Integrative Conjugative Elements
One-Sentence Definition
Integrative conjugative elements (ICEs) are self-transmissible mobile genetic elements that reside in the chromosome like a prophage/lysogen but excise and transfer by conjugation like a plasmid.
Simple Explanation
An ICE is a hybrid: it parks in the chromosome most of the time, then cuts itself out and conjugatively jumps to a new bacterium when conditions favor transfer.
Detailed Scientific Explanation
| Feature | ICE | Classical plasmid | Prophage |
|---|---|---|---|
| Main residence | Chromosome (integrated) | Extrachromosomal | Chromosome |
| Transfer | Conjugation (tra) | Conjugation / mobilization | Transduction |
| Integration | Site-specific recombinase | Usually none | Integrase |
| Cargo | AMR, virulence, metabolism | Same | Mostly phage genes |
Well-studied families: SXT/R391 (Vibrionaceae and beyond), ICEBs1 (Bacillus), Tn916/Tn1545 family (Gram-positive; tetracycline tetM), ICEKp lineages in Klebsiella.
ICEs can mobilize co-resident genomic islands or plasmids in cis/trans helper roles.
Mechanism
Integrase maintains chromosomal state → excision as circular intermediate → type IV secretion / mating pair formation → single-strand transfer → re-integration in recipient (often at tRNA or other attB sites).
Clinical Importance
- Spreads MDR and virulence without a stable free plasmid — harder to spot by plasmid typing alone.
- Tn916-like ICEs move tetracycline and sometimes other resistances among streptococci/enterococci/anaerobes.
Research Importance
- Blurs plasmid–chromosome dichotomy; requires specialized detection (conjugation genes + integrase + excision assays).
Diagnostic Relevance
- Standard plasmid finders may miss ICEs; look for tra + integrase + chromosomal junctions in WGS (Plasmid and Mobile Element Analysis).
AMR Relevance
High for Gram-positives (tet, erm contexts) and emerging Gram-negative ICE-borne resistances; contributes to Horizontal Gene Transfer beyond classic Inc plasmids.
Related Methods
- Whole-Genome Sequencing · Comparative Genomics · conjugation assays
Related MOCs
Learning Aids
Clinical Example
Example
Case: Streptococcus isolate acquires tetM without a detectable plasmid replicon.
Question: What vehicle should you suspect?
Answer: A Tn916-family ICE integrated in the chromosome — conjugative but not a classic plasmid.
Active Recall Questions
- How does an ICE differ from a conjugative plasmid?
- Why might PlasmidFinder miss an ICE?
- Name one ICE family important in Vibrio/Enterobacterales surveillance.