Plasmid
One-Sentence Definition
A plasmid is an extrachromosomal DNA molecule (usually circular) that replicates autonomously and often carries accessory genes such as antibiotic resistance or virulence factors.
Simple Explanation
Optional bonus DNA — not required for basic life, but can make a bacterium harder to kill or more dangerous.
Detailed Scientific Explanation
- Copy number: low vs high.
- Incompatibility (Inc) groups: plasmids of same Inc can’t stably coexist.
- Conjugative plasmids encode transfer machinery; mobilizable plasmids hitchhike.
- R-plasmids: resistance; virulence plasmids (e.g., some Bacillus anthracis, Shigella/E. coli systems).
- Can integrate transiently or evolve into chromosomally integrated forms; related: ICEs, integrative elements.
Mechanism
Independent replication origins; segregation systems; toxin–antitoxin modules promote persistence. Transfer mainly via Conjugation.
Clinical Importance
- Engine of MDR in Enterobacterales and beyond.
- Loss of plasmid in vitro can make AST look “susceptible” while clinical strains retain it — interpret carefully.
Research Importance
- Cloning vectors are engineered plasmids.
- Plasmidomics is a major AMR research field.
Diagnostic Relevance
- PCR/WGS detect plasmid-borne genes (blaKPC, blaNDM, mcr-1, etc.).
- Phenotype via Antimicrobial Susceptibility Testing.
AMR Relevance
- Primary storage unit for many clinically critical resistance genes.
- Co-selection: one plasmid, many resistance cassettes.
Related Methods
Related Papers
Related MOCs
- MOC - Fundamentals of Microbiology · MOC - Antimicrobial Resistance (AMR) · Horizontal Gene Transfer
Active Recall Questions
- How does a plasmid differ from the chromosome?
- What is an R-plasmid?
- Why do Inc groups matter epidemiologically?
Connections
- Transfer: Conjugation · Horizontal Gene Transfer