Quantitative polymerase chain reaction (qPCR) offers a rapid, automated, and potentially on-site method for quantifying L. pneumophila in building potable water systems, complementing and potentially replacing traditional culture-based techniques. However, the application of qPCR in assessing human health risks is complicated by its tendency to overestimate such risks due to the detection of genomic copies that do not correspond to viable, infectious bacteria. This study examines the relationship between L. pneumophila measurements obtained via qPCR and culture-based methods, aiming to understand and establish qPCR-to-culture concentration ratios needed to inform associated health risks. We developed a Poisson lognormal ratio model and a random-effects meta-analysis to analyze variations in qPCR-to-culture ratios within and across sites. Our findings indicate these ratios typically vary from 1:1 to 100:1, with ratios close to 1:1 predicted at all sites. Consequently, adopting a default 1:1 conversion factor appears necessary as a cautious approach to convert qPCR concentrations to culturable concentrations for use in models of associated health risks, for example, through quantitative microbial risk assessment (QMRA) frameworks. Where this approach may be too conservative, targeted sampling and the applications of viability-qPCR could improve the accuracy of qPCR-based QMRA. Standardizing qPCR and culture-based methods and reporting site-specific environmental factors that affect the culturability of L. pneumophila would improve the understanding of the relationship between the two methods. The ratio model introduced here shifts us beyond simple correlation analyses, facilitating investigations of temporal and spatial heterogeneities in the relationship. This analysis is a step forward in the integration of QMRA and molecular biology, as the framework demonstrated here for L. pneumphila is applicable to other pathogens monitored in the environment.

The authors have declared no competing interest.

ES was funded by postdoctoral fellowships from the Natural Sciences and Engineering Research Council of Canada (558161-2021) and the Fonds de Recherche du Quebec Nature et technologies (303866). Additional funding was provided by the Swiss Federal Food Safety and Veterinary Office (FSVO), the Federal Office of Public Health (FOPH) and the Federal Office of Energy (FOE), through the project LeCo (Legionella Control in Buildings; Aramis nr.: 4.20.01), as well as Eawag discretionary funding.

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All data produced are available online at: https://tinyurl.com/44tx6a5s