Legionella pneumophila

Legionellae are Gram-negative aerobic rods, isolated from surface water, mud, or thermally polluted lakes or streams. It is pathogenic for man and it has no known soil or animal sources. Legionella species are widespread in natural and man-made fresh water reservoirs. One of interesting characteristics of these bacteria is the ability to parasitize intracellularly on eukariotic cells. Majority of Legionella species infect free-living ameobae such as Hartmanella sp. and Acanthamoeba castellanii. The species grow best at temperatures from 20°C to 42°C. However, some of Legionella species are able to infect mammalian alveolar macrophages and cause a disease.

Legionella pneumophila are Gram-negative aerobic coccobacilii, isolated from surface water, mud, or thermally polluted lakes or streams. It is pathogenic for man and it has no known soil or animal sources. Legionella pneumophila is a main causative agent of Legionnaires' Disease. The species is responsible for more than 98% of the Legionnaires' Disease cases, and about 95% are due to serogroup 1. It has been isolated from numerous environmental sites as well as from human lung tissue, respiratory secretions, and blood.

Legionnaires' Disease (LD) is an acute, sometimes fatal, pneumonia-like bacterial infection characterized by high fever, malaise, muscle aches, respiratory disorders and headache. It is named for an outbreak at the 1976 Philadelphia convention of the American Legion. Legionnaires' disease can be very serious and can cause death in up to 5% to 30% of cases. Most cases can be treated successfully with antibiotics. The incubation period is about 7 days (from 2 to 19 days). Sporadic cases occur year around, but most cases of outbreaks happen in the warmer periods of the year. It is impossible to clinically distinguish patients with LD from patients with pneumococcal pneumonia. Culturing bacteria from biological samples, detection of antibodies in blood, detection of Legionella antigens in urine, and specific amplification of Legionella DNA by Polymerase Chain Reaction (PCR) are methods that can be applied to diagnose the disease with various degree of certainty depending on time and reagents' availability, and specific needs.

Pontiac fever is a milder infection caused by the same type of Legionella bacteria. The symptoms of Pontiac Fever (called after Pontiac in Michigan, where it was first documented) usually last for 2 to 5 days and may also include fever, headaches, and muscle aches (myalgia; however, there is no pneumonia. Symptoms usually go away without treatment.

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Transmission

Legionella come from natural fresh water reservoirs, such as lakes, ponds, and puddles, where they parasitize on a broad range of protozoan species as hosts. The availability of the hosts plays a major role in the reproduction and mass release of highly infectious Legionella forms into environments where they can be spread by airborn water caplets and inhaled by people.

Legionella thrive in man-made aquatic environments where the water temperature is higher than ambient temperature: whirlpool spas, cooling towers (air-conditioning units from large buildings), water used for drinking and bathing, water fountains, humidifiers, ice machines, and vegetable misters, etc. In fact, most cases of legionellosis can be traced to using such systems. It is a major concern for health professionals and construction and water systems maintenance workers.

Legionella bacilii enter the human body through inhalation of contaminated aerosols. They can reach the alveolae in the lungs where they are attacked and engulfed by alveolar macrophages. Instead of being killed the bacilii hijack the phagosomes, convert them into specialized membrane-bound compartments, and start living and multiplying in a manner indistinguishable from that found during growth within amoebae, their usual hosts.

Infection of the human macrophages appears to be a dead end in spreading the bacteria because transmission of Legionella pneumophila among humans has not been observed so far.

Intracellular parasitism

Interest in this bacterium stems from its ability to manipulate host cell vesicular-trafficking pathways and establish a membrane-bound replication vacuole, making it model for intracellular or, more specifically, intravacuolar pathogens.

Intracellular lifestyle of these microorganisms allows them to gain the following competitive advantages relative to other microorganisms: (1) evasion of predators in the environment and (2) evasion of host's humoral and cellular immune responses while having easy access to all necessary nutrients.

To be successful these pathogens need to solve the following important problems: (1) prevent the newly formed vacuole from merging (fusing) into antimicrobial lysosomal network; (3) manage nutrients's acquisition through the vacuolar membrane; (3) deal with space limitations during proliferative stage.

Regular phagocytosis vs. Legionella intracellular parasitism