Diverse populations of Legionella pneumophila present in the water of geographically clustered institutions served by the same water reservoir.

We cultured potable water from seven institutions (six hospitals and one medical school) every 2 weeks for 6 months for Legionella pneumophila. All of the institutions were located close to each other and received water from the same freshwater source. Two institutions (the medical school and hospital F, a maternity hospital) never had L. pneumophila isolated from their potable water. The remaining five had 17 to 72% of their water samples positive for L. pneumophila. Most of the isolates were serogroup 1; however, in hospital B serogroup 5 accounted for 56% of the isolates. Oxford and OLDA monoclonal antibody subtypes of L. pneumophila serogroup 1 coexisted in four of the five institutions, while subtype France only was found in one institution. All 10 isolates from this institution lacked plasmids. The other four institutions had Legionella populations with plasmid profiles II, III, and VI. Two of these institutions also had isolates with no plasmids. The distribution of the plasmid types was significantly different for all institutions except C and D. The distribution of monoclonal antibody subtypes was significantly different for L. pneumophila isolates recovered from institutions C and D. There were no characteristics that distinguished the culture-positive institutions from the culture-negative areas. We conclude that diverse populations of L. pneumophila exist within these institutions despite their geographic proximity and identical potable water source.