Legionella: from environmental habitats to disease pathology, detection and control

Abstract

Studies on Legionella show a continuum from environment to human disease. Legionellosis is caused by Legionella species acquired from environmental sources, principally water sources such as cooling towers, where Legionella grows intracellularly in protozoa within biofilms. Aquatic biofilms, which are widespread not only in nature, but also in medical and dental devices, are ecological niches in which Legionella survives and proliferates and the ultimate sources to which outbreaks of legionellosis can be traced. Invasion and intracellular replication of L. pneumophila within protozoa in the environment play a major role in the transmission of Legionnaires' disease. Protozoa provide the habitats for the environmental survival and reproduction of Legionella species. L. pneumophila proliferates intracellularly in various species of protozoa within vacuoles studded with ribosomes, as it also does within macrophages. Growth within protozoa enhances the environmental survival capability and the pathogenicity (virulence) of Legionella. The growth requirements of Legionella, the ability of Legionella to enter a viable non‐culturable state, the association of Legionella with protozoa and the occurrence of Legionella within biofilms complicates the detection of Legionella and epidemiological investigations of legionellosis. Polymerase chain reaction (PCR) methods have been developed for the molecular detection of Legionella and used in environmental and epidemiological studies. Various physical and chemical disinfection methods have been developed to eliminate Legionella from environmental sources, but gaining control of Legionella in environmental waters, where they are protected from disinfection by growing within protozoa and biofilms, remains a challenge, and one that must be overcome in order to eliminate sporadic outbreaks of legionellosis.

Community structure of bacteria on different types of mineral particles in a sandy soil

“…As we picked up the sand mineral particles from the soil and washed them before extraction of DNA in this study, it is possible that they lived in the soil as free-living organisms, were associated with fine mineral particles or organic matter, or attached loosely rather than firmly to the sand particles. As members of these taxa have often been detected in biofilms and organic granules (Atlas 1999;Hugenholtz et al 2001;Sekiguchi et al 2003;Yamada and Sekiguchi 2009), the latter two might be more probable as sites for these groups.…”

Detection of Legionella pneumophila in water and biofilm samples by culture and molecular methods from man-made systems in São Paulo - Brazil

“…study could be the symbiotic interaction between this bacterium and free-living amoebae. It is known that amoebae are natural hosts of legionellae and play a major role in the multiplication of L. pneumophila in some water cultures (2,3,16,29,46). In addition, amoebae were found in the water samples of all eight sources sampled in the study.…”

“…In addition, at least 20 other species of Legionella have been associated with human infections (16). Legionella bacteria do exist as free-living planktonic forms in the environment (36), intracellular parasites of protozoans (3), and/ or inhabitants of mixed-community biofilms (29).Thus, the diversity of types and the ubiquitous occurrence of legionellae in water environments or moist soil make it difficult to identify epidemic strains, and outbreaks of legionellosis have been associated mainly with contamination of man-made aquatic environments (2,16).…”

La légionellose

“…La concentration en Legionella pneumophila au niveau du réservoir est certainement un élément important dans l'intensité du risque d'infection. Cependant, la dose infectante pour l'homme n'est pas clairement définie [7]. Le risque semble plus important au-dessus du seuil de 1 000 UFC/l (unité formant colonie/litre), mais ce niveau doit être modulé en fonction du terrain de l'hôte.…”