, polypeptide, and even protein--based entities released by bacteria that are bactericidal against other bacteria, particularly ones that are closely related.

Bacteriocins differ from antibiotics in terms their spectrum of activity and specificity. That is, they display a narrow spectrum of activity, narrower even than the typical narrow spectrum antibiotic, and they are specific for close relatives rather than nonrelatives, though this narrowness is not always absolute.

Given this specificity, one of the characteristics of bacteriocins is that their producers also must produce an "antidote", that is, a protein that protects the producer from the bacteriocin. As such, bacteriocins and this resistance protein can be viewed as an example of .

Bacteriocins probably exist either as a means of enhancing a bacterium's against similar bacteria or instead as a form of toxin-antitoxin system, or both. From :

Although it has been consistently shown that bacteriocin production can provide bacteria with an advantage against competing bacteria, our results illustrate that bacteriocin production can also play an important role in plasmid maintenance. In the latter case, the bacteriocin acts as a promoting the stability of the plasmid in the population and may help explain why bacteriocins target found on the (which has been shown to decrease the overall effectiveness of the bacteriocin through neutralizing the bacteriocin ). Furthermore, this result may explain why bacteriocins are able to persist in environments where potential are often resistant . Hence, to better understand the ecology and evolution of bacteriocins and bacteriocins in general, we should not only take into account their effect on the host cell and interacting bacterial community but also their role as selfish genetic elements promoting their own .

See also the concepts of colicins, , , etc.