Comparative evaluation of the antimicrobial resistance of commensal E. coli from different cattle production systems in Germany

Commensal bacteria are recognized as a reservoir of resistance determinants and mirror of antimicrobial usage in livestock production. The European Food Safety Authority recommends monitoring of antimicrobial resistance (AMR) in commensal Escherichia coli . In Germany, monitoring of AMR in commensal E. coli has been established within the national monitoring of zoonotic agents in the food chain. Commensal E. coli from different cattle production systems were collected at farm and at slaughter between 2009 and 2011 and tested for their resistance to 13 antimicrobials using broth microdilution according to CLSI guidelines. Minimum inhibitory concentrations (MIC) were evaluated using epidemiological cut off values (www.eucast.org). Altogether, 1730 E. coli isolates were tested. 188 from bulk tank milk, 909 from fattening steers, and 633 from veal calves. The proportion of isolates resistant to one or more than one antimicrobial class differed substantially between the production systems. Isolates from veal calves at farm in 2010 displayed the highest proportion of resistant isolates (91.9% to =1 class, 89.3 % to >1 class) and those from bulk tank milk in 2009 the lowest (16.1 and 5.5 %, respectively), with beef cattle (2011) in between (24.0 and 17.5 %, respectively). Resistance to ampicillin, sulfamethoxazole, trimethoprim, tetracycline and streptomycin was most frequent in isolates of all sources albeit at different proportions. Resistance to critically important antimicrobials (CIA) such as the 3 rd generation cephalosporins cefotaxime and ceftazidime, and the fluoroquinolone ciprofloxacin was present at low proportions (0 to 6.3%) in isolates of most sources. However, it was substantially higher in veal calves, especially on farm (10.3 % for cefotaxime and 41.9 % for ciprofloxacin in 2010). Potential reasons for the difference include the age of the animals but more likely the extent of exposure to antimicrobials and the time lag between treatment and sampling. Large differences in the proportions of resistant isolates between isolates from veal calves and from beef cattle are likely to be attributed to the difference in exposure to antimicrobials. Further investigations are needed to identify strategies to reduce antimicrobial usage especially in the veal calve industry. However, resistance to CIA in beef and dairy cattle needs to be closely monitored too.