The use of bulk tank milk samples to monitor trends in antimicrobial resistance on dairy farms.

The routine monitoring of bacteria obtained from bulk tank milk (BTM) may be an important tool for detecting farm-level trends in antimicrobial resistance on dairy farms. This study describes and compares antimicrobial susceptibility patterns of Salmonella enterica subspecies enterica (Salmonella) and Escherichia coli recovered from dairy BTM. BTM from more than 400 dairies in a dairy-intense region of California were sampled eight times at 2- to 3-month intervals over a 29-month period. From Salmonella positive and Salmonella negative herds any one Salmonella and three E. coli isolates per sample were tested for susceptibility to 12 antimicrobials. The prevalence of multiple drug resistant (MDR) E. coli was assessed in relation to Salmonella on the farm, farm size, season, MDR Salmonella, and serovar. At each sampling period, 10-21% and 54-77% of the dairy farms were positive for Salmonella and E. coli, respectively. The most commonly recovered Salmonella serovars were Montevideo (33%), Typhimurium (14%), Dublin (13%), and Give (11%). Two-thirds, respectively, of 478 Salmonella and 1577 E. coli isolates were pan-susceptible. The antimicrobial resistance patterns of MDR Salmonella tended to be serovar dependent and were different from the antimicrobial resistance patterns of MDR E. coli. MDR E. coli were more likely to be recovered from dairies with MDR Salmonella. There were no associations between MDR E. coli and season, Salmonella serovar detected in the BTM, or dairy herd size. Bulk milk E. coli and Salmonella could be valuable to monitor the dynamics of antimicrobial resistance in dairy milk production.