Greater Invasion and Persistence of mcr-1-Bearing Plasmids in Escherichia coli than in Klebsiella pneumoniae.

The emergence of the plasmid-borne polymyxin resistance gene mcr-1 threatens the clinical utility of last-line polymyxins. Although mcr-1 has disseminated to various Enterobacterales species, the prevalence of mcr-1 is the highest among Escherichia coli isolates while remaining low in Klebsiella pneumoniae. The reason for such a difference in prevalence has not been investigated. In this study, we examined and compared the biological characteristics of various mcr-1 plasmids in these two bacterial species. Although mcr-1-bearing plasmids were stably maintained in both E. coli and K. pneumoniae, the former presented itself to be superior by demonstrating a fitness advantage while carrying the plasmid. The inter- and intraspecies transferability efficiencies were evaluated for common mcr-1-harboring plasmids (IncX4, IncI2, IncHI2, IncP, and IncF types) with native E. coli and K. pneumoniae strains as donors. Here, we found that the conjugation frequencies of mcr-1 plasmids were significantly higher in E. coli than in K. pneumoniae, regardless of the donor species and Inc types of the mcr-1 plasmids. Plasmid invasion experiments revealed that mcr-1 plasmids displayed greater invasiveness and stability in E. coli than in K. pneumoniae. Moreover, K. pneumoniae carrying mcr-1 plasmids showed a competitive disadvantage when cocultured with E. coli. These findings indicate that mcr-1 plasmids could spread more easily among E. coli than among K. pneumoniae isolates and that mcr-1 plasmid-carrying E. coli has a competitive advantage over K. pneumoniae, leading to E. coli being the main mcr-1 reservoir. IMPORTANCE As infections caused by multidrug-resistant "superbugs" are increasing globally, polymyxins are often the only viable therapeutic option. Alarmingly, the wide spread of the plasmid-mediated polymyxin resistance gene mcr-1 is restricting the clinical utility of this last-line treatment option. With this, there is an urgent need to investigate the factors contributing to the spread and persistence of mcr-1-bearing plasmids in the bacterial community. Our research highlights that the higher prevalence of mcr-1 in E. coli than in K. pneumoniae is attributed to the greater transferability and persistence of mcr-1-bearing plasmid in the former species. By gaining these important insights into the persistence of mcr-1 in different bacterial species, we will be able to formulate effective strategies to curb the spread of mcr-1 and prolong the clinical life span of polymyxins.

Impact of mcr-1 on the Development of High Level Colistin Resistance in Klebsiella pneumoniae and Escherichia coli.

Plasmid-mediated colistin resistance gene mcr-1 generally confers low-level resistance. The purpose of this study was to investigate the impact of mcr-1 on the development of high-level colistin resistance (HLCR) in Klebsiella pneumoniae and Escherichia coli. In this study, mcr-1-negative K. pneumoniae and E. coli strains and their corresponding mcr-1-positive transformants were used to generate HLCR mutants via multiple passages in the presence of increasing concentrations of colistin. We found that for K. pneumoniae, HLCR mutants with minimum inhibitory concentrations (MICs) of colistin from 64 to 1,024 mg/L were generated. Colistin MICs increased 256- to 4,096-fold for mcr-1-negative K. pneumoniae strains but only 16- to 256-fold for the mcr-1-harboring transformants. For E. coli, colistin MICs increased 4- to 64-folds, but only 2- to 16-fold for their mcr-1-harboring transformants. Notably, mcr-1 improved the survival rates of both E. coli and K. pneumoniae strains when challenged with relatively high concentrations of colistin. In HLCR K. pneumoniae mutants, amino acid alterations predominately occurred in crrB, followed by phoQ, crrA, pmrB, mgrB, and phoP, while in E. coli mutants, genetic alterations were mostly occurred in pmrB and phoQ. Additionally, growth rate analyses showed that the coexistence of mcr-1 and chromosomal mutations imposed a fitness burden on HLCR mutants of K. pneumoniae. In conclusion, HLCR was more likely to occur in K. pneumoniae strains than E. coli strains when exposed to colistin. The mcr-1 gene could improve the survival rates of strains of both bacterial species but could not facilitate the evolution of high-level colistin resistance.

Rapid Increase in Carbapenemase-Producing Enterobacteriaceae in Retail Meat Driven by the Spread of the blaNDM-5-Carrying IncX3 Plasmid in China from 2016 to 2018.

The presence and molecular characteristics of carbapenemase-producing Enterobacteriaceae (CPE) among meat products in China were investigated. A total of 110 carbapenem-resistant Enterobacteriaceae (CRE) isolates, including 94 Escherichia coli and 10 Klebsiella pneumoniae isolates, were identified from 105 of 794 (13.2%) samples. The positive rates markedly increased from 2016 (9.4%) to 2018 (22.2%). Only blaNDM genes were detected; 79.1% of blaNDM genes were carried by IncX3 plasmids. Routine monitoring of carbapenemase-producing Enterobacteriaceae in the animal food supply is highly recommended.