Prevalence of Antimicrobial Resistant and Extended-Spectrum Beta-Lactamase-producing Escherichia coli in Dairy Cattle Farms in East Tennessee

Antimicrobials have been widely used in dairy farms to prevent and control dairy cattle diseases since 1960s. This led to the emergence of antimicrobial resistant bacteria (ARB) that, along with their antimicrobial resistance genes (ARGs), can spread from dairy farms to humans. Therefore, regular antimicrobial resistance (AMR) monitoring is important to implement proper mitigation measures. The objective of this study was to determine the prevalence of AMR and extended-spectrum beta-lactamases (ESBLs)-producing Escherichia coli in dairy cattle. A cross-sectional study was conducted in four dairy cattle farms (A-D) in East Tennessee. A total of 80 samples consisting of 20 samples each of bulk tank milk, feces, dairy cattle manure-amended soil, and prairie soil adjacent to the farms were collected and cultured for the isolation of E. coli. Tetracycline (TETr)-, third-generation cephalosporin (TGC(r))- and nalidixic acid (NAL(r))-resistant E. coli (n = 88) were isolated and identified on agar media supplemented with TET, cefotaxime, and NAL, respectively. TGC(r) E. coli were tested for ESBLs and other coselected ARGs. TETr (74%, n = 88) was the most common, followed by TGC(r) (20%) and NAL(r) (8%). Farms had significant ( p < 0.001) differences: the highest prevalence of TGCr (55%) and TETr (100%) were observed in farm D, while all NAL(r) isolates were from farm C. Over 83% of TGC(r) isolates (n = 18) harbored ESBL gene bla(CTX-M). Majority (78%) of the E. coli isolates were multidrug-resistant (MDR), being positive for beta-lactams (bla(CTX-M)), TETs tet(A), tet(B), tet(M)), sulfonamides (sul2), aminoglycosides (strA), and phenicols (floR). This study indicated the widespread occurrence of MDR ESBLs-E. coli in dairy cattle farms. AMR surveillance of more dairy farms and identification of farm-level risk factors are important to mitigate the occurrence and spread of ARB of significant public health importance, such as ESBLs-E. coli.