Bacterial Diversity in Fish Farms: Relation to Biofilm Formation, Antibiotic Resistance, and Heavy Metal Tolerance.

The emergence of multidrug-resistant (MDR) bacteria is facilitated by the excessive and inappropriate use of antibiotics in aquaculture. This study assessed antibiotic resistance profiles and heavy metal responses of bacteria isolated on Thiosulfate Citrate Bile Salts Sucrose (TCBS) agar from six sample types across three fish farms, yielding 51 isolates belonging to nine genera and 12 bacterial classes, with Staphylococcus being the most predominant genus. Although they appeared as Gram-positive on TCBS agar, 16S rRNA sequencing confirmed their taxonomic identity, indicating that TCBS selectivity requires reevaluation. Half of the isolates were multidrug-resistant (50.98%), and MDR and MAR index >0.2 were more frequent in Gram-negative (71%) than in Gram-positive isolates (33%). The most prevalent resistance determinants were tetM (62.7%) and tetA (54.9%), while only one isolate carried a carbapenemase gene. Biofilm formation was observed in 80.4% of isolates. Heavy metal susceptibility followed the order Cd²⁺ > Cr⁶⁺ > Zn²⁺ > Pb²⁺, and co-exposure to Cd²⁺, Zn²⁺, and Cr⁶⁺ (40 ppm) significantly increased resistance to imipenem. These findings indicate that aquaculture environments may act as reservoirs for co-resistant pathogens, posing potential risks to food safety and public health.