Characterization and drug resistance of Shiga toxin-producing Escherichia coli derived from animals in Shandong Province

ObjectiveTo understand the characteristics and antibiotic resistance profiles of Shiga toxin-producing Escherichia coli （STEC） derived from animals in Shandong Province， providing a basis for the risk assessment and surveillance of STEC infection.MethodsA total of 140 STEC strains were isolated from cattle and sheep from 2017 to 2018 in Shandong Province. These strains were tested for the minimal inhibitory concentrations of 27 types of antibiotics in 15 categories using the microbroth dilution method. The detection of extended spectrum beta-lactamases （ESBLs） was conducted by using the double-disk synergy test. The O：H serotyping， Shiga toxin subtyping and antibiotic resistance gene were analysis based on whole genome sequences. The core gene SNP phylogenetic tree was constructed using the maximum likelihood method.ResultsThe 140 STEC strains constituted 45 O：H serotypes， with O157：H7 accounting for 12.86% （18/140）. Two stx1 subtypes and eight stx2 subtypes were detected， and stx1c+stx2b was the most common subtype， followed by the novel stx2k. Sixteen strains （11.43%） were resistant to at least one antibiotic， and 4 strains （2.86%） were multi-drug resistant. The highest resistance rate to tetracycline was observed in the cattle- and sheep-derived STEC strains （8.57%， 12/140）， followed by trimethoprim-sulfamethoxazole （6.43%， 9/140）， with 3 strains were ESBLs-producing strains. All strains were sensitive to meropenem and amikacin. A total of 10 categories of 25 antibiotic resistance genes were detected， including the β-lactamase resistance genes blaEC， blaCTX， blaTEM and blaOXA， and fosfomycin resistance gene fosA7 was identified for the first time in STEC. 140 STEC strains formed 38 clusters， each containing the same serotype， with the O113：H4 serotype strain being the dominant cluster.ConclusionSTEC strains carried by cattle and sheep in Shandong Province are predominantly non-O157 serogroups and exhibit high diversity. The novel fosA7-resistant gene and multidrug-resistant isolates were identified. It is essential to strengthen the management of cattle and sheep feces and enhance monitoring of pathogenic bacteria.