Antibacterial Substances

Antibacterial Substances:

Human hosts possess antibacterial substances with which they combat the continuous onslaught of bacterial pathogens. These antibacterial substances are produced either by the host itself or by certain indigenous bacteria. The important antibacterial substances are the lysozyme, bacteriocins, and beta-lysin, and other polypeptides.

Lysozyme:

Lysozyme is the enzyme that breaks the β-1, 4-glycosidic bonds between N-acetylglucosamine and N- acetylmuramic acid in peptidoglycan, the signature molecule of bacterial cell wall. This bond breakage weakens the bacterial cell wall.

Water then enters the cell, and the cell swells and eventually bursts, a process called lysis (Fig. 44.18). Lysozyme occurs in body secretions including tears, saliva, and other body fluids, and presumably functions as a major line of non-specific defence against bacterial infections.

Bacteriocins:

Many of the normal bacterial flora of the host body synthesize and release plasmid-encoded toxic proteins (e.g., colicins, staphylococcin) collectively called bacteriosins that inhibit or kill closely related bacterial species or even different and may give their producers and adaptive advantage against other bacteria.

These toxic proteins are called bacteriocins to distinguish them from the antibiotics because possess a more narrow spectrum of activity than antibiotics. Bacteriocins producing genes are often present on plasmid or a transposon.

Most bacteriocins are produced by gram-negative bacteria, and are generally named after the species of the bacterial genera that produce them; the bacteriocin produced by E. coli is colicin, by Bacillus subtilis is substilicin.

E. coli synthesizes colicins. Some colicins bind to specific receptors on the surface of susceptible cells and kill them by disrupting some critical cell function. For example, many colicins form channels in the plasma membrane that allows potassium ions and protons to leak out, leading to a loss of the cell’s energy forming ability. Colicin E2 (encoded by plasmid col E2) is a DNA endonuclease and cleaves DNA. Colicin E3 (encoded by plasmid Col E3) is a nuclease that cuts at a specific site in 16S rRNA and inactivates ribosomes.

Recently it has been discovered that some grain-positive bacteria produce bacteriocin-like peptides. For example, lactic acid bacteria produce Nisin A, which strongly inhibits the growth of a wide range of gram- positive bacteria.

Beta-lysin and other polypeptides:

Beta-lysin is a cationic polypeptide synthesized and released by blood platelets, and kills some gram-positive bacteria by disrupting their plasma membranes. Other cationic polypetides produced in host body include leukins, plakins, cecropins, and phagocytin. A zinc-containing polypeptide named ‘prostatic antibacterial factor’ is secreted by the prostate gland in males, and acts as an important antibacterial substance.