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The study assessed the efficiency of in-vitro oxytetracycline (OTC) exposure on antimicrobial resistance (AMR) development in aquatic bacteria. Aeromonas hydrophila ATCC 49140 and Edwardsiella tarda ATCC 15947 were serially passaged to OTC for short and long durations. Short duration OTC exposure (30 min) at 0, 0.097 (0.25× Minimum Inhibitory Concentration: MIC), 0.195 (0.5× MIC) and 0.39 µg mL−1 (1× MIC) elicited high mutation rate in A. hydrophila (10−4) and E. tarda (10−2). Serial passage exposure to inhibitory concentration of OTC (0.39 µg mL−1) reduced the zone of inhibition of nitrofurantoin (NIT) for A. hydrophila and of tetracycline, NIT and gentamicin for E. tarda to a clinically significant level. The upsurge in MICs of OTC was 4-folds (1.56 µg mL−1) and 8-folds (3.12 µg mL−1) in A. hydrophila and E. tarda, respectively during serial passage exposure to 0.097, 0.195 and 0.39 µg mL−1 OTC. OTC exposure also increased the MICs of biocide triclosan (TCS) >4-folds (>100 µg mL−1) in A. hydrophila without any concentration-dependent pattern. This study shows that environmental OTC levels can drive bacterial mutation and AMR, potentially leading to cross-resistance to biocides like TCS. Reducing OTC use and improving disposal could help limit AMR spread through better antimicrobial management.

The study was conducted to evaluate the vaccination effects on rohu, Labeo rohita head kidney tissues while assessing the vaccine efficacy of Aeromonas hydrophila antigens. Six acclimatized rohu groups were immunized with three antigenic formulations (outer membrane proteins, somatic and whole-cell antigen) @ 200 µg/fish and also with equal volume of Freund's incomplete adjuvant (FIA), separately. Simultaneously, two non-vaccinated groups, i.e., injected with FIA (100µl), normal saline solutions (0.85%) and one control without injection were maintained for 28 days. All rohu were challenged with median lethal dose of A. hydrophila (2.85 × 106 cells/rohu) intraperitoneally. After 7 days, highest cumulative mortality (%) of ˃88% was found for all non-vaccinated groups. During histopathological observations in head kidney tissues of all treatment and control groups, numerous histopathological changes in the nephritic cells like mild loss of typical tubular epithelial lining, necrosis, thickening of renal epithelial lining, haemorrhages, inflammation, distorted and widening of the lumen with vacuolated surrounding and the constricted lumen of nephritic tubules were noticed for vaccinated rohu in contrast to non vaccinated groups before A. hydrophila challenge. In case of all non-vaccinated fish, including control, extensive degenerated and necrotized head kidney tissues were observed, whereas it was least observed in vaccinated rohu after 7 days A. hydrophila challenge. Results suggest that OMPs antigen along with FIA was the premier vaccine approach for improving resistance to Aeromonas disease and reduce mortality in rohu. Similarly, vaccination with all three antigenic formulations, preferably when applied along with FIA, can effectively protect the head kidney against A. hydrophila infection.