Legionella pneumophila: Morphology and Identification, Antigens and Cell Products, Pathology and Pathogenesis, Clinical Findings, Diagnostic Laboratory

Question 1

Legionella pneumophila: morphology and identification

Answer 1

Legionella pneumophila is a gram-negative, pleomorphic, facultatively intracellular bacterium that stains poorly with Gram stain but can be identified using silver stain. It appears as short rods or coccobacilli. It grows best on buffered charcoal yeast extract (BCYE) agar, which contains iron and cysteine. It is oxidase-positive, catalase-positive, and urease-negative. It is non-fermentative and requires special growth conditions.

Question 2

Legionella pneumophila: antigens and cell products

Answer 2

Legionella pneumophila contains a lipopolysaccharide (LPS) endotoxin, which contributes to its virulence. The bacterium also produces proteases, phosphatases, and other enzymes that enable it to invade and survive within macrophages. It produces the type IV secretion system, which is critical for intracellular survival. The bacteria have a unique ability to replicate inside amoebae, which they use as a reservoir.

Question 3

Legionella pneumophila: pathology and pathogenesis

Answer 3

Legionella pneumophila causes Legionnaires’ disease (severe pneumonia) and Pontiac fever (a milder flu-like illness). The bacteria are inhaled as aerosols and enter the lungs, where they are engulfed by macrophages. Inside these cells, they inhibit phagosome-lysosome fusion and replicate within a protective vacuole. The infection triggers a severe inflammatory response, causing lung tissue damage. The disease can result in multi-organ failure, particularly in immunocompromised individuals. The bacteria are primarily found in water reservoirs, such as air-conditioning units and hot tubs.

Question 4

Legionella pneumophila: clinical findings

Answer 4

Legionnaires’ disease presents with fever, cough, shortness of breath, and chest pain. Symptoms may progress to pneumonia, with a productive or non-productive cough, pleuritic pain, and respiratory failure. Gastrointestinal symptoms (diarrhea, nausea, vomiting) and confusion are also common. Pontiac fever is a milder, self-limiting flu-like illness without pneumonia. Complications may include sepsis and organ failure.

Question 5

Legionella pneumophila: diagnostic laboratory tests

Answer 5

Diagnosis is made by detecting Legionella pneumophila in sputum, bronchoalveolar lavage (BAL), or tissue samples. Urinary antigen tests (detecting Legionella LPS) are commonly used and highly sensitive. Culturing the organism requires special media (BCYE agar). Direct fluorescent antibody (DFA) testing and PCR can also detect the pathogen. Serology is less useful in acute cases.

Question 6

Legionella pneumophila: immunity

Answer 6

Cell-mediated immunity is crucial for controlling Legionella infection, as the bacteria replicate within macrophages. Individuals with impaired T-cell function (e.g., transplant patients, HIV-infected) are at higher risk for severe disease. Protective immunity after infection is not complete, and reinfection can occur, though severity is typically reduced.

Question 7

Legionella pneumophila: treatment

Answer 7

Legionella pneumonia is treated with antibiotics that penetrate cells, such as macrolides (azithromycin) or fluoroquinolones (levofloxacin). Rifampin can be added in severe cases. Beta-lactams are ineffective, as Legionella is intracellular. Early treatment reduces mortality.

Question 8

Legionella pneumophila: epidemiology and control

Answer 8

Legionella pneumophila is found in natural and artificial water systems (e.g., cooling towers, hot tubs, fountains). Outbreaks are typically linked to contaminated water sources. It is not transmitted person-to-person. Prevention includes proper maintenance of water systems to prevent biofilm formation and regular disinfection. Cases are often reported in summer and fall, when water systems are most active.