Legionella pneumophila in Domestic Hot Water Systems: Evaluation of Detection Methods and Environmental Factors Affecting Survival

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Virginia Tech


Legionella is the causative agent of Legionnaires' disease which hospitalizes 8,000 to 18,000 people in the United States each year. The disease in transmitted through inhalation or aspiration of water containing the bacterium and can be acquired within the home. Studies have found that 0-37% of domestic water heaters contain Legionella, making household hot water systems a potential route of exposure.

The objective of this research was to evaluate different methods for testing environmental samples for Legionella pneumophila and to analyze potable water conditions that affect survival of free living Legionella pneumophila in hot water tanks. Three heat pretreatment methods (50ºC for 30 minutes, 55ºC for 15 minutes, and 60ºC for 3 minutes) were not effective at recovering Legionella in this study. There was no statistically significant difference between the three acid pretreatment methods that were tested (pH 2.0 with a neutralizing solution, pH 2.2, and the CDC method). Six media (BCYE, DGVP, PCV, GPCV, CCVC, and GPVA) exhibited similar Legionella recovery, except for when high levels of non-Legionella organisms were present, in which case BCYE demonstrated lower recovery. When disinfectant was present, if sodium thiosulfate was not added before the disinfectant, Legionella recovery was lower. However, this result was not statistically significant for free chlorine until after 5 minutes. Pseudomonas aeruginosa (up to 67.5 cfu/ml) and pyocyanin (up to 9 mg/l) did not have an effect on Legionella recovery under the tested conditions.

Environmental factors affecting survival of free living Legionella pneumophila in hot water tanks were also studied. After one day exposure in small-scale simulated water heaters at 55ºC, viable Legionella could not be recovered. At 44ºC, Legionellae were recovered after one day but only at very low levels after eight days. Between 23 and 37ºC, Legionella could survive longer than eight days. Copper (Cu²⁺) concentrations above 2160 ppb were found to be toxic to Legionella, but iron (Fe³⁺) between 1 and 2160 ppb did not affect survival. Above pH 11 survival was greatly reduced. No effect was observed between pH 5-10. When glass fiber filters were added to the reactors and they were seeded with tap water and sediment slurry, Legionellae were retained in 7 of 16 reactors for 327 days.

The results of this work will assist in optimal identification of Legionella via microbial analysis of potable water samples, thereby assisting in prevention and diagnosis of factors contributing to Legionnaires' disease, especially in settings with high-risk patients (e.g. hospitals). Water systems studying Legionella amplification in domestic hot water systems can use simulated or real distribution system sampling to reproduce and study factors that prevent or reduce Legionella growth and persistence.



potable water, Legionella