Regrowth of Legionella pneumophila in a heat-disinfected plumbing system.

We examined the factors involved in the occurrence of Legionellaceae in a hospital water system and the recontamination by Legionella pneumophila after a thermal disinfection procedure was studied. Three months after the heat treatment (70 degrees C), the regrowth of the two prevalent Legionella strains (L. pneumophila serogroup 1 [Oxford-like] and L. pneumophila serogroup 2) reached the original level of cell numbers. Genomic analysis (pulsed-field gel electrophoresis) revealed the strains to be survivors of the decontamination. Temperature tolerance experiments showed that the serogroup 1 strain exhibited a higher tolerance to 60 degrees C than the serogroup 2 strain, which could account for the order of reappearance of the strains after the heat treatment. Potential host amoebae, including Acanthamoeba spp. and Vahlkampfia spp., which are known to play a critical role in the amplification process of Legionella, were isolated from the plumbing system. In-vitro studies demonstrated both Legionella strains for a similar rate of multiplication in A. castellanii. In competitive coinfections, however, the serogroup 1 strain achieved a higher rate of multiplication if compared with the serogroup 2 strain.