Regrowth of Legionella pneumophila in a heat-disinfected plumbing system

We examined the factors involved in the occurrence of Legionellaceae in a h ospital water system and the recontamination by Legionella pneumophila afte r a thermal disinfection procedure was studied. Three months after the heat treatment (70 degrees C), the regrowth of the two prevalent Legionella str ains (L. pneumophila serogroup 1 [Oxford-like] and L, pneumophila serogroup 2) reached the original level of cell numbers. Genomic analysis (pulsed-fi eld gel electrophoresis) revealed the strains to be survivors of the decont amination. Temperature tolerance experiments showed that the serogroup 1 st rain exhibited a higher tolerance to 60 degrees C than the serogroup 2 stra in, 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 amplifica tion process of Legionella, were isolated from the plumbing system. In-vitr o studies demonstrated both Legionella strains for a similar rate of multip lication in A. castellanii. In competitive coinfections, however, the serog roup 1 strain achieved a higher rate of multiplication if compared with the serogroup 2 strain.