LAUSR

BACKGROUND Our study aimed to prospectively analyze the occurrence of colistin-resistant and extended-spectrum ß-lactamase (ESBL)-producing Enterobacterales colonizing pigs in two farms in Portugal, and to evaluate the putative correlations with different antibiotic usages. MATERIAL AND METHODS One-hundred and two fecal samples recovered from two different Portuguese pig farms were screened for polymyxin-resistant and ESBL-positive Enterobacterales. First farm was the same as the one where we had performed a study two years ago, but the use of colistin has been banned, with zinc oxide and amoxicillin being kept as prophylactic and curative molecules, respectively. Second farm used zinc oxide only. RESULTS Ninety-three ESBL-producing isolates (62 E. coli, 29 K. pneumoniae, one E. aerogenes and one E. cloacae) and seventeen colistin-resistant isolates (12 E. coli, 4 K. pneumoniae and one E. cloacae) were recovered. Among the ESBL producers, a majority (84%) produced CTX-M-15, while others produced CTX-M-1 or CTX-M-9. Many different strain and plasmid backgrounds were identified, ruling out a massive dissemination of one major clone. A total of 17 colistin-resistant isolates were recovered, being only from the first farm. All produced MCR-1, corresponding to 12 E. coli (ten clones) and three K. pneumoniae (two clones). Those MCR-1 producers were all recovered from the first farm, where colistin had actually used two years before. CONCLUSION This study showed a surprising high rate of CTX-M-15 producers in two Portuguese pigs farms. A link between the antibiotic selective pressure (ß-lactam or polymyxin) and the rate of corresponding resistance was evidenced here.