Colicins are toxic exoproteins produced by bacteria of colicinogenic strains of Escherichia coli and some related species of Enterobacteriaceae, during the growth of their cultures. They inhibit sensitive bacteria of the same family. About 35% E. coli strains appearing in human intestinal tract are colicinogenic. Synthesis of colicins is coded by genes located on Col plasmids. Until now more than 34 types of colicins have been described, 21 of them in greater detail, viz. colicins A, B, D, E1-E9, Ia, Ib, JS, K, M, N, U, 5, 10. In general, their interaction with sensitive bacteria includes three steps: (1) binding of the colicin molecule to a specific receptor in the bacterial outer membrane; (2) its translocation through the cell envelope; and (3) its lethal interaction with the specific molecular target in the cell. The classification of colicins is based on differences in the molecular events of these three steps.