Bacteriocins
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Bacteriocins are proteinaceous toxins produced by bacteria to inhibit the growth of similar or closely related bacterial strain(s). They are typically considered to be narrow spectrum antibiotics, though this has been debated ^[1] 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.^[2][3] 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.

Classification of bacteriocins

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 ^[4], the microcins, and the bacteriocins from Archaea. The bacteriocins from E. coli are called colicins (formerly called 'colicines,' meaning 'coli killers'). 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. The bacteriocins of lactic acid-fermenting bacteria are called lantibiotics. This naming system is problematic for a number of reasons. First, naming bacteriocins by what they putatively kill would be more accurate if their killing spectrum were contiguous with genus or species designations. The bacteriocins frequently possess spectra that exceed the bounds of their named taxa and almost never kill the majority of the taxa for which they are named. Further, the original naming is generally derived not from the sensitive strain the bacteriocin kills, but instead the organism that produces the bacteriocin. This makes the use of this naming system a problematic basis for theory; thus the alternative classification systems.

Methods of Classification

Alternative methods of classification include: method of killing (pore forming, dnase, nuclease, murein production inhibition, etc), genetics (large plasmids, small plasmids, chromosomal), molecular weight and chemistry (large protein, polypeptide, with/without sugar moiety, containing atypical amino acids like lanthionine) and method of production (ribosomal, post ribosomal modifications, non-ribosomal).

One method of classification fits the bacteriocins into Class I, Class IIa/b/c, and Class III. ^[5]

Class I bacteriocins

The class I bacteriocins are small peptide inhibitors and include nisin.

Class II bacteriocins

The class II bacteriocins are small heat-stable proteins. 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.

Class III bacteriocins

Large, heat-labile protein bacteriocins.

Databases

Two databases of bacteriocins are available: BAGEL^[6] and BACTIBASE^[7].

Medical significance

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 opportunistic pathogenic bacteria to invade the human body.

Bacteriocins have also been suggested as a cancer treatment ^[8][9]. They have shown distinct promise as a diagnostic agent for some cancers,^{[10][11][12][13][14][15]}, but their status as a form of therapy remains experimental and outside the main thread of cancer research. Partly this is due to questions about their mechanism of action and the presumption that anti-bacterial agents have no obvious connection to killing mammalian tumor cells. Some of these questions have been addressed, at least in part^[16][17].

In the long quest for medical applications, bacteriocins have also been tested as AIDS drugs ^[18].

Production

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 way. 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 (ammonium 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
• Class III

Bacteriocins by Name

agrocin alveicin carnocin colicin curvaticin divercin enterocin enterolysin epidermin erwiniocin glycinecin halocin lactococin lacticin leucoccin mesentericin nisin pediocin plantaricin sakacin subtilin sulfolobicin vibriocin

References

External links

• Bagel Bacteriocin Database
  
• BACTIBASE Database

See also

Peripheral membrane proteins