Multidrug-resistant bacteria in wastewater-impacted zones of Ashtamudi Lake, a Ramsar site in Kerala, India

Antimicrobial resistance (AMR) in aquatic ecosystems is a growing global concern. This study investigated AMR profiles of bacteria isolated from hospital-, poultry-, and aquaculture-adjacent sectors of Ashtamudi Lake, Kerala, India. Twelve isolates (Bacillus cereus and Citrobacter freundii) were isolated and tested against ten antibiotics. Isolates from all sectors showed consistent resistance to β-lactam antibiotics, including amoxicillin, ampicillin, amoxicillin–clavulanic acid, and ceftriaxone. Notably, ceftriaxone resistance and intermediate resistance to imipenem were detected in C. freundii, phenotypes which correspond to WHO designated Critical Priority pathogens, raising serious public health concerns. Multidrug resistance (≥ four antibiotics) was observed in 75% of the isolates, with aquaculture strains exhibiting the highest resistance. Gentamicin, chloramphenicol, and tetracycline retained full activity against all the isolates. These findings identify Ashtamudi Lake as a reservoir of multidrug-resistant bacteria and highlight the need for integrated One Health surveillance and improved effluent management.