MCR-1-dependent lipid remodelling compromises the viability of Gram-negative bacteria

The global dissemination of the mobilized colistin resistance gene, <i>mcr-1</i>, threatens human health. Recent studies by our group and others have shown that the withdrawal of colistin as a feed additive dramatically reduced the prevalence of <i>mcr-1</i>. Although it is accepted that the rapid reduction in <i>mcr-1</i> prevalence may have resulted, to some extent, from the toxic effects of MCR-1, the detailed mechanism remains unclear. Here, we found that MCR-1 damaged the outer membrane (OM) permeability in <i>Escherichia coli</i> and <i>Klebsiella pneumonia</i> and that this event was associated with MCR-1-mediated cell shrinkage and death during the stationary phase. Notably, the capacity of MCR-1-expressing cells for recovery from the stationary phase under improved conditions was reduced in a time-dependent manner. We also showed that mutations in the potential lipid-A-binding pocket of MCR-1, but not in the catalytic domain, restored OM permeability and cell viability. During the stationary phase, PbgA, a sensor of periplasmic lipid-A and LpxC production that performed the first step in lipid-A synthesis, was reduced after MCR-1 expression, suggesting that MCR-1 disrupted lipid homeostasis. Consistent with this, the overexpression of LpxC completely reversed the MCR-1-induced OM permeability defect. We propose that MCR-1 causes lipid remodelling that results in an OM permeability defect, thus compromising the viability of Gram-negative bacteria. These findings extended our understanding of the effect of MCR-1 on bacterial physiology and provided a potential strategy for eliminating drug-resistant bacteria.

可移动黏菌素耐药基因<em>mcr-1</em>（mobilized colistin resistance gene, mcr-1）的全球传播对人类健康构成威胁。本团队及其他学者的近期研究表明，停用黏菌素作为饲料添加剂后，<em>mcr-1</em>的流行率显著下降。尽管学界普遍认为，<em>mcr-1</em>流行率的快速下降在一定程度上可能与MCR-1的毒性效应有关，但其具体分子机制仍不明确。本研究发现，MCR-1会损伤大肠埃希菌（<i>Escherichia coli</i>）与肺炎克雷伯菌（<i>Klebsiella pneumoniae</i>）的外膜（outer membrane, OM）通透性，且该现象与静止期内MCR-1介导的细胞皱缩及死亡相关。值得注意的是，在优化培养条件下，表达MCR-1的细菌从静止期恢复的能力随时间推移逐渐降低。本研究还证实，MCR-1的潜在脂质A（lipid-A）结合口袋发生突变后，可恢复外膜通透性与细胞活力，但催化结构域（catalytic domain）的突变则无此效果。在静止期内，表达MCR-1后，作为周质脂质A传感器及参与脂质A合成第一步的LpxC产生调控因子的PbgA的表达量下调，提示MCR-1会扰乱脂质稳态。与此一致的是，过表达LpxC可完全逆转MCR-1诱导的外膜通透性缺陷。本研究提出，MCR-1可引发脂质重塑，进而导致外膜通透性缺陷，最终削弱革兰氏阴性菌（Gram-negative bacteria）的生存活力。本研究结果加深了我们对MCR-1调控细菌生理活动机制的理解，并为清除耐药菌提供了潜在策略。