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Kusnetsov J, Torvinen E, Perola O, Nousiainen T, Katila ML. Colonization of hospital water systems by legionellae, mycobacteria and other heterotrophic bacteria potentially hazardous to risk group patients. APMIS 2003;111:546–56. Occurrences of legionellae and nontuberculous mycobacteria were followed in water systems of a tertiary care hospital where nosocomial infections due to the two genera had been verified. The aim was to examine whether their occurrence in the circulating hot water can be controlled by addition of a heat-shock unit in the circulation system, and by intensified cleaning of the tap and shower heads. One hot water system examined had an inbuilt heat-shock system causing a temporary increase of temperature to 80 °C, the other was an ordinary system (60 °C). The heat-shock unit decreased legionella colony counts in the circulating hot water (mean 35 cfu/l) compared to the ordinary system (mean 3.6×103 cfu/l). Mycobacteria constantly present in the incoming cold water (mean 260 cfu/l) were never isolated from the circulating hot water. Water sampled at peripheral sites such as taps and showers contained higher concentrations of legionellae, mycobacteria, and mesophilic and Gram-negative heterotrophs than the circulating waters. The shower water samples contained the highest bacterial loads. The results indicate the need to develop more efficient prevention methods than the ones presently used. Prevention of mycobacteria should also be extended to incoming cold water.