Occurrence of CTX-M-15- and MCR-1-producing Enterobacterales in pigs in Portugal: Evidence of direct links with antibiotic selective pressure.

To undertake a prospective analysis of the occurrence of colistin-resistant and extended-spectrum ß-lactamase (ESBL)-producing Enterobacterales colonizing pigs at two farms in Portugal, and to evaluate the putative correlations with usage of different antibiotics. One hundred and two faecal samples recovered from two different Portuguese pig farms were screened for polymyxin-resistant and ESBL-positive Enterobacterales. The authors had undertaken a study at one of the farms previously, but the use of colistin has since been banned; zinc oxide and amoxicillin are used as prophylactic and curative drugs, respectively, at this farm. The other farm included in this study used zinc oxide alone. Ninety-three ESBL-producing isolates (62 Escherichia coli, 29 Klebsiella pneumoniae, one Enterobacter aerogenes and one Enterobacter cloacae) and 17 colistin-resistant isolates (12 E. coli, four K. pneumoniae and one E. cloacae) were recovered. Among the ESBL producers, the majority (84%) produced CTX-M-15, while the 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. In total, 17 colistin-resistant isolates were recovered, all from the first farm. All produced MCR-1, corresponding to 12 E. coli (10 clones) and three K. pneumoniae (two clones). The MCR-1 producers were all recovered from the farm where colistin had been used 2 years previously. This study showed a surprisingly high rate of CTX-M-15 producers at two Portuguese pig farms. A link was found between antibiotic selective pressure (ß-lactam or polymyxin) and the corresponding resistance rate.

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