Characterization of virulence genes, serogroups and antimicrobial susceptibility of Shiga toxin producing Escherichia coli isolated from bovine mastitic milk in Tehran, Iran

Clinical mastitis caused by Enterobacteriaceae accounts for significant economic loss in dairy herds. One of the important pathogens that causes mastitis is Shiga toxinproducing Escherichia coli (STEC). Moreover, mastitis caused by STEC can be considered as a source of transmission of STEC strains to humans through unpasteurized milk. The aim of the current study was to determine of the prevalence, the identification of serogroups, the molecular characterization of virulence factors, and the antibiotic resistance properties of STEC isolates from bovine mastitic milk in dairy cattle in Tehran. A total of 325 milk samples from dairy cattle with clinical signs of mastitis were collected. All E. coli isolates (n: 87, 26.7%) were subjected to multiplex PCR for the detection of stx1, stx2, eaeA, and ehly genes and serogroups. Antibiotic susceptibility testing was carried out by the disc diffusion method for all the STEC isolates. Eighty-seven (26.8%) E. coli and 9 (2.8%) STEC strains were isolated from the bovine mastitic milk samples. Shiga-like toxin genes (stx1 and stx2 or one of them), eaeA and ehly were detected in 100%, 66.6%, and 33.3% of STEC isolates, respectively. O26 (22.2%) and O111 (22.2%) were the most commonly detected STEC serogroups. Other serogroups included O145, O121, O128, O157 and O113. High resistance rate to ampicillin and tetracycline (100%) was observed, followed by trimethoprim/sulfamethoxazole (66.6%) and chloramphenicol (55.5%). STEC isolates were found in bovine mastitic milk in Tehran and most of the STEC isolates in our study were non-O157 strains