Difference in virulence of environmental isolates of Legionella pneumophila

Endemic nosocomial Legionnaires disease has occurred at our medical center for several years. Two subtypes of Legionella pneumophila serogroup 1 (UH-1 and RH-1) have been isolated in approximately equal numbers from hospital potable water. However, almost all clinical isolates have been UH-1. To assess potential differences in virulence, 50% lethal doses (LD50) and 50% infective doses (ID50) of UH-1 and RH-1 were determined by intraperitoneal infection in guinea pigs. The UH-1 LD50 was 7.41 X 10(6) CFU, which was significantly lower than the RH-1 LD50 of 9.12 X 10(7) CFU (P = 0.0001). The mean time to death in UH-1-infected guinea pigs was also significantly shorter than in RH-1-infected animals (P = 0.0008). The UH-1 ID50 was 5.8 X 10(3) CFU, and although it was lower than the RH-1 ID50 of 1.4 X 10(4) CFU, this difference was not statistically significant (P = 0.21). This study demonstrates a difference in virulence between UH-1 and RH-1 in guinea pigs. Differences in strain virulence, as demonstrated between these two subtypes, may help to explain the widespread isolation of L. pneumophila from the environment contrasted with the limited occurrence of human Legionnaires disease.