Bacteriocin

Bacteriocins are proteinaceous toxins produced by bacteria to inhibit the growth of similar or closely related bacterial strain(s). They are phenomenologically analogous to yeast and paramecium killing factors, and are structurally, functionally, and ecologically diverse. Bacteriocins were first discovered by A. Gratia in 1925. He was involved in the process of searching for ways to kill bacteria, which also resulted in the development of antibiotics and the discovery of bacteriophage, all within a span of a few years. He called his first discovery a colicine because it killed E. coli.

Bacteriocins are categorized in several ways, including producing strain, common resistance mechanisms, and mechanism of killing. There are several large categories of bacteriocin which are only phenomenologically related. These include the bacteriocins from gram-positive bacteria, the colicins, the microcins, and the bacteriocins from Archaea. The bacteriocins from E. coli are called colicins (formerly called 'colicines'). They are the longest studied bacteriocins. They are a diverse group of bacteriocins and do not include all the bacteriocins produced by E. coli. In fact, one of the oldest known so-called colicins was called colicin V and is now know as microcin V.  It is much smaller and produced and secreted in a different manner than the classic colicins.

Virtually all colicins are carried on plasmids. There are two general classes of colicinogenic plasmids, large, low-copy number plasmids, and small high copy number plasmids. The larger plasmids carry other genes as well as the colicin operon. The colicin operons are generally organized with several major genes. These include an immunity gene, a colicin structural gene, and a BRP (bacteriocin release protein), or lysis, gene. The immunity gene is often produced constitutively, while the BRP is generally produced only as a read-through of the stop codon on the colicin structural gene. The colicin itself is repressed by the SOS response and may be regulated in other ways as well.

Research indicates that retaining the colicin plasmid is very important for cells that live with their relatives, because if a cell loses the immunity gene, it quickly becomes subject to destruction by circulating colicin. At the same time, colicin is only released from a producing cell by the use of the lysis protein, which results in that cell's death. This suicidal production mechanism would appear to be very costly, except for the fact that it is regulated by the SOS response, which responds to significant DNA damage. In short, colicin production may only occur in terminally-ill cells. Still these matters require further research.

The colicins themselves are composed of three globular domains. One domain regulates the target and binds to the receptor on the sensitive cell. The second is involved with translocation, co-opting the machinery of the target cell. The third is the 'killing' domain and may produce a pore in the target cell membrane, or act as a nuclease to chop up the DNA or RNA of the target cell. Because they target specific receptors and use specific translocation machinery, cells can make themselves resistant to the colicin by repressing or deleting the genes for these proteins. Such resistant cells may suffer the lack of a key nutrient (such as iron or vitamin B) but benefit by not being killed. Colicins exhibit a '1-hit killing kinetic' which doesn't necessarily mean a single molecule is sufficient to kill, but certainly that it only takes a small number. In his Nobel Laureate speech, Salvador E. Luria, 1969, speculated that colicins could only be this toxic by causing a domino effect that destabilized the cell membrane He was not entirely correct, but pore-forming colicins do de-polarize the membrane and thus eliminate the energy source for the cell. The colicins are highly effective toxins.

The bacteriocins of lactic acid-fermenting bacteria are well studied because of the commercial use of these bacteria in the food industry for making dairy products such as cheese. Bacteriocins are classified according to their extent of posttranslational modification. The lantibiotics are a class of more extensively modified bacteriocins, also called Class I. Bacteriocins for which disulfide bonds are the only modification to the peptide are Class II bacteriocins. Most bacteriocins are biologically active single-chain peptides. Some are only active as partners with a second peptide (see Class IIb, below).

Nisin and epidermin are members of a family of lantibiotics that bind to a cell wall precursor lipid component of target bacteria and disrupt cell wall production. The duramycin family of lantibiotics binds phosphoethanolamine in the membranes of its target cells and seem to disrupt several physiological functions.

The action of Class IIa bacteriocins seems to involve disruption of mannose transport into target cells. Class IIb bacteriocins form pores in the membranes of target cells and disrupt the proton gradient of target cells. Other bacteriocins can be grouped together as Class IIc. These have a wide range of effects on membrane permeability, cell wall formation and pheromone actions of target cells.

A database of bacteriocins is available (see link below).

Bacteriocins are of interest in medicine because they are made by non-pathogenic bacteria that normally colonize the human body. Loss of these harmless bacteria following antibiotic use may allow oportunistic pathogenic bacteria to invade the human body.

There are many ways to demonstrate bacteriocin production, depending on the sensitivity and labor intensiveness desired. To demonstrate their production, technicians stab inoculate multiple strains on separate multiple nutrient agar Petri dishes, incubate at 30 °C for 24 h., overlay each plate with one of the strains (in soft agar), incubate again at 30 °C for 24 h. After this process, the presence of bacteriocins can be inferred if there are zones of growth inhibition around stabs. This is the simplest and least sensitive. It will often mistake phage for bacteriocins. Some methods prompt production with UV radiation, Mitomycin C, or heat shock. UV radiation and Mitomycin C are used because the DNA damage they produce stimulates the SOS response. Cross streaking may be substituted for lawns. Similarly, production in broth may be followed by dripping the broth on a nascent bacterial lawn, or even filtering it. Precipitation (ammonia sulfate) and some purification (e.g. column or HPLC) may help exclude lysogenic and lytic phage from the assay.

Bacteriocins by Class
Class I Class IIa Class IIb Class IIc

Bacteriocins by Name
alveicin carnocin colicin curvacin divercin enterocin epidermin halocin lactococin lacticin leucoccin mesentericin nisin pediocin plantaricin sakacin subtilin sulfolobicin vibriocin