Water ecology of Legionella and protozoan: environmental and public health perspectives

Ecological studies on Legionella spp. are essential to better understand the germ sources in the natural environments, the mechanism of entry into man-made water systems and the factors enabling survival and growth in aquatic habitats. Legionella spp exhibits peculiar and multiple strategies to adapt to stressful environment conditions which normally impair other germ survival. These strategies include the ability to enter in a viable but non cultivable (VBNC) state, to multiply intracellularly within a variety of protozoa, such as amoebae, to survive as a free organism within biofilms and to be enhanced/inhibited by the presence of other aquatic bacteria. The host-parasite interaction has been shown to be central in the pathogenesis and ecology of L.pneumophila. The bacterial-protozoan interaction contributes to the amplification of Legionella population in water systems, represents a shelter against unfavourable environmental conditions, acts as a reservoir of infection, and contributes to virulence as primes the pathogen to infect human cells. Legionella is able to survive as a free organism for long periods within biofilms which are widespread in man-made water systems. Biofilm provides shelter and nutrients, exhibits a remarkable resistance to biocide compounds and chlorination, thus representing ecological niches for legionella persistence in the environments. A most extensive knowledge on biofilm-associated legionellae may lead to most effective control measures to prevent legionellosis. Lastly, new perspectives in controlling legionella contamination can arise from investigations on aquatic bacteria able to inhibit legionella growth in natural and artificial water systems.