LAUSR

INTRODUCTION In the Swiss veal calf production, antimicrobials and disinfectants are used to control bacterial infectious diseases, leading to a risk of selecting for a resistant bacterial population. While the prevalence of antibiotic resistance in E. coli from calves has been monitored at slaughterhouses in Switzerland since 2006, the resistance situation of E. coli from young calves entering the fattening period is not known. A total of 100 calves entering the fattening period in 20 geographically distant farms in Switzerland were screened for the presence of E. coli using rectal swabs in 2017. Genetic diversity between isolates was determined using repetitive palindromic Polymerase Chain Reaction (rep-PCR) revealing a genetically diverse E. coli population. Susceptibility to 13 antibiotics and to alkyldimethylbenzylammonium (ADBAC) was determined by the measurement of the minimal inhibitory concentration. Antibiotic and quaternary ammonium compound (QAC) resistance genes were identified using microarray and Polymerase Chain Reaction (PCR). Sixty-four percent of the isolates were resistant to at least one antibiotic, and 52% also exhibited decreased susceptibility to ADBAC. Resistance to more than 3 antibiotics was found in 40% of the isolates. Isolates exhibited resistance to tetracycline (57%) associated with the presence of tet genes (tet(A), (B), (E), (G)), to sulfonamides (61%) (sul1, sul2, sul3), ampicillin (56%) (blaTEM-1), trimethoprim (32%) (dfrA), phenicols (31%) (catA1, cmlA1, floR), gentamicin (27%) (ant(2")-Ia, aac(3)-IVa, aac(3)-VIa), and cefotaxime (2%) (blaCTX-M-14 (ESBL)). Mutations in GyrA (S83L) and ParC (S80I) were found in the fluoroquinolone-resistant isolates (6%). All isolates were susceptible to colistin, tigecycline and meropenem. No association between the presence of decreased susceptibility to ADBAC and qac genes was observed. In conclusion, antibiotic and QAC resistant E. coli are present in the gastrointestinal tract of young calves at the beginning of the fattening period, emphasizing the need for appropriate and reduced use of antibiotics and QAC-containing disinfectants in order to limit further selection of these bacteria during the fattening period.