Segregational instability of multicopy plasmids: A population genetics approach

Abstract

Plasmids are extra‐chromosomal genetic elements that encode a wide variety of phenotypes and can be maintained in bacterial populations through vertical and horizontal transmission, thus increasing bacterial adaptation to hostile environmental conditions like those imposed by antimicrobial substances. To circumvent the segregational instability resulting from randomly distributing plasmids between daughter cells upon division, nontransmissible plasmids tend to be carried in multiple copies per cell, with the added benefit of exhibiting increased gene dosage and resistance levels. But carrying multiple copies also results in a high metabolic burden to the bacterial host, therefore reducing the overall fitness of the population. This trade‐off poses an existential question for plasmids: What is the optimal plasmid copy number? In this manuscript, we address this question by postulating and analyzing a population genetics model to evaluate the interaction between selective pressure, the number of plasmid copies carried by each cell, and the metabolic burden associated with plasmid bearing in the absence of selection for plasmid‐encoded traits. Parameter values of the model were estimated experimentally using Escherichia coli K12 carrying a multicopy plasmid encoding for a fluorescent protein and bla TEM‐1 , a gene conferring resistance to β ‐lactam antibiotics. By numerically determining the optimal plasmid copy number for constant and fluctuating selection regimes, we show that plasmid copy number is a highly optimized evolutionary trait that depends on the rate of environmental fluctuation and balances the benefit between increased stability in the absence of selection with the burden associated with carrying multiple copies of the plasmid.

A mathematician’s guide to plasmids: an introduction to plasmid biology for modellers

“…The copy number is not exactly identical from cell to cell, but is subject to some noise, which can be reduced by a partitioning system [89]. The plasmid copy number may of course be subject to evolution, even over short time scales when the plasmid carries an important host function [90][91][92].…”

“…Models that account for the plasmid copy number, despite appearing early on [129, 198–202], have remained overall rare. Recently the topic started to receive growing attention, mostly (but not exclusively) in the context of allele dynamics on multicopy plasmids (see and [92]). Several of the early models of multicopy plasmids consider segregation of two incompatible plasmids and thus include coinfection of cells by multiple plasmid types.…”

Evolutionary rescue of bacterial populations by heterozygosity on multicopy plasmids

“…Recently, interest in the evolution of multicopy plasmids and their hosts has gained momentum (e.g. San Millan et al, 2016;Rodriguez-Beltran et al, 2018;Santer and Uecker, 2020;Rodríguez-Beltrán et al, 2021;Garoña et al, 2021;Hernandez-Beltran et al, 2022;Santer et al, 2022;Garoña et al, 2023). This includes experimental studies of multicopy plasmids with accompanying models: some of these focus on segregation of plasmid copies as a limitation on the fixation of novel beneficial alleles on plasmids (Ilhan et al, 2018;Garoña et al, 2023), while others focus on the effect of selection for heterozygotes on maintaining plasmid variants together despite segregation (Rodriguez-Beltran et al, 2018).…”