pathogenesis and clinical presentation
Legionellae are facultative intracellular parasites. Upon entering the lungs, they selectively target and invade alveolar macrophages, actively multiplying within them.
The bacteria utilize a set of virulence factors:
- Cytotoxin and superoxide dismutase protect against the macrophage's "respiratory burst".
- Metalloprotease (cytolysin) prevents the fusion of the phagosome with the lysosome and damages tissue.
- Endotoxin is released upon the destruction of the microbial cell and causes intoxication.
After destroying the macrophage, the bacteria emerge and infect new cells, triggering severe inflammation.
Two main forms of legionellosis:
- Legionnaires' disease: severe pneumonia. Incubation period is 2–10 days. Manifests with high fever, chest pain, and dyspnea. Frequently complicated by respiratory and renal failure. Fatality rates can reach 25% (and higher in immunodeficiency).
- Pontiac fever: acute respiratory illness without pneumonia. Short incubation period (36–48 hours). Milder course with dry cough and fever; there are no fatalities, but contagiousness (within the aerosol zone) reaches 100%.
Diagnostics and treatment
Sputum, pleural effusion, and bronchoscopy specimens are used to confirm the diagnosis. Cell-mediated immunity provides bodily defense (antibodies lack protective properties).
Diagnostic methods:
- Rapid diagnostics: detection of soluble antigen in urine (ELISA or immunochromatography) on days 2–10. DFA and PCR are also used (for bronchoscopy specimens).
- Serology (retrospective): detection of rising antibody titers in paired sera.
- Bacteriological method on complex media.
Treatment involves antibiotics capable of penetrating human cells, since the pathogen hides inside them. The main drugs are macrolides and fluoroquinolones. There is no specific vaccine; the basis of prevention is sanitary control of water supply and ventilation systems.
Briefly: answers to topical questions
Which serogroups of Legionella pneumophila have the greatest clinical significance?
Of greatest clinical significance is Legionella pneumophila serogroup 1. This particular variant of the pathogen is identified most frequently and accounts for about 80% of all legionellosis cases. In total, 16 different serogroups are distinguished within this bacterial species. Their antigenic specificity is determined by the lipopolysaccharide antigen. When diagnosing, it is important to consider that standard rapid tests detect only the first serogroup, so a negative result does not rule out infection caused by other Legionella serogroups.
Which protozoan species serve as a natural reservoir for legionellae in nature?
Amoebae and other protozoa inhabiting freshwater bodies serve as the natural reservoir for legionellae in nature. Legionellae exist in a symbiotic relationship with them. Specific protozoan species are not specified in the sources.
What complications can develop in Legionnaires' disease?
The following complications can develop in Legionnaires' disease: acute respiratory failure in 20–30% of cases, toxic shock syndrome, renal failure, and central nervous system damage. The fatality rate is 8–25% in the general population and can reach 60% in immunocompromised individuals.
Class and exam questions
Why is it impossible to contract legionellosis from an infected person?
The pathogen is transmitted exclusively through the inhalation of fine water aerosols contaminated with the bacteria. Person-to-person transmission is impossible.
What is the main difference between Legionnaires' disease and Pontiac fever?
Legionnaires' disease is a severe pneumonia with systemic lesions and a risk of death. Pontiac fever presents as an acute respiratory infection without pneumonia and always ends in recovery.
What culture media are required for growing Legionella?
They grow only on complex media (e.g., buffered charcoal-yeast extract agar) supplemented with cysteine and iron pyrophosphate.
What else to study on this topic
- History of the discovery of L. pneumophila
- Antigenic structure and serogroups of L. pneumophila
- Mechanisms of biofilm formation
- Pathogenetic action of cytolysin and superoxide dismutase
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