Aya El Badawy, Dalia Hamza, Zeinab Ahmed, Maha A. Sabry
Abstract Aquaculture is vital for global food security, but climate change threatens fish health by altering environmental conditions. Pseudomonas aeruginosa is a pathogenic bacterium causing significant losses in fish farms. This study investigated the effects of seasonal water temperature and pH variations on its prevalence, virulence, and antibiotic resistance in Nile Tilapia. A total of 328 Nile tilapia samples from two Egyptian farms were pooled into 82 composite samples. P. aeruginosa was isolated on selective agar, identified through biochemical tests, and confirmed by PCR targeting the gyrB gene. Virulence genes ( opr L, exo S, phz M, tox A) and resistance genes (ESBL: bla TEM , bla SHV, bla CTX−M , bla OXA−1 ; carbapenemases: bla KPC , bla NDM , bla VIM , bla OXA−48 ) were detected, and correlations with water temperature and pH were analyzed. Phylogenetic analysis of opr L sequences assessed genetic relatedness to human strains. Pseudomonas aeruginosa was detected in the vast majority of samples, with prevalence remaining high throughout the year, peaking numerically in summer. The prevalence of virulence genes in P. aeruginosa varied across seasons. exo S was consistently detected in all seasons, whereas tox A and opr L were most prominent in summer. phz M showed marked seasonal variation, with the highest occurrence in summer. Antibiotic resistance genes were most prominent in the warmer months, with bla TEM consistently present, bla CTX−M highly prevalent, and bla OXA−48 showing a noticeable increase. Phylogenetic analysis revealed genetic similarity between fish-derived isolates and previously reported human strains, suggesting potential zoonotic relevance. These findings highlight the widespread occurrence of P. aeruginosa and the seasonal distribution of virulence and antimicrobial resistance genes in aquaculture systems. Continuous monitoring and improved management strategies are recommended to limit the spread of resistant strains.