What Is Colicin? Definition and What Research Reports

Colicin is the collective name for a family of protein bacteriocins produced by Escherichia coli and some closely related enterobacteria, which kill or inhibit competing bacterial strains of the same or nearby species while leaving the producing cell protected by a dedicated immunity protein. Individual colicins are named by letter — colicin A, D, E1 through E9, M, U, and F(Y), among others — and each letter designates a distinct protein with its own receptor, import route, and killing mechanism. This page is a definitional reference entry: it explains what the word denotes, where the molecules come from, how the term appears in peptide and bacteriocin research, and what the published literature has reported. This page is for educational purposes only and is not medical advice; consult a licensed physician with questions about any compound.

What class of molecule a colicin is

Colicins belong to the bacteriocin class — antibacterial proteins made by bacteria that target other bacteria. Unlike the short, chemically synthesised chains that dominate much of the peptide literature, colicins are relatively large, genetically encoded, multi-domain proteins. A typical colicin is built from three functional regions: an N-terminal translocation domain, a central receptor-binding domain, and a C-terminal cytotoxic domain that carries the actual killing activity. Structural work on colicin D resolved the central and C-terminal RNase domains by crystallography and used that structure to propose a translocation pathway through the inner membrane of the target cell (PMID 29905832).

This modular architecture is the reason colicins appear so often in protein-engineering papers. Researchers reported that the N-terminal and central domains of colicin A could be fused to a phage lysin so that the lysin was able to lyse E. coli from outside the cell (PMID 28730370), an example of the translocation and receptor-binding modules being repurposed as a delivery scaffold.

Where colicins come from

Colicin genes sit on plasmids carried by a subset of E. coli strains, together with the gene for the immunity protein that shields the producer and, usually, a lysis gene involved in release. Production is classically tied to the SOS stress response, and release requires the producing cell to give up its own integrity. Work on colicin E2 in E. coli biofilms revisited how expression is induced and regulated in that setting (PMID 36518173), and a separate study followed the dynamics of colicin E9 release from E. coli under native conditions (PMID 38653724).

Colicin-producing strains are also found among clinical isolates. A 2025 report described the molecular characterisation and antibacterial activity of colicin-producing E. coli recovered from urinary tract infections (PMID 41082035). Related bacteriocins occur in other genera under the same naming convention — colicin U, for instance, was described from Shigella boydii (PMID 31548276).

How the term is used in peptide and bacteriocin research

In the wider antimicrobial-peptide literature, "colicin" is normally used in three ways:

• As a named molecule. A specific paper studies colicin E7, colicin M, or colicin U as a defined protein with a defined target.
• As a mechanistic category. Authors group bacteriocins by killing mode — pore-forming colicins, nuclease colicins (DNases, rRNases, tRNases), and enzymatic colicins acting on cell-wall precursors.
• As a research tool. Colicin domains are used as import handles, and colicin–immunity protein pairs are used as model systems for studying protein–protein recognition and outer-membrane transport.

An important definitional caveat: colicins are proteins in the tens of kilodaltons, not short peptides. They are sometimes filed alongside antimicrobial peptides because of shared function, but they differ from small peptides in size, in genetic encoding, and in the fact that they require dedicated receptor and translocation machinery in the target cell rather than acting by generic membrane disruption.

Reported mechanisms of action

Pore formation

Some colicins insert into the inner membrane of the target cell and form ion-conducting channels that collapse the membrane potential. A study of colicin U from Shigella boydii reported that the protein formed voltage-dependent pores in planar membrane systems (PMID 31548276).

Nuclease activity

Nuclease colicins carry a C-terminal enzymatic domain that cleaves DNA, ribosomal RNA, or transfer RNA once delivered into the cytoplasm. Structural analysis of colicin D characterised its RNase domains and the route by which the toxic domain is proposed to cross the inner membrane (PMID 29905832). Target cells are not defenceless: researchers reported that an RtcB2–PrfH operon protected E. coli ATCC25922 from colicin E3 toxin (PMID 35742896).

Cell-wall precursor degradation

Colicin M acts differently again. An earlier study reported that colicin M exerted its bacteriolytic effect through enzymatic degradation of undecaprenyl phosphate-linked peptidoglycan precursors (PMID 16777846).

Import across the envelope

Before any of this happens, the colicin must cross the outer membrane using host transport systems. Work on colicin E1 examined its TolC-binding conformer and asked whether TolC serves a "pillar" or a "pore" function during colicin import (PMID 27536862).

Named colicins in the cited literature

What the published literature reports

Beyond mechanism, the cited literature spans three broad settings. In an animal model, the study of colicin F(Y) reported inhibition of pathogenic Yersinia enterocolitica in mice (PMID 30115964). In an ecological setting, researchers modelled how colicin production rates affected allelopathic interactions within E. coli populations (PMID 31739595), reflecting the long-standing view of colicins as tools of intraspecies competition. Outside antibacterial work, a 2021 report examined the anticancer properties of colicin E7 against colon cancer (PMID 34976246).

Adverse Events: What Studies Report

The verified papers summarised here are microbiological, structural, cell-based, and animal studies; none of them is a human safety or tolerability trial, and none reported a human adverse-event profile for a colicin. The mouse work on colicin F(Y) reported inhibition of Yersinia enterocolitica (PMID 30115964), and the colicin E7 work was framed around anticancer properties in a colon cancer context (PMID 34976246), but neither establishes human safety. Readers evaluating claims about colicins should note the difference between in vitro activity, animal models, and clinical evidence, and should read the underlying abstracts directly.

Summary of the definition

1. Class: protein bacteriocin, plasmid-encoded, multi-domain.
2. Source: E. coli and related enterobacteria, including clinical isolates described in urinary tract infection samples (PMID 41082035).
3. Targets: other strains of the same or closely related species, via specific surface receptors.
4. Killing modes reported: pore formation, nucleic-acid cleavage, and peptidoglycan precursor degradation, as described in the studies cited above.

References

• Colicin F(Y) inhibits pathogenic Yersinia enterocolitica in mice (Scientific Reports, 2018)
• Colicin E2 expression in Escherichia coli biofilms: Induction and regulation revisited (Current Research in Microbial Sciences, 2022)
• Anticancer properties of colicin E7 against colon cancer (Przeglad Gastroenterologiczny, 2021)
• Molecular characterization and antibacterial activity of colicin-producing Escherichia coli isolated from urinary tract infections (Folia Microbiologica, 2025)
• RtcB2-PrfH Operon Protects E. coli ATCC25922 Strain from Colicin E3 Toxin (International Journal of Molecular Sciences, 2022)
• The dynamics of colicin E9 release from Escherichia coli in native conditions (Letters in Applied Microbiology, 2024)
• The N-terminal and central domain of colicin A enables phage lysin to lyse Escherichia coli extracellularly (Antonie van Leeuwenhoek, 2017)
• Crystal structure of the central and the C-terminal RNase domains of colicin D implicated its translocation pathway through inner membrane of target cell (Journal of Biochemistry, 2018)
• Colicin U from Shigella boydii Forms Voltage-Dependent Pores (Journal of Bacteriology, 2019)
• The Colicin E1 TolC-Binding Conformer: Pillar or Pore Function of TolC in Colicin Import? (Biochemistry, 2016)
• Colicin M exerts its bacteriolytic effect via enzymatic degradation of undecaprenyl phosphate-linked peptidoglycan precursors (Journal of Biological Chemistry, 2006)
• The Effects of Colicin Production Rates on Allelopathic Interactions in Escherichia coli Populations (Microorganisms, 2019)