Screening and molecular characterization of an indigenous antibiotic and heavy metal-resistant Aeromonas hydrophila with bioremediation potential
Heavy metal contamination arising from municipal, industrial, and biomedical waste disposal has emerged as a major environmental concern, contributing not only to ecosystem degradation but also to the co-selection and dissemination of antimicrobial resistance among environmental microorganisms. The present study aimed to isolate, characterize, and molecularly identify an antibiotic- and heavy metal-resistant (AHMR) bacterium from contaminated solid waste disposal sites in West Bengal, India, and to evaluate its potential for bioremediation of heavy metal-polluted environments. Soil samples collected from hospital, industrial, and municipal waste disposal sites were screened on nutrient agar supplemented with nickel (Ni), lead (Pb), and cadmium (Cd) (50 µg mL⁻¹), resulting in the recovery of seven morphologically distinct bacterial isolates. Antibiotic susceptibility profiling using the Kirby–Bauer disc diffusion method identified isolate S23 as the most resistant strain, exhibiting a multidrug resistance (MDR) index of 0.8 with resistance to gentamicin, tetracycline, penicillin, and amphotericin B. Heavy metal tolerance assays demonstrated a minimum inhibitory concentration (MIC) of approximately 100 µg mL⁻¹ for all three metals, with nickel producing the strongest inhibitory effect on bacterial growth. Physiological and biochemical characterization revealed that isolate S23 was a mesophilic, halotolerant, catalase-positive bacterium capable of utilizing multiple carbohydrates, including glucose and sucrose. Growth kinetics further demonstrated its ability to maintain sustained proliferation under heavy metal stress, indicating a high degree of environmental adaptability. Molecular identification based on 16S rRNA gene sequencing confirmed the isolate as Aeromonas hydrophila. The concurrent occurrence of multidrug antibiotic resistance and heavy metal tolerance supports the hypothesis of co-selection of resistance determinants in contaminated environments. Furthermore, the remarkable metal tolerance exhibited by A. hydrophila suggests its potential application in the bioremediation of heavy metal-contaminated soils and wastewater through mechanisms such as biosorption, bioaccumulation, and biotransformation. Overall, this study highlights the ecological significance of environmental A. hydrophila as a resilient microorganism with promising applications in sustainable remediation strategies while emphasizing the environmental role of polluted habitats as reservoirs for multidrug-resistant bacteria