Source-to-Tap Monitoring of Legionella in Drinking Water Distribution System: Comparison of Culture-Based and qPCR Detection Within a Risk-Based Framework

The occurrence of Legionella bacteria in drinking water distribution systems poses an important public health concern, particularly in the context of opportunistic premise plumbing pathogens (OPPPs) and the implementation of EU Directive 2020/2184. This study evaluated the prevalence of Legionella using a “source-to-tap” approach and compared culture-based and molecular detection methods. A total of 120 water samples were collected between February and November 2025 from water intakes, treatment plants, distribution networks, and premise plumbing systems in Poland. Analyses were performed using the ISO 11731 culture method and quantitative PCR (qPCR) targeting the ssrA, mip, and wzm genetic markers. The highest prevalence of Legionella was observed in premise plumbing systems, with positivity rates of 64.0% by qPCR and 30.0% by culture. Overall, qPCR detected Legionella in 37.5% of samples compared with 14.2% using the culture method, demonstrating significantly higher sensitivity (p < 0.001). Seasonal variation was observed, with the highest detection frequencies during summer months. Positive samples were associated with higher water hardness as well as elevated iron and manganese concentrations. Significant differences in Legionella occurrence were found among sampling points and seasons (p < 0.05). The results indicate that premise plumbing systems constitute the primary reservoir of Legionella within drinking water distribution systems. Furthermore, qPCR proved to be a rapid and sensitive tool supporting microbiological risk assessment and monitoring within the “source-to-tap” framework recommended by EU Directive 2020/2184.