BACTERIOCIN

Bacteriocin was first observed by Gratia but the term bacteriocin was coined by Jacob et al. Bacteriocins are antibiotic-like agents produced by some bacteria which are encoded in the plasmids, with the purpose of killing or inhibiting closely related species or even different strains of the same species. Bacteriocins are ribosomally synthesised antimicrobial peptides produced by bacteria. They are defined as extracellular bioactive peptides or peptide complexes that are bacteriocidal, due to the combined action of the bacteriocin and the host autolysin, or bacteriostatic against other species, usually closely related to the producer strain. 

Bacteriocins are named after the species it is produced by adding �cin� to the species or genus of the bacterium. For example; E.coli produces colicin, P.aeruginosa produces pyocin, S.marcesans produces marcesin, Staphylococcus epidermidis produces staphylococcin, C.diphtheriae produces dipththericin. A naturally occurring plasmid only encodes for one type of bacteriocin and the plasmids are named after the bacteriocin they produce. Plasmid encoding colicin is designated as col.  Significance of bacteriocin: Bacteriocins probably have arisen from the need for survival in over-populated environment, thus cells containing the plasmid encoding for a bacteriocin have the capability of destroying surrounding cells. 

Types of bacteriocins: 
Class I: They are probably defective phage particles comprising only tail elements. They exert their effect on cytoplasmic membrane of sensitive cells after binding to specific receptors on outer membrane. They do not lyse cells but inhibit protein synthesis. Example: Pyocin produced by P.aeruginosa. 
Class II: These are phospholipases that attack membrane phospholipids, resulting in leakage of intracellular components. Example: Megacin produced by Bacillus megaterium. 
Class III: These are the classic bacteriocins. They are proteins of molecular weight 50000-100000. The structural genes coding for these bacteriocins reside on plasmids. Loss of receptor proteins renders a cell resistant to action by bacteriocin. Colicins function by disrupting the cell membrane of surrounding Esherichia cells lacking immunity protein. Some colicins have DNase activity, some have RNase activity and some inhibit membrane transport. Staphylococcin acts by producing pores in the membranes of susceptible cells. 

The cells that produce bacteriocins are immune themselves due to the production of �immunity protein� coded by imm gene. This protein protects itself from its own bacteriocin but also from the bacteriocin produced by related strain or species. Bacteriocin production can be induced in a cell by exposure to certain physical and chemical agents. In contrast to bacteriocins produced by gram negative bacteria, its production can not be induced in gram positive bacteria.  

Bacteriocin typing:
Strains can be typed by bacteriocin typing. While phage typing is based on sensitivity of the test strain to the lytic action of standard phages, bacteriocin typing involves ability of the test strain to kill standard indicator strains of bacteria. Bacteriocin typing is variously known as colicin or pyocin typing, depending on the bacteria. 
The test bacterium is inoculated as a broad streak on the center of a culture medium and incubated. The bacteriocin produced by the isolate would have diffused into the medium. The growth is then scraped off and remaining viable cells are killed by exposure to chloroform vapor. Standard indicator strains of bacteria are then inoculated at right angles to the original inoculum and the plates are incubated. The bacteriocin type is indicated by the pattern of susceptibility (or inhibition) of the standard strains. 
Bacteriocin typing is an important epidemiological typing tool in typing isolates of P.aeruginosa, E.coli, Serratia, S.sonnei, Proteus etc.