The evolution of infectious transmission promotes the persistence of mcr-1 plasmids.

Conjugative plasmids play a vital role in bacterial evolution and promote the spread of antibiotic resistance. They usually cause fitness costs that diminish the growth rates of the host bacteria. Compensatory mutations are known as an effective evolutionary solution to reduce the fitness cost and improve plasmid persistence. However, whether the plasmid transmission by conjugation is sufficient to improve plasmid persistence is debated since it is an inherently costly process. Here, we experimentally evolved an unstable and costly mcr-1 plasmid pHNSHP24 under laboratory conditions and assessed the effects of plasmid cost and transmission on the plasmid maintenance by the plasmid population dynamics model and a plasmid invasion experiment designed to measure the plasmid's ability to invade a plasmid-free bacterial population. The persistence of pHNSHP24 improved after 36 days evolution due to the plasmid-borne mutation A51G in the 5'UTR of gene traJ. This mutation largely increased the infectious transmission of the evolved plasmid, presumably by impairing the inhibitory effect of FinP on the expression of traJ. We showed that increased conjugation rate of the evolved plasmid could compensate for the plasmid loss. Furthermore, we determined that the evolved high transmissibility had little effect on the mcr-1-deficient ancestral plasmid, implying that high conjugation transfer is vital for maintaining the mcr-1-bearing plasmid. Altogether, our findings emphasized that, besides compensatory evolution that reduces fitness costs, the evolution of infectious transmission can improve the persistence of antibiotic-resistant plasmids, indicating that inhibition of the conjugation process could be useful to combat the spread of antibiotic-resistant plasmids. IMPORTANCE Conjugative plasmids play a key role in the spread of antibiotic resistance, and they are well-adapted to the host bacteria. However, the evolutionary adaptation of plasmid-bacteria associations is not well understood. In this study, we experimentally evolved an unstable colistin resistance (mcr-1) plasmid under laboratory conditions and found that increased conjugation rate was crucial for the persistence of this plasmid. Interestingly, the evolved conjugation was caused by a single-base mutation, which could rescue the unstable plasmid from extinction in bacterial populations. Our findings imply that inhibition of the conjugation process could be necessary for combating the persistence of antibiotic-resistance plasmids.