UTTAR PRADESH JOURNAL OF ZOOLOGY

The use of Growth Promoting Antibiotics (GPAs) in large scale poultry production pose significant risks to the intestinal microbial homeostasis in those chickens and contributes towards the development of resistance towards these antibiotics. This study investigated the antimicrobial susceptibility and resistance pattern of eight major GPAs - Oxytetracycline, Tetracycline, Vancomycin, Streptomycin, Enrofloxacin, Tylosin, Neomycin and Gentamycin, against E. coli and Lactobacillus sp. isolated from the jejunum of commercially slaughtered broiler chickens. Agar Well Diffusion assay was carried out to evaluate the antibacterial sensitivity followed by Resazurin-enhanced MIC assay for the determination of MIC, EC₅₀ and MBC. Non-linear regression analysis was carried out for curve analysis of Concentration vs Response. Freshly isolated bacteria from the jejunum showed a significant concentration-related response towards all antibiotics tested, except Tylosin, against E. coli and Lactobacillus sp.  and the most effective antibiotics, Enrofloxacin and Oxytetracycline, showed a drastically low EC₅₀ value towards E. coli (0.038 ± 0 µg/µl and 46 ± 0 µg/µl, respectively) and Lactobacillus sp. (0.037 ± 0.00 µg/µl and 0.046 ± 0.00 µg/µl, respectively). In contrast, the inhibitory activity of Tylosin was negligible and its EC₅₀ values were considerably higher, signifying innate and/or developed resistance. Lactobacillus sp. was highly sensitive to all GPAs, pointing to the unintended vulnerability of beneficial gut microbiome and dysbiosis. All Concentration-Response models demonstrated excellent goodness of fit and all p < 0.01, validating the Concentration-dependent antimicrobial activity of GPAs against pathogenic and commensal jejunal bacteria, thus underscoring the warranted prudent use of antibiotic GPs and the need for the development of sustainable alternatives to mitigate antimicrobial resistance in broiler raised regimes.