Quantification of Legionella pneumophila by qPCR and culture in tap water with different concentrations of residual disinfectants and heterotrophic bacteria.

Legionellosis prevalence is increasing in the United States. This disease is caused primarily by the bacterium Legionella pneumophila found in water and transmitted by aerosol inhalation. This pathogen has a slow growth rate and can "hide" in amoeba, making it difficult to monitor by the traditional culture method on selective media. Tap water samples (n = 358) collected across the United States were tested for L. pneumophila by both culture and quantitative Polymerase Chain Reaction (qPCR). The presence of other bacteria was quantified by heterotrophic plate counts (HPC). Residual disinfectant concentrations (free chlorine or monochloramine) were measured in all samples. Legionella pneumophila had the highest prevalence and concentration in the chlorinated water samples that had a free‑chlorine value of less than 0.2 mg Cl/L. In total, 24% (87/358) of the samples were positive for L. pneumophila either by qPCR or 3% (11/358) were positive by culture. In chloramine-treated samples, L. pneumophila was detected by qPCR in 21% (31/148) and 1% (2/148) by culture, despite a high monochloramine residual >1 mg Cl/L. Despite the presence of a high disinfectant residual (>1 mg Cl/L), HPC counts were substantial. This study indicates that both culture and qPCR methods have limitations when predicting a potential risk for disease associated with L. pneumophila in tap water. Measuring disinfectant residuals and quantifying HPC in water samples may be useful adjunct parameters for reducing Legionellosis' risk from public water supplies at high-risk locations.