Fitness Costs of Plasmids: a Limit to Plasmid Transmission

Abstract

Plasmids mediate the horizontal transmission of genetic information between bacteria, facilitating their adaptation to multiple environmental conditions. An especially important example of the ability of plasmids to catalyze bacterial adaptation and evolution is their instrumental role in the global spread of antibiotic resistance, which constitutes a major threat to public health. Plasmids provide bacteria with new adaptive tools, but they also entail a metabolic burden that, in the absence of selection for plasmid-encoded traits, reduces the competitiveness of the plasmid-carrying clone. Although this fitness reduction can be alleviated over time through compensatory evolution, the initial cost associated with plasmid carriage is the main constraint on the vertical and horizontal replication of these genetic elements. The fitness effects of plasmids therefore have a crucial influence on their ability to associate with new bacterial hosts and consequently on the evolution of plasmid-mediated antibiotic resistance. However, the molecular mechanisms underlying plasmid fitness cost remain poorly understood. Here, we analyze the literature in the field and examine the potential fitness effects produced by plasmids throughout their life cycle in the host bacterium. We also explore the various mechanisms evolved by plasmids and bacteria to minimize the cost entailed by these mobile genetic elements. Finally, we discuss potential future research directions in the field.

Phylogenetic background and habitat drive the genetic diversification of Escherichia coli

“…In the present context, this suggests that either many integrations of genetic material are slightly deleterious or that there is rapid deletion of neutral genes. The first hypothesis is consistent with the fitness costs associated with the acquisition of many MGEs 80–82 , and with our observation that most MGEs present in a genome were very recently acquired. The second hypothesis is consistent with the previous works suggesting the existence of mechanistic biases towards gene deletion in bacteria 83, 84 .…”

Plasmid persistence: costs, benefits, and the plasmid paradox

“…For example, a plasmid may be 93 beneficial when its bacterial host is exposed to a novel stressor, but detrimental once the stressor 94 is removed (Vogwill and MacLean 2015). Under these conditions, it is expected that bacterial 95 strains lacking the costly plasmid outcompete the plasmid-bearing strain due to the fitness cost of 96 plasmid carriage, and this cost may be a result of any one of a number of proposed mechanisms 97 (Enne et al 2004;San Millan and MacLean 2017). Thus, costly plasmids are expected to be lost 98 from a population due to competition with plasmid-free cells.…”

How genetic parasites persist despite the purge of natural selection

“…Because intracellular replication of genetic parasites consumes resources without providing any functional benefit to the host, cells that host genetic parasites experience a reduction in growth (that is, a fitness cost) compared to parasite-free cells [20,21]. In a population with parasite-bearing and parasite-free cells, the higher fitness of the latter will lead to the decline of the parasite, unless the parasite evolves the ability to infect new cells at a rate that compensates for the purge of infected cells [22].…”