Bacteriocins are proteinaceous toxins given off by bacteria to inhibit the growth of similar bacterial strain(s). E. coli bacteriocins are called colicins.

The bacteriocins of lactic acid-fermenting bacteria are well studied because of the commercial use of these bacteria in the food industry for making dairy products such as cheese. Bacteriocins are classified according to their extent of posttranslational modification. The lantibiotics are a class of more extensively modified bacteriocins, also called Class I. Bacteriocins for which disulfide bonds are the only modification to the peptide are Class II bacteriocins. Most bacteriocins are biologically active single-chain peptides. Some are only active as partners with a second peptide (see Class IIb, below).

Nisin and epidermin are members of a family of lantibiotics that bind to a cell wall precursor lipid component of target bacteria and disrupt cell wall production. The duramycin family of lantibiotics binds phosphoethanolamine in the membranes of its target cells and seem to disrupt several physiological functions.

The action of Class IIa bacteriocins seems to involve disruption of mannose transport into target cells. Class IIb bacteriocins form pores in the membranes of target cells and disrupt the proton gradient of target cells. Other bacteriocins can be grouped together as Class IIc. These have a wide range of effects on membrane permeability, cell wall formation and pheromone actions of target cells.

Bacteriocins are of interest in medicine because they are made by non-pathogenic bacteria that normally colonize the human body. Loss of these harmless bacteria following antibiotic use may allow oportunistic pathogenic bacteria to invade the human body.

To demonstrate their production, technicians stab inoculate multiple strains on separate multiple nutrient agar Petri dishes, incubate at 30 °C for 24 h., overlay each plate with one of the strains (in soft agar), incubate again at 30 °C for 24 h. After this process, the presence of bacteriocins can be inferred if there are zones of growth inhibition around stabs.