Background and Aim: The bovine industry is threatened by one of the most serious and deadly enteric diseases, calf diarrhea, particularly in developing nations like Bangladesh. In this context, bacterial resistance to antimicrobial drugs and its detrimental consequences have become a critical public health issue that is difficult to address globally. This study aimed to isolate and identify Escherichia coli and Salmonella spp. with their antibiogram and antibiotic resistance gene detection from sulfonamide-treated diarrheic calves.

Materials and Methods: Twelve diarrheic calves suffering from calf diarrhea in a dairy farm were selected and a total of 36 fecal samples were aseptically collected directly from rectum before, during, and at the end of treatment for each calf to determine the total viable count, total E. coli count and total Salmonella count. A polymerase chain reaction was used for the specific detection of E. coli and Salmonella genus targeting fliC and invA genes, respectively. Antibiotic sensitivity test of the isolated E. coli and Salmonella spp. were performed by the disk diffusion method for eight commonly used antibiotics.

Results: A total of 36 E. coli (100%) and 12 Salmonella spp. (33%) were isolated from the samples and were confirmed by polymerase chain reaction. Total viable count was found to be ranged from 35 × 10⁷ to 99 × 10¹⁰ colony-forming unit (CFU)/g fecal sample before starting sulfonamide treatment, 34 × 10⁵ to 25 × 10¹⁰ CFU/g during treatment with sulfonamide, and 48 × 10³ to 69 × 10¹⁰ CFU/g immediately after completion of sulfonamide treatment. Total E. coli count was found to be ranged from 4 × 10⁴ to 36 × 10¹⁰ CFU/g, 24 × 10⁴ to 23 × 10⁸ CFU/g, and 13 × 10⁴ to 85 × 10¹⁰ CFU/g, whereas total Salmonella count was found to be ranged from 16 × 10⁶ to 18.5 × 10¹¹ CFU/g, 15 × 10⁴ to 44 × 10⁷ CFU/g, and 13.2 × 10⁵ to 21 × 10¹⁰ CFU/g fecal sample before starting sulfonamide treatment, during treatment with sulfonamide immediately after completion of sulfonamide treatment, respectively. The in vitro antibiotic sensitivity test showed that all the E. coli and Salmonella spp. isolated from diarrheic calves (100%) contained multidrug-resistant (MDR) phenotypes. Escherichia coli isolates were found 100% resistant to amoxicillin (AMX), cefuroxime, cephalexin (CN), erythromycin (ERY), and tetracycline (TET); whereas 94.4%, 86.1%, and 77.8% isolates were resistant to doxycycline (DOX), moxifloxacin (MOF), and gentamycin (GEN), respectively. In case of Salmonella isolates, all were found 100% resistant to AMX, CN, and ERY; whereas 91.7% of resistance was observed for DOX, MOF, cefuroxime, GEN, and TET. Based on the molecular screening of the antibiotic resistance genes, tetA gene was present in 83.3% of the isolated E. coli and 75% of the isolated Salmonella strains, whereas 83.3% E. coli and 79.2% Salmonella isolates contained blaTEM gene.

Conclusion: These findings suggest that MDR E. coli and Salmonella spp. might be responsible for calf scouring, which is challenging to treat with antibiotics or sulfonamide drugs alone. Therefore, it is important to check the antibiotic sensitivity pattern to select a suitable antibiotic for the treatment of calf scoring. A suitable antibiotic or combination of an antibiotic and sulfonamide could be effective against E. coli and Salmonella spp. responsible for calf scouring.

Keywords: antimicrobial resistance, calf, diarrhea, Escherichia coli, multidrug-resistant, Salmonella spp.