Plasmid Costs Explain Plasmid Maintenance, Irrespective of the Nature of Compensatory Mutations

Area of Science:

• Microbiology
• Evolutionary Biology
• Genetics

Background:

• Conjugative plasmids frequently harbor virulence and antibiotic resistance genes, influencing bacterial evolution and spread.
• The ubiquity of plasmids despite a known replication cost suggests strong selective advantages for host bacteria.
• Existing hypotheses for plasmid maintenance do not fully account for the diverse interactions between bacteria, plasmids, and environments.

Purpose of the Study:

• To investigate the selective advantages conferred by conjugative plasmids to host bacteria, particularly under conditions where compensatory mutations arise.
• To elucidate the mechanisms maintaining plasmids in bacterial populations despite potential fitness costs.

Main Methods:

• Utilized computer simulations to model bacterial populations with and without conjugative plasmids under various conditions.
• Analyzed the impact of compensatory mutations occurring within plasmids versus chromosomal mutations on bacterial fitness and plasmid maintenance.

Main Results:

• Donor bacterial cells benefit from harboring conjugative plasmids, even when compensatory mutations occur within the plasmid.
• Key factors contributing to this advantage include the time required for mutations to appear, the inherent cost of many plasmids, and the spatial separation of re-transfer events.
• This challenges the notion that plasmid costs solely hinder antibiotic effectiveness, showing they can aid antibiotic-resistant bacteria's competitiveness.

Conclusions:

• Conjugative plasmids provide a competitive edge to bacteria, facilitating their persistence in populations.
• The study refines understanding of plasmid maintenance, emphasizing the role of mutation dynamics and spatial factors.
• Findings offer a new perspective on the interplay between antibiotic resistance, plasmid costs, and bacterial competition.