Prevalence and zoonotic risk of multidrug-resistant Escherichia coli in bovine subclinical mastitis milk: Insights into the virulence and antimicrobial resistance

The emergence of antibiotic-resistant microorganisms has made AMR a major issue on a global scale, and milk has the potential to be a possible source for the propagation of resistant bacteria causing zoonotic diseases. SCM cases, often overlooked and mixed with normal milk in dairy farms, frequently involve E. coli, which can spread through contaminated milk. We conducted this study to determine the prevalence of virulence genes, ARGs, antimicrobial susceptibility, and the genetic relatedness of MDR-STEC isolated from SCM milk. SCM-positive bovine milk was subjected to E. coli detection using cultural, biochemical, and molecular methods. Further, we detected STEC virulence genes including stx1, stx2, and eaeA. STEC isolates were tested for ARGs including blaSHV, CITM, tetA, and aac(3)-IV, and underwent AST. Moreover, we performed a phylogenetic analysis of the stx1 gene of MDR-STEC. SCM was detected in 47.2% of milk samples of which 50.54% were E. coli positive. About 17.20% of E. coli isolates contained STEC virulence genes, and stx2 was the most prevalent. Moreover, all STEC isolates harbored at least one of the ARGs, while about 43.75% of the isolates carried multiple ARGs. Additionally, all the STEC isolates showed multidrug resistance, and were found to be fully resistant against amoxicillin, followed by ampicillin (87.50%) and gentamycin (75%); and were mostly sensitive to aztreonam (81.25%) and meropenem (68.75%). We found that MDR-STEC isolated from SCM were closely related to disease-causing non-O157 and O157 strains of different sources including cattle, humans, and food. These findings indicate a significant zoonotic risk.