Fitness Cost of bla(NDM-5)-Carrying p3R-IncX3 Plasmids in Wild-Type NDM-Free Enterobacteriaceae

The wide dissemination of New Delhi metallo-β-lactamase genes (bla(NDM)) has resulted in the treatment failure of most available β-lactam antibiotics, with IncX3-type bla(NDM-5)-carrying plasmids recognised as having spread worldwide. In China, bacteria carrying these plasmids are increasingly being detected from diverse samples, including hospitals, communities, livestock and poultry, and the environment, suggesting that IncX3 plasmids are becoming a vital vehicle for bla(NDM) dissemination. To elucidate the fitness cost of these plasmids on the bacterial host, we collected bla(NDM)-negative strains from different sources and tested their ability to acquire the bla(NDM-5)-harboring p3R-IncX3 plasmid. We then measured changes in antimicrobial susceptibility, growth kinetics, and biofilm formation following plasmid acquisition. Overall, 70.7% (29/41) of our Enterobacteriaceae recipients successfully acquired the bla(NDM-5)-harboring p3R-IncX3 plasmid. Contrary to previous plasmid burden theory, 75.9% (22/29) of the transconjugates showed little fitness cost as a result of plasmid acquisition, with 6.9% (2/29) of strains exhibiting enhanced growth compared with their respective wild-type strains. Following plasmid acquisition, all transconjugates demonstrated resistance to most β-lactams, while several strains showed enhanced biofilm formation, further complicating treatment and prevention measures. Moreover, the highly virulent Escherichia coli sequence type 131 strain that already harbored mcr-1 also demonstrated the ability to acquire the bla(NDM-5)-carrying p3R-IncX3 plasmid, resulting in further limited therapeutic options. This low fitness cost may partly explain the rapid global dissemination of bla(NDM-5)-harboring IncX3 plasmids. Our study highlights the growing threat of IncX3 plasmids in spreading bla(NDM-5).