Legionella ecophysiology in drinking water distribution systems

Legionella pneumophila cause water-based infections resulting in severe pneumonia. Our aim was to improve knowledge regarding Legionella ecology in Drinking Water Distribution Systems (DWDS). A three years survey was conducted and seasonal samples were taken from a small DWDS system. Legionella was isolated and genotyped using Multiple-Locus Variable number of tandem-repeat Analysis (MLVA). Different physiological traits related with the bacterial life cycle and virulence were studied; temperature-dependent growth kinetics modeling, amoebae and macrophage infectivity rates, hemolytic activity and antimicrobial susceptibility. Five Legionella MLVA-genotypes (Gt4, Gt6, Gt15, Gt17, Gt18) were identified. The presence of a specific genotype, Gt4, consistently co-occurred with high Legionella counts and seemed to “trigger” high Legionella counts in cold water. Mathematical modeling of the bacterial growth at different temperatures revealed significant differences between the growth traits of the genotypes. Gt4 strains exhibited superior growth and adaptation to lower temperatures (25-30ºC) while Gt15 strains were better adapted to higher temperatures (42-45ºC). Moreover, Gt4 strains were more infective towards amoebae and macrophages and were significantly more resistant to antimicrobials compared to Gt6 and Gt15 strains. We have shown for the first time that different genotypes possess significantly different physiological traits. These traits may influence their colonization at specific ecological niches within DWDSs. Moreover, differences in virulence traits suggest that each genotype has a different pathogenicity potential towards humans. Our findings highlight the importance of understanding the eco-physiology of Legionella genotypes and may improve the assessment of public health risk of Legionella in DWDSs.