Plasmids as epidemiological markers in nosocomial Legionnaires' disease.

Plasmid analysis was used in the investigation of an outbreak of nosocomial Legionnaires' disease in four patients. Serogroup 1 strains were isolated from two patients, the air-conditioning cooling tower, and two hot-water tanks. All serogroup 1 strains contained two plasmids with approximate molecular masses of 21 and 48 megadaltons (Mdal). The serogroup 1 strain found in the cooling-tower isolate also contained an additional 1.9 Mdal-plasmid. Restriction-endonuclease analysis of the 21-Mdal plasmid that was present in patient and hot water-tank isolates revealed identical EcoRI and HaeIII fragment patterns. Digestion of the similarly sized plasmid in the cooling-tower isolate resulted in a unique fragment pattern. The data provide direct bacteriologic evidence implicating the hot-water tanks rather than the cooling tower as the source of the infecting strain.