Abstract

Effect of oxytetracycline (OTC) on the intestinal bacterial load and development of antibiotic-resistance in monosex (all-male) Nile tilapia(Oreochromis niloticus) fries were investigated under laboratory condition. The Nile tilapia fries were fed with OTC-diets at 80 (1X) and 800 mg/kg biomass/day (10X) for 10 consecutive days and compared with control. The OTC-dosing at 1X and 10X levels caused 0.23% and 1.52% mortalities, respectively. The feed intake was reduced by 8.75% in 1X and 31.25% in 10X groups during the OTC-dosing period. The control, 1X and 10X-dosed fries had total plate counts (TPCs) in the range of 8.68±0.05 to 9.72±0.02, 8.86±0.08 to 9.83±0.05 and 8.56±0.11 to 9.73±0.02 log10 cfu/g of intestine, respectively. In the 1X and 10X groups, the proportion of OTC-resistant bacteria increased significantly to 0.02276% (22 folds) and 0.017439% (17 folds), respectively from 0.001004% of the TPCs on day 10 OTC-dosing. The proportion of chloramphenicol-resistant bacteria increased significantly from 0.000082% to 0.016053% (195 folds) and 0.006208% (75 folds) of the TPCs on day 10 OTC-dosing in the 1X and 10X groups, respectively. On day 42 post-OTC-dosing, OTC-resistant bacteria were almost 37.50 and 16.90 folds higher in the 1X and 10X groups than the control; while the respective increases were 70 and 55 folds in chloramphenicol-resistant bacteria. The OTC-dosing also favoured cross-resistance to chloramphenicol among the intestinal bacterial flora of Nile tilapia fries. The release of these resistant bacteria may act as a reservoir for the horizontal transmission of antimicrobial resistance gene determinants to the microflora of the aquaculture environment, which is a serious cause for concern.

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