R93P Substitution in the PmrB HAMP Domain Contributes to Colistin Heteroresistance in Escherichia coliIsolates from Swine

Here, the mechanisms of colistin heteroresistance (CHR) were assessed in 12 Escherichia coliisolates from swine in China. CHR was investigated by population analysis profile tests. CHR stability was studied by culturing isolates for five overnight incubation periods in colistin-free medium. Subsequently, the mcr-1gene and mutations in PmrAB, PhoPQ, and MgrB were screened in parental isolates and resistant subpopulations. Additionally, the expression levels of phoPQ, its target gene pagP, and its negative regulator gene mgrB, as well as pmrABand its target genes pmrHFIJKLMand pmrC, were determined by real-time relative quantitative PCR.