Fitness costs of mobilised colistin resistance gene 3 (mcr-3): systematic review, epidemiological study, and functional analysis.

Background: The rapid evolution and dissemination of mobilised colistin resistance gene (mcr) family has revealed as a severe threat to the global public health. Nevertheless, dramatic reduction in the prevalence of mcr-1, the major member of mcr family, was observed after the withdrawal of colistin in animal fodder in China since 2017, demonstrating that colistin acts as a selective stress to promote the dissemination of mcr-1. As the second largest lineage, mcr-3 was firstly discovered in 2017 and has been identified from numerous sources. However, whether the spreading of mcr-3 is driven by colistin remains unknown.

Methods: To this end, we investigated the global prevalence of mcr-3 from 2005 to 2022 by an up-to-date systematic review, along with a nation-wide epidemiological study to establish the change of mcr-3 prevalence in China before and after 2017. To investigate the fitness cost imposed by MCR-3 upon bacterial host, in vitro and in vivo competitive assays were employed, along with morphological study and fluorescent observation. Moreover, by replacing non-optimal codons with optimal codons, synonymous mutations were introduced into the 5'-coding regions of mcr-3 to study mechanisms accounting for the distinct fitness cost conferred by MCR-1 and MCR-3. Furthermore, by combining AlphaFold and molecular dynamics (MD) simulation, we provided a complete characterisation on the putative lipid A binding pocket localised at the linker domain of MCR-3. Crucially, inhibitors targeting at the putative binding pocket of MCR-1 or MCR-3 were identified from small molecules library using the pipeline of virtual screening.

Findings: The global prevalence of mcr-3 increased continuously from 2005 to 2022. The average prevalence was 0.18% during 2005-2014 and rapidly increased to 3.41% during 2020-2022. The prevalence of mcr-3 in China increased from 0.79% in 2016 to 5.87% in 2019. We found that the fitness of mcr-3-bearing Escherichia coli and empty plasmid control was comparable but higher than that of mcr-1-positive strain. Although the putative lipid A binding pocket of MCR-3 was similar to that of in MCR-1, mcr-3 occupies remarkable codon bias at the 5'-end of coding region that disrupted the stability of mRNA, further reduced its protein expression in E. coli, resulting in the low fitness burden of bacterial host. Moreover, the 5'-end codon usage frequency appeared as a critical factor related with the evolution of mcr family. Furthermore, based on the similar lipid A binding pocket among MCR family protein, we identified three MCR inhibitors targeting at such pocket by screening from small-molecule library, which effectively restored the colistin susceptibility of mcr-bearing E. coli.

Interpretation: We found that the prevalence of mcr-3 increased continuously during 2016-2019 in China, demonstrating that the withdrawal of colistin in husbandry failed to prevent the dissemination of mcr-3. Our study evidenced that the 5'-end codon bias appeared as a crucial regulator upon the fitness cost conferred by horizontally transferred genes. Most importantly, the putative lipid A binding pocket verified from current study was a promising target site for designing inhibitors against mcr-positive strains.

Funding: Natural Science Foundation of China, National Key Research and Development Programme of China.