Establishing a mechanistic basis for the plasmid acquisition cost

Project Summary/Abstract This project will investigate the previously understudied phenomenon termed plasmid acquisition cost, in the context of horizontal gene transfer (HGT). Plasmid transfer is a dominant way that pathogens adapt to environmental stressors. These plasmids often exert a sustained burden on cells, associated with continued expression of plasmid genes, known as the plasmid fitness cost. They also induce a complementary, independent, transient burden reflective of the metabolic adaptation immediately following plasmid acquisition. However, whereas fitness costs are well-studied, the mechanistic factors underlying these transient effects, referred to as the plasmid acquisition costs, are currently unknown. This information is critical to developing fundamental insights into pathogen dynamics and potential downstream interventional strategies. Based on our preliminary data, our central hypothesis is that acquisition costs arise as a result of metabolic dysregulation immediately following plasmid transfer. We will investigate this hypothesis with two Specific Aims: (1) We will elucidate the transcriptional basis of the plasmid acquisition cost, by undertaking a detailed characterization of the well-characterized RP4 conjugative plasmid in a standard laboratory Escherichia coli strain; we note that our preliminary work illustrating the existence and extent of acquisition costs was originally done using this combination. Specifically, we established a novel experimental protocol that reliably quantifies the growth defect in new plasmid recipients. Here, we will pair time series RNA-seq measurements with metabolic modeling to elucidate the metabolic dysregulation that occurs immediately following plasmid acquisition; we will validate these results using a rationally selected set of gene knockout strains and representative plasmids. Overall, this will establish a mechanistic explanation for observed acquisition costs. (2) We will determine the genetic determinants of the acquisition cost in diverse plasmids, using naturally relevant plasmids and strains isolated from both environmental and clinical samples, as well as standard laboratory plasmids. Specifically, we will leverage available whole genome sequences with assembled plasmids from our collaborators, along with the corresponding samples, to isolate plasmids from two species: E. coli and Klebsiella pneumoniae. We will then quantify acquisition costs for each plasmid; multiple variable regression will be used to understand which plasmid characteristics are most predictive of observed acquisition costs. This will result in the first rigorous quantification of plasmid acquisition costs in naturally occurring environmental and clinical plasmids. Beyond demonstrating the generality of this phenomenon, these results will serve as the foundation for predicting and modulating HGT dynamics in more complex populations. Moreover, all plasmids will be made available, which will serve as an invaluable research and educational tool throughout the wider academic community.