Integrated Genomic and Phenotypic Characterization of an -Harboring Multidrug Resistant Strain From Migratory Birds in China.

The global rise in antibiotic resistance among multidrug resistant (MDR) Gram-negative (GN) bacteria has posed significant health challenges, leading to the resurgence of colistin as a key defense against these bacteria. However, the widespread use of colistin has resulted in the rapid emergence of colistin resistance on a global scale. Ten members of the (mobile colistin resistance) gene family, through , have been reported and documented. Currently, bacteria reported to carry the gene are sensitive to colistin, but the mechanism underlying the low-level resistance phenomenon mediated by remains unclear. In this study, antimicrobial susceptibility testing (AST) was conducted on (E.coli) isolated from Chinese migratory birds, resulting in the selection of 87 strains exhibiting MDR phenotypes. Whole-genome sequencing (draft) was performed on these 87 MDR strains, and for one of the strains carrying the gene, whole-genome sequencing, phenotypic characterization, AST and conjugation experiments were conducted to identify its resistance phenotypes and genetic characteristics. Whole-genome sequencing (draft) of 87 MDR isolates revealed a diverse array of resistance genes, predominantly including aminoglycoside, -lactam, tetracycline, and sulfonamide resistance genes. Remarkably, one isolate, despite being sensitive to colistin, harbored the gene. Further sequencing showed that was located in the conserved region of , a hotspot for movable elements with various insertion sequences (ISs) or transposons nearby. Phenotypic characterization indicated that the MDR plasmid pGN25- had no significant effect on the growth of GN25 and its derivatives but reduced the number of bacterial flagella. It is particularly important to note that bacteria harboring the gene may exhibit low minimum inhibitory concentration (MIC) values, but that the MIC values under colistin selective pressure can become progressively higher and exacerbate the difficulty of treating infections caused by -associated bacteria. Therefore, vigilance for such "silent transmission" is warranted, and continuous monitoring of the spread of is necessary in the future.