Many bacteria can produce agents that inhibit or kill closely related species or even different strains of the same species in order to reduce competetion. These agents are called bacteriocins in order to differentiate them from antibiotics which generally have a much wider spectrum of activity.

Bacteriocins are generally proteinaceous agents which are encoded genomically but more often on plasmids. Bacteriocins are named according to the species from which the originate. For example, in E. coli, they are called colicins, produced on the Col plasmid. Bacillus subtilis produces subtilisin, a serine protease that hydrolyzes peptide bonds, etc. The colicins are the best studied of these.

The Col plasmid of E. coli encodes various colicins. Colicins released from a cell bind to specific receptors on the surface of suceptible neighboring target cells. The receptors for colicins normally are designed to function as a transport mechanism for some substance the bacterium needs, such as a growth factor or a micronutrient. Colicins, once in the cell, can kill them by disrupting some important cell function, and the range of targets is quite large.

Properties of some colicins

  • Colicin B: inhibits transcription and translation by binding the receptor involved in enterochelin transport.
  • Colicin I: binds to the iron transport receptor and causes the cell membrane to leak.
  • Colicin E1: involves the Vitamin B12 receptor and can cause membrane leakage.
  • Colicin E2: also binds the Vitamin B12 receptor and inhibits DNA synthesis.
  • Colicin E3: binds the Vitamin B12 receptor and cleaves 16S rRNA which affects ribosome function and tranlation.
  • Colicin K: binds receptors involved in nucleoside transport and can cause membrane leakage.

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