Bacteriocins and Bacterial Antagonism: What They Are, How They Kill, and Why They Matter

There is a good chance you have eaten a bacteriocin. Nisin, a small protein made by the dairy bacterium Lactococcus lactis, has been added to cheese and other foods as a preservative for decades. It is on food labels as E234. The same molecule that keeps cheese safe is now being studied as a weapon against drug-resistant bacteria, including MRSA.

That is the useful thing about bacteriocins: they are proteins that bacteria make to kill other bacteria. Understanding what they are, why bacteria produce them, and how they work explains a concept that runs from basic microbial ecology all the way to the search for new antibiotics.

What is a bacteriocin?

A bacteriocin is a protein or peptide, made by a bacterium on its ribosomes, that kills or inhibits the growth of other bacteria, usually strains closely related to the producer. That one sentence hides three ideas that are easiest to learn by contrast, because a bacteriocin is often confused with two other things: antibiotics and bacterial toxins.

Bacteriocin vs antibiotic. An antibiotic is a small-molecule secondary metabolite, and it usually acts on a broad range of bacteria. A bacteriocin is a protein, made directly by ribosomes from a gene, and it usually acts on a narrow range, often just strains of the same or a closely related species. So the difference is both chemical (protein vs small molecule) and in reach (narrow vs broad).

Bacteriocin vs bacterial toxin (exotoxin). A bacterial exotoxin is aimed at the cells of a host, and it is a weapon of disease. A bacteriocin is aimed at other bacteria, competitors in the same environment, not at a host. This is the reason the topic sits right after bacterial toxins in most syllabi: both are proteins a bacterium secretes, but they point in completely different directions. A toxin attacks the host; a bacteriocin attacks the neighbors.

Why do bacteria make them? Bacterial antagonism

Bacteria live in crowded places, the gut, the skin, the soil, where nutrients and space are limited. One way an organism competes is by chemically suppressing its rivals. This general phenomenon, one microbe inhibiting or killing another, is called bacterial antagonism, and bacteriocins are one of its main tools. A bacterium that can kill nearby competitors of the same species keeps more of the niche and its nutrients for itself and its own descendants.

This raises an obvious question: if a bacterium releases a protein that kills its close relatives, why does it not kill itself? The answer is self-immunity.

A bacteriocin-producing cell also makes a matching immunity protein, encoded right next to the bacteriocin gene, that protects it from its own weapon. The producer is immune; its unprotected competitors are not. A student who can state this, that the producer survives because it carries a specific immunity protein, has understood the mechanism, not just the definition.

Classification of bacteriocins

Bacteriocins are produced by both gram-positive and gram-negative bacteria, and the two groups are organized differently.

Gram-negative bacteriocins. The named archetype is the colicin, produced by Escherichia coli. Colicins are the classic, best-studied model for how a bacteriocin works.

Gram-positive bacteriocins. These are grouped into classes. The two a student is usually asked about are:

• Class I, the lantibiotics: small peptides that contain unusual modified amino acids. Nisin (from Lactococcus lactis) is the famous example.
• Class II: small, heat-stable peptides that are not modified in the same way. Many of them kill by forming pores in the target membrane.

For this level, the useful thing is to know the archetype (colicin) and the named gram-positive example (nisin), not an exhaustive table of subclasses.

How bacteriocins kill: mechanism of action

Bacteriocins kill in a few distinct ways. Each route is worth learning as: the action, then what it does to the target cell.

Pore formation in the membrane. Many colicins and many Class II bacteriocins insert into the target cell's membrane and punch a channel through it. Once the membrane leaks, the cell can no longer hold its membrane potential (the charge difference it needs to make energy). The gradient collapses, and the cell dies. This is the most common killing route.

Degrading nucleic acids. Some colicins are enzymes. Instead of making a pore, they enter the target cell and cut its DNA or RNA. With its genetic material destroyed, the cell cannot survive.

Blocking cell-wall synthesis, plus pore formation (nisin's dual attack). Nisin is the memorable one, because it does two things at once. It binds lipid II, the building-block molecule the cell needs to construct its wall, which both blocks wall synthesis and pulls nisin into position to form pores in the membrane. Stopping wall-building and puncturing the membrane together make nisin especially effective, and this dual mechanism is the non-obvious detail worth remembering.

Named bacteriocins worth knowing

Bacteriocin typing (a historical applied use)

Because different strains of one species differ in which bacteriocins they produce and which they are susceptible to, these patterns were once used to tell strains apart. This is bacteriocin typing, and it was used, alongside phage typing, to sub-type outbreak strains of organisms such as staphylococci and Salmonella Typhi during epidemiological tracing.

The principle is simple: match strains by their bacteriocin production and susceptibility patterns. In practice, this approach has largely been replaced by molecular typing methods, which are more reproducible and can tell strains apart more finely. So bacteriocin typing is best understood as an early tool that molecular methods have now overtaken, not a current bench routine.

For where it sits among other bacterial typing methods, see the article on bacterial typing methods.

Why bacteriocins matter now

The syllabus concept has a live modern relevance that is worth knowing:

• As alternatives to antibiotics. Because bacteriocins kill bacteria by mechanisms different from most antibiotics, and because some can be aimed at a narrow target, they are being actively studied as treatments for antibiotic-resistant infections. Nisin, for example, is being tested against MRSA and other resistant gram-positive organisms.
• As a food preservative already in use. Nisin is not a future idea; it is an approved, widely used food preservative (E234), valued because it is safe for people to eat while suppressing spoilage and pathogenic bacteria in food.
• In shaping the microbiome. Bacteriocins help decide which strains win a place in a crowded niche such as the gut. This is one of the ways some probiotic bacteria are thought to work: by producing bacteriocins that suppress competing, less desirable strains.

How to Remember

Bacteriocin is a sniper rifle; an antibiotic is a shotgun. A bacteriocin is a protein aimed at a narrow target, usually the producer's own close relatives. An antibiotic is a small molecule that sprays across a broad range of bacteria. Protein vs small molecule, narrow vs broad, that single contrast carries most of the topic.

Named after the producer, mostly ending in -cin or -in. Colicin comes from E. coli, pyocin from Pseudomonas (pyo-), nisin from Lactococcus lactis. If you know the producer, you can usually recall the name.

The producer survives because it carries its own shield. The immunity protein is the answer to "why doesn't it kill itself." Producer makes weapon and shield together; competitors get only the weapon.

Nisin does two jobs. It binds lipid II, which both blocks wall-building and lets it form membrane pores. Two hits at once is why it is the standout example.

Key exam facts

Named bacteriocins:

Other essentials: producer self-immunity is via a specific immunity protein; bacterial antagonism is the ecological "why"; bacteriocin typing is an epidemiological tool that is now largely historical.

Where Students Get Confused

• "A bacteriocin is just a kind of antibiotic." It is not. An antibiotic is a small-molecule secondary metabolite with usually broad reach. A bacteriocin is a protein made directly from a gene on the ribosome, and it usually kills only a narrow set of bacteria, often just close relatives of the producer. Different chemistry, different reach.
• "A bacteriocin is a type of toxin, so it harms the host." No. A bacterial exotoxin is aimed at host cells and causes disease. A bacteriocin is aimed at competing bacteria, not at the host. Both are secreted proteins, which is why they get confused, but they point at completely different targets.
• "Narrow spectrum means it kills only one species." Roughly, narrow spectrum means it kills the producer's own species or closely related strains, rather than bacteria in general. It is not a single-species rule; it is "close relatives," which is exactly what makes bacteriocins useful for telling strains apart (typing) and attractive as targeted antibacterials.
• "If a bacterium makes a bacteriocin, it would kill itself too." It would, except that the producer also makes a matching immunity protein that protects it. The immunity protein is the reason the producer survives its own weapon.
• "Bacteriocin typing is a treatment." It is not treatment. It is an identification and epidemiology tool: a way to tell strains of one species apart when tracing an outbreak. It has largely been replaced by molecular typing.

References

• Madigan MT, Bender KS, Buckley DH, Sattley WM, Stahl DA. Brock Biology of Microorganisms. 16th ed. Pearson; 2021.
• Tille PM. Bailey & Scott's Diagnostic Microbiology. 15th ed. Elsevier; 2022.
• Cotter PD, Ross RP, Hill C. Bacteriocins: a viable alternative to antibiotics? Nature Reviews Microbiology. 2013;11(2):95-105. DOI: 10.1038/nrmicro2937
• Chandrakasan G, et al. Nisin and its applications: an overview of a natural food preservative and its antimicrobial potential.