Colicins are proteins produced by some strains of Escherichia coli that are lethal for related strains of E.coli.
The first colicin was identified by Gratia in 1925 as a heat-labile product present in cultures of E.coli V and toxic for E.coli φ.
The name ‘colicin’ was coined by Gratia and Fredericq in 1946, who demonstrated their protein nature and the specificity of their activity spectra.
These are produced by strains of E. coli carrying a colcinogenic plasmid (pCol) that bears the genetic determinants for colicin synthesis, immunity, and release.
Colicins are organized into three domains, each one involved in a different step of the process of killing sensitive bacteria. The three functional domains found in all colicins are:
the N-domain required for translocation across the outer membrane
the central domain for binding to the receptor
the C-domain in which the enzymatic activity is located
Colicins exert their lethal action by first binding to specific receptors, which are outer membrane proteins used for the entry of specific nutrients. They are then translocated through the outer membrane and transit through the periplasm by either the Tol or TonB system. Colicins then reach their lethal target and act either by forming a voltage-dependent channel in the inner membrane or by using their endonuclease activity on DNA, rRNA, or tRNA.
No colicin acts on its own producing bacteria since each bacterium produces a specific inhibitor called the immunity protein. The immunity protein of pore-forming colicins is located in the inner membrane of producing cells, blocking colicin when it reaches its target after its entry into sensitive cells.
Colicins are a type of bacteriocins with pronounced antimicrobial activity, which justifies their biotechnological potential. The use of colicins has been reported for the following:
food preservation
therapeutic purposes such as treatment of peptic ulcer
