Water quality influences Legionella pneumophila determination

Legionellosis is a respiratory disease of public health concern. The bacterium Legionella pneumophila is the etiologic agent responsible for >90% of legionellosis cases in the United States. Legionellosis transmission primarily occurs through the inhalation or aspiration of contaminated water aerosols or droplets. Therefore, a thorough understanding of L. pneumophila detection methods and their performance in various water quality conditions is needed to develop preventive measures. Two hundred and nine potable water samples were collected from taps in buildings across the United States. L. pneumophila was determined using three culture methods: Buffered Charcoal Yeast Extract (BCYE) culture with Matrix-assisted Laser Desorption/Ionization Mass Spectrometry (MALDI-MS) identification, Legiolert® 10- and 100-mL tests, and one molecular method: quantitative Polymerase Chain Reaction (qPCR) assay. Culture and molecular positive results were confirmed by secondary testing including MALDI-MS. Eight water quality variables were studied, including source water type, secondary disinfectant, total chlorine residual, heterotrophic bacteria, total organic carbon (TOC), pH, water hardness, cold- and hot-water lines. The eight water quality variables were segmented into 28 categories, based on scale and ranges, and method performance was evaluated in each of these categories. Additionally, a Legionella genus qPCR assay was used to determine the water quality variables that promote or hinder Legionella spp. occurrence. L. pneumophila detection frequency ranged from 2 to 22% across the methods tested. Method performance parameters of sensitivity, specificity, positive and negative predictive values, and accuracy were >94% for the qPCR method but ranged from 9 to 100% for the culture methods. Water quality influenced L. pneumophila determination by culture and qPCR methods. L. pneumophila qPCR detection frequencies positively correlated with TOC and heterotrophic bacterial counts. The source water-disinfectant combination influenced the proportion of Legionella spp. that is L. pneumophila. Water quality influences L. pneumophila determination. To accurately detect L. pneumophila, method selection should consider the water quality in addition to the purpose of testing (general environmental monitoring versus disease-associated investigations).