Emergence of High-level Colistin Resistance Mediated by Multiple Determinants, Including , and Mutations, Combined with Tigecycline Resistance in an ST656

Colistin and tigecycline are usually regarded as the last resort for multidrug-resistant infection treatment. Emergence of colistin and tigecycline resistance poses a global healthcare challenge and is associated with high mortality due to limited therapeutic options. Here, we report the ST656 extensively drug-resistant strain KP15-652, which was isolated from a patient's urine in China. Antimicrobial susceptibility testing showed it to be resistant to tigecycline, amikacin, levofloxacin, ciprofloxacin, and high-level colistin resistance (> 2048 mg/L). Whole-genome sequencing revealed that it harbors one chromosome and seven plasmids, including four plasmids carrying multiple acquired resistance genes. Transformation/conjugation tests and plasmid curing assays confirmed that , and mutations are responsible for the high-level colistin resistance and that a series of efflux pump genes, such as , (A) and (M), contribute to tigecycline resistance. and (M) are located on an IncX1 plasmid, which has conjugation transfer potential. and (A) are located on a multireplicon IncR/IncN plasmid but unable to be transferred conjugation. Moreover, another conjugable and fusion plasmid carries the gene cluster, which may have arisen due to IS-mediated replicative transposition based on 8-bp target-site duplications. Importantly, a complex class 1 integron carrying various resistance genes was detected on this fusion plasmid. In conclusion, it is possible that the high-level of colistin resistance is caused by the accumulated effect of several factors on the chromosome and -carrying plasmids, combined with many other resistances, including tigecycline. Effective surveillance should be performed to prevent further dissemination.