Aims: To characterize antimicrobial resistance (AMR) and determine the seasonal prevalence of Escherichia coli O157:H7 isolated from commercial feedlots.
Methods and results: Escherichia coli O157:H7 were isolated from faecal and oral samples collected at monthly intervals from three commercial feedlots over a 12-month period. A total of 240 isolates were characterized using pulsed-field gel electrophoresis (PFGE) technique. A subset of 205 isolates was analysed for AMR using Sensititre system and AMR genes (tet, sul and str) by PCR. Seven PFGE clusters (>or=90% Dice similarity) were identified, and two clusters common to all three feedlots were recovered year-round. The majority of isolates (60%) were susceptible to all antimicrobials and were closely related (P < 0.001), whereas isolates with unique AMR patterns were not related. The prevalences of AMR from feedlots A, B and C were 69%, 1% and 38%, respectively. Resistance to tetracycline (69%) and sulfisoxazole (68%) was more prevalent in feedlot A than other two feedlots. The presence of strA and strB genes was linked in the majority of isolates, and tet(A) and tet(B), and sul1 and sul2 genes were present individually. Escherichia coli O157:H7 were genetically diverse during summer and fall, and strains from winter and spring months were more closely related.
Conclusions: Antimicrobial resistance was more common in E. coli O157:H7 obtained from two of the three commercial feedlots, and the phenotypic expression of resistance was correlated with the presence of resistant genes. A highly diverse E. coli O157:H7 population was found during summer and fall seasons.
Significance and impact of the study: Information would help understanding the dynamics of AMR in E. coli O157:H7 from commercial feedlots.