Genetic diversity of Legionella pneumophila in hospital water systems.

It has been shown that different patients who had acquired legionellosis in a hospital setting were infected with the same strain even years apart. However, there are no longitudinal data describing the molecular epidemiology of Legionella pneumophila strains that contaminate a water system. This raised the question if there are any shifts of L. pneumophila strains over time, or after carrying out control measures. Using genotyping on a large collection of isolates, we investigated in a retrospective study the distribution of L. pneumophila serogroups and PFGE types in six different hospitals of the University of Heidelberg between 1991 and 2001. A total of 2012 water samples were drawn for routine testing and for evaluation of control measures, 747 samples were positive for L. pneumophila. Serogroups were determined by latex agglutination or by direct fluorescence assay; and 515 L. pneumophila isolates from water systems and six from patients underwent PFGE typing after SfiI-restriction. We identified seven serogroups and 19 genotypes among the water isolates. Each hospital had one to four predominating PFGE types that were stable over the investigation period. The oldest buildings in hospitals 4 and 5 (built 1876 and 1907) had more types than the newest one (built 1986). In all hospitals PFGE types were identified that could be found only sporadically. Although each hospital had its own warm water supply, we identified types that could be found in more than one hospital. However, there was no overlap of types in buildings that were fed from different wells. Infrequently occurring nosocomial legionellosis (n=3) were only caused by predominant strains. Contamination of water supplies seemed to be dominated by stable genotypes, even after various control measures. Additional genotypes could be isolated sporadically, however, their pathogenetic relevance seemed to be questionable.