The Virulence Gene and Drug Resistance Analysis of Shiga Toxin–Producing Escherichia coli from Diarrhea Calves in Xinjiang, China

This investigation aimed to examine the virulence genes and antimicrobial resistance profiles of Shiga toxin-producing Escherichia coli (STEC) strains found in diarrheal calves in Xinjiang between 2016 and 2022. A total of 800 samples, including 232 fecal and 568 rectal swabs from calves under 2 months old with diarrhea, were analyzed for Shiga toxin (Stx)-encoding genes using polymerase chain reaction. The study characterized the isolates based on their stx subtypes, virulence genes, O serogroups, phylogenetic groups, hemolytic phenotypes, antibiotic resistance, and resistance genes. This investigation detected 32 STEC isolates, with stx1 and stx2 genes in 37.5% and 12.5% of the isolates, respectively. Additionally, half of the isolates contained both genes. The combination of stx subtypes was predominantly observed as follows: stx1a/stx2d (31.3%), stx1a only (31.3%), stx1a/stx2a/stx2d (12.5%), stx2d only (9.4%), stx1a/stx1d/stx2d (6.2%), stx1a/stx1d (6.2%), and stx2a/stx2d (3.1%). The eae gene was found in one isolate that carried stx1, stx2, and hlyA, defining it as enterohemorrhagic E. coli. Additionally, six STEC carrying the astA gene were identified as EAST1⁺ STEC. These isolates did not correspond to the serotypes in the "top seven" category. The phylogenetic analysis revealed that most strains belonged to groups B1 (43.8%) and A (40.6%), with 56.3% exhibiting hemolytic activity. Moreover, among the 34.4% resistant isolates (2021∼2022), the drug resistance rates of STEC isolates to tetracycline, florfenicol, ceftazidime, ampicillin, cefotaxime, gentamicin, and sulphamethoxazole ranged from 21.9% to 34.4%, to streptomycin, piperacillin, ceftazidime, aztreonam-polymyxin B, and cefepime ranged from 3.1% to 18.8%. All isolates exhibited multiple drug resistance. The most common resistance genes identified were floR (31.3%), tetA (25.0%), sulR (15.6%), strA and strB (12.5%), and bla_CTX-M-1, bla_TEM-1, and mph(A) (9.4%). These findings offer valuable information that might aid in developing preventative measures and enhance the comprehension of the epidemiological characteristics of STEC in calves.