Persistence under pressure: exploring the impact of conjugation rate evolution on the stability of plasmids

Abstract

Plasmids are small, extrachromosomal DNA elements commonly found in bacteria, often carrying accessory genes such as antibiotic resistance genes. They play a pivotal role in disseminating antibiotic resistance within bacterial populations through the process of conjugation, enabling transfer between different bacterial strains or species horizontally, rather than vertically through cellular division. In the absence of selection for the plasmid, its presence in the population tends to decrease due to associated fitness costs. However, coevolution between hosts and plasmids can lead to enhanced plasmid persistence, allowing them to persist even after the selective pressure is removed. There are a variety of ways that plasmids become more persistent, including acquiring compensatory mutations to reduce the cost of carriage, minimizing segregational loss, and increasing conjugation rates. In this study, we specifically investigated the impact of increased conjugation rates, a less explored yet significant factor contributing to plasmid persistence. We employed the Luria-Delbrück Method (LDM) to estimate the conjugation rate of an ancestral and descendant plasmid-host pair consisting of an Escherichia coli host and an IncP- plasmid that coevolved under antibiotic selection favoring plasmid maintenance. Remarkably, we observed a significant increase in conjugation rates, which suggests that the increase in plasmid persistence in a population can be partially explained by an increase in the transfer rate after plasmid-host coevolution. To understand the drivers behind this increase, we formulated two hypotheses: (i) a pleiotropic effect of cost reduction and (ii) direct selection for heightened conjugation rates. While the pleiotropy hypothesis is attractive, our findings lacked robust evidence for it, as there was no significant change in growth rate that would indicate a reduction in plasmid cost. Consequently, we explored the direct selection hypothesis. Although we did not find any conjugation-related mutations in the plasmid, our theoretical model suggested that mutations impacting conjugation rates could potentially drive the mutant plasmid's invasion into the population under certain conditions. Our results shed light on the complexities of plasmid persistence and conjugation rates, indicating that selection under antibiotic pressure not only favors retaining antibiotic resistance genes and alleviating associated plasmid costs, but in some cases, may also promote an increase in horizontal transmission of the plasmid. Thus, selective antibiotic conditions may enhance the spread of antibiotic resistance through horizontal gene transfer.