Serratia fonticola isolates in water treatment
Ensuring microbial safety in drinking water remains a challenge, particularly for ageing infrastructure and complex treatment processes. Here, we investigated the repeated detection of Serratia fonticola (a coliform) across multiple stages of a water treatment works (WTW). Sixty-seven isolates, obtained from raw, interstage, and final waters, were analysed via whole-genome sequencing. Phylogenomic and average nucleotide identity analyses (>99.94%) confirmed a single, near-clonal strain that was adapted to the WTW being genetically distinct from raw water isolates. Variant calling revealed minimal differences, suggesting the organism's repeated detection was not caused by multiple sources but rather a single persistence variant. The variant had antibiotic resistance genes (e.g., blaFONA, blaSFDC), and plasmid-encoded toxin–antitoxin systems, however, none of the isolates possessed known high-level chlorine-resistance genes. Flow cytometry–based disinfection assays demonstrated that free-floating S. fonticola was rapidly inactivated by modest chlorine doses, achieving a >3-log reduction with a Ct of around 10 mg·min/L. However, floc-associated cells showed markedly higher resilience, requiring a Ct of approximately 100 mg·min/L to achieve a ~2.5-log reduction, indicating strong physicochemical protection of the organism within flocs. These findings suggest that physical shielding, rather than intrinsic chlorine tolerance facilitates S. fonticola survival through the treatment process. The invariable presence of a specific plasmid in nearly all of the all the WTW isolates was an indication that it might confer traits enhancing resilience to chlorine and confer the ability to shield itself within biofilms. Optimizing filtration and minimizing floc carryover alongside strategies to remove biofilm may therefore be key to preventing final-water detections of this opportunistic organism.