The Battle Against Antibiotic Resistance: Unveiling the Secrets of IncI2 Plasmids
In the ongoing war against antibiotic resistance, a major global health threat, the emergence of plasmid-mediated colistin resistance, particularly driven by the mcr-1 gene, has become a focal point of concern. Among the carriers of mcr-1, IncI2 plasmids have been frequently encountered in both clinical and agricultural settings. However, their behavior and adaptability in the absence of antibiotic pressure, as well as their responses to colistin exposure, remain shrouded in mystery.
Unraveling the Mystery: A 60-Day Laboratory Evolution Experiment
To shed light on this enigma, researchers conducted a 60-day laboratory evolution experiment using Escherichia coli C600 carrying the mcr-1-harboring IncI2 plasmid pBD110. The experiment was designed with three colistin concentrations: 0, 2, and 4mg/L, representing no pressure, moderate pressure, and strong selection, respectively. The stability of the plasmid was evaluated using polymerase chain reaction (PCR), while bacterial fitness was assessed through growth curve analysis and competition assays. Antimicrobial susceptibility was determined by the broth microdilution method, and conjugation potential was examined through conjugation experiments. To delve deeper, the researchers employed whole-genome sequencing combined with bioinformatic analysis to investigate genomic alterations.
Results: A Tale of Two Strategies
The findings revealed that pBD110 was stably maintained for 120 passages under all conditions, with no significant loss observed in the absence of colistin. Interestingly, under strong selection (4mg/L), the plasmid abundance increased, while moderate pressure (2mg/L) led to fitness costs and a reduction in plasmid copy number. Whole-genome sequencing uncovered distinct adaptive strategies employed by the plasmids. In the absence of selective pressure, the plasmids accumulated mutations in conjugation-related genes, enhancing their transfer frequency. However, when exposed to colistin, the plasmids retained structural stability but acquired shufflon inversions, impairing their conjugation ability. Meanwhile, the host genomes accumulated numerous chromosomal mutations, particularly in metabolic and stress response pathways, to compensate for the burdens associated with resistance.
Dual Evolutionary Strategies: Horizontal Transfer vs. Vertical Inheritance
The results highlight the dual evolutionary strategies adopted by IncI2 plasmids. In the absence of colistin, they optimized horizontal transfer, ensuring their spread and persistence. However, under selective pressure, they prioritized the stability and vertical inheritance of mcr-1, focusing on maintaining resistance. These findings provide valuable insights into the persistence and dissemination of colistin resistance and emphasize the evolutionary trade-offs that shape the coadaptation between plasmids and their hosts.
Data Availability and Further Exploration
The raw data from this study has been deposited in the NCBI GenBank genomic DNA database under BioProject accession number PRJNA1003636, allowing further exploration and analysis by the scientific community. The findings presented here contribute to our understanding of antibiotic resistance and offer a glimpse into the complex world of plasmid evolution.
Thoughts and Questions for Discussion
The adaptive strategies employed by IncI2 plasmids raise intriguing questions. How do these plasmids balance the need for horizontal transfer with the stability required for vertical inheritance? Are there specific conditions or environmental factors that trigger these strategies? Furthermore, the impact of chromosomal mutations on host fitness and the potential long-term consequences of these adaptations remain open areas for exploration. We invite readers to share their thoughts and engage in a discussion on these fascinating aspects of plasmid-host coevolution.
