Contamination of domestic water heaters with Legionella pneumophila: Impact of water temperature on growth and dissemination of the bacterium

Sediment and tap water from 255 randomly selected domestic water heaters were cultured on buffered charcoal yeast extract agar and two semiselective media to detect the presence of Legionellaceae . Different serogroups of L. pneumophila , the only species of Legionellaceae identified in this study, were found in 84 of the 255 water heaters and this contamination occurred only in electric water heaters ( n = 205). Sediment contamination by L. pneumophila was strongly associated with water temperature. The bacterium was detected in 45% (80/176) of the sediments with temperature lower than 40°C and in only 14% (4/29) of the sediments at this or higher temperatures. When the peripheral water temperature was lower than 56°C, 23% (9/39) of the tap water samples were contaminated with Legionella while only one of the 161 samples that were above this temperature contained Legionella . These results do suggest that reducing the water temperature in electric water heaters could play a role in the multiplication of L. pneumophila in the water heaters and in its dissemination in the household.