P BRAND¹, S GOBELI¹, V PERRETEN <sup>1

1</sup>Institute of Veterinary Bacteriology, Vetsuisse Faculty, University of Bern, Längassstrasse 122, CH-3001 Bern, Switzerland

Antibiotics like sulphonamides, tetracyclines and colistin are frequently used to treat diarrhoea in pigs caused by enteropathogenic E. coli posing the risk of selecting antibiotic resistant strains. The recent report of plasmid-mediated colistin resistance in E. coli raised the question whether this resistance is also emerging in enteropathogenic E. coli from pigs in Switzerland. We therefore screened a collection of 95 strains isolated in 2014 and 2015 by measurement of the minimal inhibitory concentration (MIC) of 14 different antibiotics and detection of resistance genes using a microarray. Forty-five percent of the isolates were susceptible to all of the antimicrobial agents tested with the MICs situated below the EUCAST resistance breakpoints. All isolates were susceptible to 3rd generation cephalosporins, carbapenems, colistin and tigecycline. Except for colistin, these antibiotics are not used in medicated feed in pigs. However, the most commonly found resistances were those to classes of antibiotics which are routinely used in pig husbandry. Indeed, resistance to sulfamethoxazole [44%, sul1, sul2, sul3], tetracycline [43%, tet(A), tet(B)] and trimethoprim [32%, dfr (A)] were the most frequent resistance traits, similar to those already observed in 2003 in Switzerland (1). This study showed that resistance to antibiotics of the latest generation as well as to colistin did not emerge yet in enteropathogenic E. coli from pigs in Switzerland. The most common resistances are those to antibiotics which are commonly used for the treatment of pigs via medicated feed like sulphonamides and tetracyclines. It is therefore of major importance to avoid selection of further resistance by inappropriate use of antibiotics, like e.g. colistin. Consequently, a correct identification of the causative agent and accurate antibiotic susceptibility testing prior to therapy of diarrhoeic pigs is strongly recommended for an adequate therapy of bacterial infections. Additionally, continuous efforts in reducing the use of antibiotics in animal husbandry have to be made to keep the level of resistance low.

1)Lanz R, Kuhnert P, Boerlin P. Veterinary Microbiology 2003; 91: 73–84