Antimicrobial Protein – Bacteriocin

Bacteriocin is ribosomally synthesized antimicrobial peptides produced by most species of lactic acid bacteria. Most of them are small cationic membrane-active compounds that form pores in the target cells, disrupting membrane potentials and causing death. The production of small cationic peptides with antimicrobial activity is a defense strategy found not only in bacteria, but also in plants and animals. The antagonistic interaction between competing bacteria was described early in 1877, when Pasteur and Joubert noticed that some Escherichia coli strains interfered with the growth of Bacillus anthracis present in infected animals.

Bacteriocins was first detected in 1925 by Andre Gratia, who observed that the growth of some E.coli strains was inhibited by the presence of an antibacterial compound that he called colicin v, released into the medium by E.coli V (virulent strain). Colicin v was later characterized as a heat-stable and dialyzable peptidic compound and in 1954; Pierre Frederic found its genetic determinates in a conjugation – transmissible element similar to the F factor. The antimicrobial peptides produced by bacteria have been grouped into different classes on the basis of the producer organism, molecular size, chemical structure and mode of action.

Bacteriocin Classification:

Jack et. al considered the presence of disulphide and monosulfide (lanthionine) bonds as the basis for their classification as a landmark for their activity spectrum. Accordingly Bacteriocins were classified into four groups:

Antibiotics containing unusual post translationally modified amino acids such as dehydroalanine, dehydrobutirine, lanthionine or B-methyl lanthionine (lantibiotics);
Antibiotics containing at least one disulphide bridge essential for their activity;
Compounds with a single-SH residue that should be in a reduced form for the antibiotic to the active (thiolbiotics); and
Antibiotics without cycteine residues.

According to Klaenhammer, bacteriocins can be classified into four groups on the basis of their molecular mass, thermo stability, enzymatic sensitivity, presence of post translationally modified amino acids and mode of action.

Physical and Chemical characteristics of Bacteriocin:

To perform the bacteriocins lethal activity, it must fulfill two principal requirements:

- to be cationic
- highly hydrophobic

Most bacteriocins are active over a wide pH range (3.0 – 9.0), and while resistance to extreme pH values of 1.0 (acidocin B) and 11.0 (bavaricin A) has been observed. Most of these bacteriocins are cationic at pH 7.0. The high isoelectric point allows them to interact at physiological pH values with the anionic surfaces of bacterial membranes. Heat stability is another major feature of the bacteriocins (stable at 60oc for 120mins at pH 4.5).

They are partially inactivated by autoclaving or heating at 100oC for 120mins. Heat stability was decreased at pH 6.8. Totally inactivated by proteolytic enzymes, pronase, proteinase K, trypsin, and chemotrypsin within 25mins incubation period, whereas treatment with lipase, catalase, and α-amylase did not affect the activity of bacteriocin. The molecular mass was estimated to be 2.5 to 3.0kda based on SDS-PAGE analysis.