The role of environmentally relevant concentrations of oxytetracycline in the emergence of antimicrobial resistance in Aeromonas hydrophila and Edwardsiella tarda

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.