2023 Volume 79 Issue 25 Article ID: 23-25044
Antimicrobials are frequently used on livestock farms to treat animal diseases and to promote its growth. Therefore, livestock farms are widely recognized as hotspots for the emergence of antibiotic-resistant bacteria and their relevant genes caused by the frequent administration of antimicrobials. In this study, we investigated the prevalence of antibiotic-resistant Escherichia coli (AR-EC) strains isolated from fatting cattle with minimum use of antimicrobials in a livestock farm affiliated with Yamagata University, Japan. The fecal samples were monthely collected from 6 pairs of fatting cattles, including < 7-month calves and their parents, from January to December 2022. A total of 384 isolates were collected from fattig cattle through the periods and identified as E. coli based on uidA detection. Antimicrobial susceptibility test for the isolates found only two strains (0.52%, 2/384 isolates), both of which were isolated from calves, were resistant to ampicillin (ABPC) or tetracycline (TC), whereas all other strains were susceptible to all other antimicrobials. This percentage of AR-EC was extremely low compared to reported cases in Japan. The two AR-EC strains isolated from calves, ABPCR-EC and TCR-EC, were characterized by the typing of antibiotic-resistance genes using PCR analysis. The result showed that ABPCR-EC and TCR-EC possessed blaTEM and tetB, respectively. Both strains were evaluated on the transmission route in livestock farms using the genotyping method such as multi-locus sequencing typing (MLST) and pulsed-field gel electrophoresis (PFGE). According to the MLST analysis, the ABPCR-EC was identified as ST8333 which is commonly isolated from cattle, whereas TCR-EC was not assigned to the database (NSM, no strain match). The PFGE analysis was performed on E. coli isolates in the months when ABPCR-EC and TCR-EC were obtained. The PFGE type of the ABPCR-EC isolate did not match those of the seven E. coli strains isolated during the same period. In addition, the PFGE type same as ABPCR-EC was not detected in the months before and after the month of its detection, suggesting that the AR-EC came from outside the cattle body. In contrast, the PFGE type of the TCR-EC was determined to be identical (= difference of 0.0) among the seven isolates collected at the same time. This indicated that a part of the dominant strains of E. coli in the feces has acquired TCR with tetB in the intestinal tract. AR-EC were less likely to occur in this farm with minimum use of antimicrobials, and even when they occured, they seemed to be quickly eliminated.
