Co-colonization of blaCTX-M and mcr-1 in Avian Pathogenic Escherichia coli in Southern Xinjiang: Current Status and Antimicrobial Resistance Characteristics

Extended-spectrum β-lactamases (ESBLs) constitute one of the principal mechanisms underlying multidrug resistance (MDR) in avian pathogenic Escherichia coli (APEC). To clarify the current epidemiological status and potential transmission risks associated with the co-occurrence of ESBL and mcr-1 genes in southern Xinjiang, 133 liver samples from diseased chickens were collected, from which 104 E. coli strains were isolated; 100 of these isolates (96.15%, 100/104) were confirmed as APEC. Double-disc diffusion testing with cefotaxime-clavulanate (CTL)/cefotaxime (CTX) and ceftazidime-clavulanic acid (CAL)/ceftazidime (CAZ) showed that 65 strains (65.00%, 65/100) were ESBL producers. Among these, five isolates (7.69%, 5/65) were resistant to colistin and carried the mcr-1 gene. All five isolates exhibited MDR phenotypes and were assigned to phylogenetic groups A (n = 1), B1 (n = 3), and D (n = 1). Whole-genome sequencing (WGS) revealed that the five mcr-1 -positive APEC strains belonged to four sequence types (STs): ST6792 (n = 1), ST1196 (n = 2), ST155 (n = 1), and ST162 (n = 1), and harbored three b laCTX-M subtypes ( blaCTX-M-55 [n = 3], blaCTX-M-64 [n = 1], and blaCTX-M-65 [n = 1]). These data suggest that MDR APEC strains co-harboring mcr-1 and ESBL genes particularly those classified as phylogenetic group D (ST162) and ST155 may serve as important reservoirs and potential vehicles for antimicrobial resistance dissemination.