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

Studies on Legionella show a continuum from environment to human disease. L egionellosis is caused by Legionella species acquired from environmental so urces, principally water sources such as cooling towers, where Legionella g rows intracellularly in protozoa within biofilms. Aquatic biofilms, which a re widespread not only in nature, but also in medical and dental devices, a re ecological niches in which Legionella survives and proliferates and the ultimate sources to which outbreaks of legionellosis can be traced. Invasio n and intracellular replication of L. pneumophila within protozoa in the en vironment play a major role in the transmission of Legionnaires' disease. P rotozoa provide the habitats for the environmental survival and reproductio n of Legionella species. L. pneumophila proliferates intracellularly in var ious 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 g rowth requirements of Legionella, the ability of Legionella to enter a viab le non-culturable state, the association of Legionella with protozoa and th e occurrence of Legionella within biofilms complicates the detection of Leg ionella and epidemiological investigations of legionellosis. Polymerase cha in reaction (PCR) methods have been developed for the molecular detection o f Legionella and used in environmental and epidemiological studies. Various physical and chemical disinfection methods have been developed to eliminat e Legionella from environmental sources, but gaining control of Legionella in environmental waters, where they are protected from disinfection by grow ing within protozoa and biofilms, remains a challenge, and one that must be overcome in order to eliminate sporadic outbreaks of legionellosis.