Integrons
One-Sentence Definition
Integrons are genetic platforms that capture, stockpile, and express gene cassettes (often antibiotic-resistance genes) using a site-specific recombinase (IntI) and a dedicated promoter (Pc).
Simple Explanation
Think of an integron as a cassette player built into bacterial DNA: it can plug in new resistance “tapes” and play them from one shared promoter.
Detailed Scientific Explanation
| Part | Function |
|---|---|
| intI | Integrase that inserts/excises cassettes at attI |
| attI | Primary recombination site |
| Pc | Promoter driving cassette expression |
| Gene cassettes | Usually promoterless ORFs + attC sites |
Classes of clinical importance:
| Class | Typical context | Classic cargo |
|---|---|---|
| Class 1 | Plasmids, transposons in Gram-negatives | aadA, dfr, bla, qac |
| Class 2 | Often Tn7-associated | Similar cassette repertoire |
| Class 3 | Less common clinically | Resistance cassettes |
| Super-integrons | Chromosomal, vibrios etc. | Hundreds of cassettes (ecology/virulence) |
Cassettes form arrays; position relative to Pc strongly affects expression (proximal > distal).
Mechanism
Free circular cassette ↔ integrase-mediated recombination at attI/attC → ordered array under Pc. Selection with antibiotics enriches arrays that express useful cassettes.
Clinical Importance
- Explains co-resistance packages on a single mobile platform.
- Class 1 integrons are a hallmark of hospital MDR Enterobacterales and Pseudomonas.
Research Importance
- Model of modular genome evolution and cassette metagenomics in wastewater/One Health studies.
Diagnostic Relevance
- Detected in WGS annotation and specialized tools (IntegronFinder); PCR for intI1 used in surveillance.
AMR Relevance
High. Many aminoglycoside, trimethoprim, β-lactamase, and quaternary-ammonium resistance genes travel as integron cassettes nested inside Transposons and Insertion Sequences on Plasmids.
Related Methods
Related MOCs
Learning Aids
Clinical Example
Example
Case: P. aeruginosa isolate resistant to gentamicin and trimethoprim-sulfamethoxazole; WGS shows aadA and dfrA as consecutive cassettes in a class 1 integron on an IncP plasmid.
Question: Why might both resistances appear together?
Answer: Shared Pc expression of an integron array — one acquisition event, multiple phenotypes.
Active Recall Questions
- What does IntI do?
- Why does cassette order matter clinically?
- Integron vs transposon — which captures cassettes?