Legionella pneumophila

The Legionella genus comprises some fifty currently identified species amongst which Legionella pneumophila serogroup 1 is the most commonly found in human pathology. A few other species are also involved, including L. micdadei, L. longbeachae, L. dumoffii, L. anisa, L. gormanii and more recently Legionella waltersii .

Legionella are freshwater bacteria that live, survive and multiply in natural aquatic media (surface water and underground water in communication with surface water). They are naturally present in drinking water systems, which they may contaminate and colonise, especially as they are more resistant to chlorine than Escherichia coli, the standard bacterial indicator for water intended for human consumption.

Heat encourages the growth of Legionella (optimum growing temperature: 32 – 35°C). At temperatures between 20 and 43°C, they will multiply and colonise domestic hot water supplies.

To prevent the water from being colonised, it must be stored and distributed at a temperature of at least 50°C, or even higher, or the entire system must be given an effective chlorine treatment (0.2 mg/L free chlorine) provided the water does not lie stagnant for long periods.
In case of legionnaire’s disease or significant and widespread contamination, disinfection treatments can significantly lower the level of contamination in the water, provided there is no scaling, corrosion products or dead legs.

Such treatment may be thermal (circulating water at a temperature of 70°C for 30 minutes) or chemical using products approved by the Ministry of Health, followed by careful rinsing of the pipes. Chlorine concentrations of 50 mg/l do not destroy the Legionella present in cysts whereas a concentration of 2 mg/l is sufficient to kill free Legionella.

Legionella have the ability to be present and persist in water systems. They find ecological niches in biofilms, where they take root and can proliferate, in association with cyanobacteria, green algae or bacteria.

After being ingested by a protozoa, Legionella acquire the ability to set up residence and multiply.

Legionella infections take two clinical forms:

1. Legionellosis or legionnaire’s disease, an acute pneumonia, with a high death rate (around 15%), and
2. Pontiac fever, a milder infection that is not progressive and is of short duration.