Multiple types of Legionella pneumophila serogroup 6 in a hospital heated-water system associated with sporadic infections

Five sporadic cases of nosocomial Legionnaires' disease were documented fro m 1989 to 1997 in a hospital in northern Italy. Two of them, which occurred in a 75-year-old man suffering from ischemic cardiopathy and in an 8-year- old girl suffering from acute leukemia, had fatal outcomes. Legionella pneu mophila serogroup 6 was isolated from both patients and from hot-water samp les taken at different sites in the hospital. These facts led us to conside r the possibility that a single clone of L. pneumophila serogroup 6 had per sisted in the hospital environment for 8 years and had caused sporadic infe ctions. Comparison of clinical and environmental strains by monoclonal subt yping, macrorestriction analysis (MRA), and arbitrarily primed PCR (AP-PCR) showed that the strains were clustered into three different epidemiologica l types, of which only two types caused infection. An excellent corresponde nce between the MRA and AP-PCR results was observed, with both techniques h aving high discriminatory powers. However, it was not possible to different iate the isolates by means of ribotyping and analysis of rm operon polymorp hism. Environmental strains that antigenically and chromosomally matched th e infecting organism were present at the time of infection in hot-water sam ples taken from the ward where the patients had stayed. Interpretation of t he temporal sequence of events on the basis of the typing results for clini cal and environmental isolates enabled the identification of the ward where the patients became infected and the modes of transmission of Legionella i nfection. The long-term persistence in the hot-water system of different cl ones of L. pneumophila serogroup 6 indicates that repeated heat-based contr ol measures were ineffective in eradicating the organism.