Legionella pneumophila strains isolated from six patients, three air-conditioning- and cooling tower-derived strains, and three hot water supply-derived strains were analyzed by three genetic typing methods. The results of the whole-cell DNA restriction endonuclease analysis and the restriction patterns based on genes coding for rRNA correlated with each other and demonstrated that the patient isolates were indistinguishable from the air-conditioning- and cooling tower-derived isolates but differed markedly from the hot water supply-derived isolates. The patient and air-conditioning- and cooling tower-derived strains contained plasmids of the same molecular weight; the hot water supply-derived strains were plasmidless. These results indicated that the cooling tower or the air-conditioning system was the environmental source for the examined cluster of Legionnaires disease strains
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