Colicins

Colicins are bacteriocins, which are a class of ribosomally synthesized antimicrobial peptides produced by Gram-negative bacteria, particularly *Escherichia coli*. These proteins are secreted by bacteria to inhibit or kill closely related strains that compete for the same ecological niche. The term "colicin" specifically refers to those produced by *E. coli*. Colicins are encoded on plasmids, which facilitates their spread within bacterial populations.

The mechanism of action for most colicins involves targeting essential cellular processes in sensitive bacteria. They often enter the target cell through specific outer membrane receptors, which are typically involved in nutrient uptake. Once inside the periplasm, colicins can translocate to the cytoplasm or inner membrane. Different colicins exhibit distinct cytotoxic activities. Some form pores in the cytoplasmic membrane, disrupting ion gradients and leading to cell death. Others are enzymes that degrade peptidoglycan, cleave ribosomal RNA, or inhibit DNA replication.

The specificity of colicins is a key characteristic. Each colicin typically targets a narrow range of bacterial strains, often within the same species or closely related species. This specificity is due to the interaction between the colicin's N-terminal domain and specific receptors on the surface of the target bacterium. The immunity proteins, also encoded by the colicin-encoding plasmid, protect the producing bacterium from self-intoxication. Colicins play a significant role in bacterial ecology and evolution, contributing to the competitive advantage of producer strains and influencing bacterial community dynamics.