Polymorphism of genes encoding PmrAB in colistin-resistant strains of Escherichia coli and Salmonella enterica isolated from poultry and swine

Objectives: To detect the occurrence of low susceptibility to colistin (polymyxin E), a last-resort antimicrobial, among enterobacteria isolated from samples of animal origin (poultry and swine) and to find out the molecular basis of colistin resistance. Methods: Salmonella enterica and Escherichia coli were isolated from eggs and swine samples. Bacterial strains were screened for colistin resistance by using MIC determinations interpreted according to EUCAST recommendations. pmrAB genes were amplified by PCR from bacterial isolates and their sequences were characterized. Results: Nine colistin-resistant strains were detected in a collection of 739 enterobacteria (S. enterica and E. coli) isolated from animal samples taken in different environments. Sequences encoding the PmrAB two-component sensor-regulator from two colistin-resistant E. coli strains isolated from swine faeces presented three non-synonymous polymorphisms, producing the variants 39S -> I and 81R -> S of PmrA and 161V -> G of PmrB, among which the involvement of mutations in PmrA-81 and PmrB-161 in resistance to the antimicrobial had been previously shown. No variation at the protein level was detected after analysis of PmrAB sequences from seven colistin-resistant S. enterica strains. Conclusions: E. coli strains carrying mutations in PmrAB that confer resistance to polymyxins, which might have evolved in vivo and have been rarely detected, are described for the first time in enterobacteria isolated from animals.