Differences in vertical and horizontal transmission dynamics shape plasmid 1 distribution in clinical enterobacteria 2

Conjugative plasmids can transfer both vertically and horizontally in bacterial communities, playing a key role in the dissemination of antimicrobial resistance (AMR) genes across bacterial pathogens. AMR plasmids are widespread in clinical settings, but their distribution is not random, and certain associations between plasmids and bacterial clones are particularly successful. However, knowledge remains limited about the contribution made by vertical and horizontal transmission dynamics to plasmid distribution and maintenance in clinically relevant bacterial communities. In this study, we used a collection of wild type enterobacterial strains isolated from hospitalized patients to perform a comprehensive analysis of the transmission dynamics of the globally spread carbapenem resistance plasmid pOXA-48. We combined in vitro and in vivo experimental approaches to quantify key traits responsible for vertical (the level of AMR) and horizontal (conjugation frequency) plasmid transmission. Our results reveal significant variability in these traits across different bacterial hosts, with Klebsiella spp. strains showing higher pOXA-48-mediated AMR and conjugation frequencies than Escherichia coli strains. Using experimentally determined parameters, we developed a simple mathematical model to interrogate the contribution of vertical and horizontal transmission to plasmid distribution in bacterial communities. These simulations revealed that a small subset of clones, combining high vertical and horizontal plasmid transmission ability, play a critical role in stabilizing the plasmid in different polyclonal microbial communities. Our results indicate that strain-specific differences in plasmid transmission dynamics dictate successful associations between plasmids and bacterial clones, shaping AMR evolution. Significance statement Conjugative plasmids are the main vehicle for the dissemination of AMR genes across many bacterial pathogens, contributing to one of the most concerning public health problems facing modern societies. Understanding the rules governing plasmid dynamics is therefore crucial to controlling the global AMR crisis. In this study, we show that the plasmid-associated traits responsible for vertical and horizontal plasmid transmission in bacterial communities vary across different bacterial hosts. This information can be used to predict which specific plasmid-bacteria associations are more likely to spread in bacterial communities, thus enabling health care authorities to predict, and potentially control, the evolution of AMR in clinical settings.