Conjugative plasmids are mobile elements that spread horizontally between bacterial hosts and often confer adaptive phenotypes, including antimicrobial resistance (AMR). Theory suggests that opportunities for horizontal transmission favor plasmids with higher transfer rates, whereas selection for plasmid carriage favors less-mobile plasmids. However, little is known about the mechanisms leading to variation in transmission rates in natural plasmids or the resultant effects on their bacterial host. We investigated the evolution of AMR plasmids confronted with different immigration rates of susceptible hosts. Plasmid RP4 did not evolve in response to the manipulations, but plasmid R1 rapidly evolved up to 1,000-fold increased transfer rates in the presence of susceptible hosts. Most evolved plasmids also conferred on their hosts the ability to grow at high concentrations of antibiotics. This was because plasmids evolved greater copy numbers as a function of mutations in the copA gene controlling plasmid replication, causing both higher transfer rates and AMR. Reciprocally, plasmids with increased conjugation rates also evolved when selecting for high levels of AMR, despite the absence of susceptible hosts. Such correlated selection between plasmid transfer and AMR could increase the spread of AMR within populations and communities.

接合质粒是在细菌宿主之间水平传播的移动元件，通常赋予适应性表型，包括抗菌素耐药性（AMR）。理论表明，水平传播的机会有利于具有较高转移率的质粒，而质粒运输的选择则有利于移动性较低的质粒。然而，人们对导致天然质粒传播率变化的机制或对其细菌宿主产生的影响知之甚少。我们研究了 AMR 质粒在易感宿主不同迁移率下的进化情况。质粒 RP4 并没有响应这些操作而进化，但质粒 R1 迅速进化，在易感宿主存在的情况下，转移率提高了 1,000 倍。大多数进化的质粒还赋予宿主在高浓度抗生素下生长的能力。这是因为控制质粒复制的copA基因突变导致质粒进化出更大的拷贝数，从而导致更高的转移率和 AMR。相反，尽管不存在易感宿主，但在选择高水平 AMR 时，接合率增加的质粒也会进化。质粒转移和 AMR 之间的这种相关选择可能会增加 AMR 在人群和社区内的传播。