Antimicrobial Resistance of Escherichia coli and Salmonellae Isolates from Diarrhoeic Calves.

Escherichia coli (E. coli) and Salmonella are considered the most common bacterial causes of enteritis in calves, often leading to severe diarrhea, dehydration, and even death. This study investigated the prevalence, antimicrobial susceptibility, and resistance genes of E. coli and Salmonella in diarrheic calves from Beni Suef governorate, Egypt. Faecal samples were collected from 80 diarrheic calves (1–3 months of age); the isolation results showed that E. coli was the predominant isolate, identified in 62 (77.5%) samples, while Salmonella was isolated from 11 (13.75%) samples. The most prevalent serogroups among E. coli isolates were O26 (17.7%), O123 (14.5%), and O15 (12.9%). The Salmonella isolates were mostly S. Typhimurium (45.5%) and S. Enteritidis (36.4%). The antimicrobial resistance profile indicated that 69.4% of E. coli and 63.6% of Salmonella isolates demonstrated multidrug resistance. E. coli isolates showed high resistance to erythromycin (90.3%), penicillin (72.6%), and florfenicol and sulfamethoxazole-trimethoprim (71%). Salmonella isolates showed high resistance to colistin sulphate (72.7%), amoxicillin-clavulanic acid, kanamycin, doxycycline HCl, and erythromycin (63.6%). The blaTEM gene, conferring resistance to beta-lactam antibiotics, was detected in all E. coli isolates and in 75% of Salmonella isolates. The aadA1 and aacC genes, conferring resistance to aminoglycosides, were detected in 75% of the E. coli isolates, while the dfrA gene, conferring resistance to trimethoprim, and aadB, conferring resistance to aminoglycosides were detected in 100% and 50% of Salmonella isolates, respectively. These findings highlight the risk of E. coli and Salmonella as etiological agents of diarrhea in calves and underscore the growing threat of antimicrobial resistance in these pathogens, highlighting the need for responsible antimicrobial usage in veterinary medicine.