Prevalence of virulent Escherichia coli pathotypes in calf diarrhoea: molecular characterisation and antibiotic resistance profiles in Türkiye.

Neonatal calf diarrhoea caused by Escherichia coli (E. coli) pathotypes remains a major health concern in livestock. This study aimed to characterise E. coli pathotypes isolated from diarrhoeic calves in the Küçük Menderes Basin, Türkiye, and to evaluate their antimicrobial resistance profiles. Faecal samples were collected from 100 calves across five districts. Twenty-five E. coli isolates were obtained and analysed by PCR for virulence genes. Antimicrobial susceptibility was tested using the disk diffusion method, following guidelines from the Clinical and Laboratory Standards Institute (CLSI). Enterotoxigenic E. coli (ETEC) was the most prevalent pathotype (64%). Multiple virulence gene combinations were detected, including one isolate (4%) that carried virulence genes associated with ETEC, enterohaemorrhagic E. coli (EHEC), and enteropathogenic E. coli (EPEC). Antimicrobial susceptibility testing revealed high effectiveness of florfenicol (80%) and tetracycline (76%), while resistance was highest to penicillin G (88%), streptomycin (52%), and aztreonam (64%). Multidrug resistance (MDR), defined as phenotypic resistance to three or more antimicrobial classes, was observed in 36% (9/25) of the isolates. The high prevalence of ETEC confirms its dominant role in neonatal calf diarrhoea, whereas substantial β-lactam resistance underscores the need for prudent antibiotic use and regionally adapted management strategies.