Prevalence and Profiles of Antibiotic Resistance Genes mph (A) and qnrB in Extended-Spectrum Beta-Lactamase (ESBL)-Producing Escherichia coli Isolated from Dairy Calf Feces.

The use of antibiotics to treat dairy calves may result in multidrug-resistant extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli . This study investigated fluoroquinolone and macrolide resistance genes among ESBL-producing E. coli isolated from dairy calves. Fresh fecal samples from 147 dairy calves across three age groups were enriched to select for ESBL-producing E. coli. Plasmid-mediated fluoroquinolone ( qnrB ), macrolide ( mph (A)), and beta-lactam ( bla CTX-M groups 1 and 9) resistance genes were identified by PCR and gel electrophoresis in ESBL-producing E. coli . Beta-lactamase variants and antibiotic resistance genes were characterized for eight isolates by whole-genome sequencing. Seventy-one (48.3%) samples were positive for ESBL-producing E. coli , with 159 (70.4%) isolates identified as bla CTX-M variant group 1 and 67 (29.6%) isolates as bla CTX-M variant group 9. Resistance gene mph (A) was more commonly associated with bla CTX-M variant group 1, while resistance gene qnrB was more commonly associated with variant group 9. E. coli growth was quantified on antibiotic media for 30 samples: 10 from each age group. Significantly higher quantities of ceftriaxone-resistant E. coli were present in the youngest calves. Results indicate the dominant bla CTX-M groups present in ESBL-producing E. coli may be associated with additional qnrB or mph (A) resistance genes and ESBL-producing E. coli is found in higher abundance in younger calves.