Flood-derived sludge as a reservoir of viable opportunistic pathogens after the October 2024 DANA in Valencia (Spain).

UNLABELLED: Extreme flooding events generate large volumes of contaminated sludge that may act as environmental reservoirs of viable microorganisms, yet their capacity to harbour viable bacteria with pathogenic potential remains poorly understood. Following the October 2024 DANA event in Valencia (Spain), flood-derived sludge was collected one month after the episode and analysed using enrichment-based culture, molecular detection, and phenotypic characterization. High loads of culturable bacteria (> 10⁶ CFU·g⁻¹ after enrichment) were recovered, with microbial communities dominated by Enterobacterales, together with Aeromonas spp. and Pseudomonas spp., consistent with contamination from sewage overflows and disturbed urban environments. Culture-based approaches did not yield Vibrio spp.; however, weak qPCR signals for Vibrio vulnificus suggest its presence at low abundance and/or in a viable but non-culturable state. Species-level identification revealed the presence of opportunistic pathogenic taxa, including Escherichia coli, Enterobacter cloacae, Salmonella spp. and Aeromonas spp. Functional characterization showed widespread proteolytic activity and biofilm formation among isolates. In selected strains, haemolytic activity, resistance to human serum, and multidrug resistance were also observed. Notably, one E. coli isolate combined multiple virulence-associated traits and antibiotic resistance, highlighting its potential clinical relevance. Overall, these results demonstrate that flood-derived sludge deposited after extreme rainfall events can function as a reservoir of viable bacteria with pathogenic potential. This study highlights the importance of incorporating microbiological surveillance and risk-oriented approaches into post-flood management strategies under a One Health framework.

SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10123-026-00821-4.