A study by the Institute of Integrative Systems Biology (I2SysBio), a joint centre of the Spanish National Research Council (CSIC) and the University of Valencia (UV), and the Carlos III Health Institute (ISCIII), has documented for the first time how a building’s water system can generate and sustain new variants of the bacterium Legionella pneumophila with greater environmental persistence and potential for human infection. The research, the results of which have been published in the journal Water Research, provides new evidence on the role of water infrastructure in the evolution of pathogens of public health significance.

The article provides an in-depth analysis of an exceptional case of contamination that persisted for two decades in the water supply network of a state-run residential facility. Despite the systematic application of conventional control measures, such as hyperchlorination and thermal shock treatments, as well as other extreme measures, including the replacement of one of the infrastructure components, the bacterium managed to persist in the system.

The variant that emerged in this water supply network is a new recombinant variant, identified by the research team, which eventually accounted for more than 80 per cent of the detected bacterial population and was the only variant detected in cases of illness associated with the building.

By integrating genomic surveillance, comparative genomics, cellular infection models and three-dimensional characterisation of biofilms, the scientific team reconstructed the evolution of this variant. Phylodynamic analysis revealed that the new variant, a recombinant lineage derived from previously existing variants, emerged when the building first became operational, suggesting that the selection and establishment of this new evolutionary lineage took place within the residential building’s water infrastructure.

“Unlike the original variants, which remained confined to minority niches, the new variant showed marked differences, both in its ability to disperse in water, being more readily dispersed and less adherent, and in its ability to infect in the laboratory, where it shows higher replication rates,” explains Mireia Coscolla, a researcher at I2SysBio and one of the authors of the article. “This combination of environmental persistence, efficient dispersal and greater infectivity could explain its predominance amongst the clinical isolates linked to the facility,” she adds.

For his part, Fernando González Camacho, a researcher at the ISCIII’s National Centre for Microbiology, notes: “This research combines 20 years of real-world epidemiological surveillance with techniques from genomics, transcriptomics, biofilm modelling and cellular infection modelling. We have identified a strain that is unique to this facility which, through a process of recombination, has acquired new capabilities that allow it to persist; it reshapes the architecture of the biofilm, the biological layer on the pipes, creating a three-dimensional matrix that functions as a continuous reservoir and source of bacteria that contaminate the water and may put people at risk.”

Water Infrastructure and Public Health

These findings highlight that water systems are not merely passive reservoirs of Legionella, but dynamic ecosystems where environmental pressure, prolonged persistence and genetic recombination can favour the emergence of new variants adapted to local conditions. This phenomenon, which links environmental, microbiological and human exposure factors, reinforces the importance of One Health approaches by integrating environmental health, human health and infrastructure management, thereby enabling the anticipation of risks, the improvement of water safety plans and the protection of the most vulnerable populations.

The research team highlights the need to implement long-term genomic surveillance in building water systems, complemented by phenotypic analyses. Combining these tools would enable the early detection of newly emerging variants before they undermine management strategies, facility safety or public health.

Reference: Cancino-Muñoz, I., González-Rubio, J. M., Coscollá, M., & González-Camacho, F. (2026). Genomic and phenotypic characterization of a novel recombinant Legionella pneumophila lineage associated with long-term persistence in a building water system and recurrent legionellosis outbreaks. Water Research, 308. DOI: https://doi.org/10.1016/j.watres.2026.127035