GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

6. MICROBIAL GROWTH

6.2. MICROBIAL ADAPTIVE RESPONSES TO STRESS

6.2.3. Antiradical defense

The foundational concepts concerning the mechanisms of free-radical oxidation (FRO) cytotoxicity were formulated by Soviet scientists B.M. Tarusov and N.M. Emanuel. It is believed that stress conditions trigger an Amplification of FRO in microbial Cells. The primary initiator of intracellular FRO appears to be the superoxide anion O₂. In addition, toxic oxygen derivatives include such reactive oxygen species as hydrogen peroxide H₂O₂, the hydroperoxyl radical HO₂, the hydroxyl radical OH⁺, and singlet oxygen *O₂. According to current understanding, singlet oxygen and hydroxyl radicals are the most toxic to The Cell. Hydrogen peroxide is the least reactive in this regard; however, its accumulation within cells can also be lethal. The inactivation of cells by hydrogen peroxide is believed to be associated with the generation of hydroxyl radicals via the so-called Fenton reaction. In this process, H₂O₂ reacts with DNA, playing a major role in the damage of these macromolecules.

Aside from DNA damage, the accumulation of toxic peroxide compounds can induce other cellular lesions: peroxidation of Unsaturated Fatty acids in Membrane Lipids, oxidation of protein SH groups, destruction of Tryptophan residues within Proteins, depolymerization of acidic Polysaccharides, etc.

Nevertheless, cells possess a robust enzymatic antiradical defense system. Specifically, the enzyme superoxide dismutase catalyzes The conversion of the superoxide anion into hydrogen peroxide, while hydrogen peroxide is decomposed by catalase and peroxidase.

In addition, a protective function in cells is performed by various antioxidants (tocopherols, ubiquinones, Vitamins K and A, Sulfur-Containing Amino Acids, etc.). It should be emphasized that bioantioxidants are effective agents for inhibiting FRO in the Organism. Their primary role is to suppress free-radical Lipid Peroxidation in cell membranes. Upon The addition of exogenous antioxidants (phenosan, iotsol) to the medium, the survival rate of E. coli cells increased by 1.6–2.0 folds compared to the control. Interesting data indicate that the lipids of certain Bacteria may act as antioxidants when the organism transitions into a dormant state. During the encystment process of Azotobacter species, There is a decrease

in The rate of phospholipid synthesis and an increase in the content of specialized lipids—alkylresorcinols and trones. It is possible that these unique lipids replace a portion of the membrane Phospholipids, forming a Structure that ensures dormancy and resistance of mature cysts to external factors, while also acting as bioantioxidants that protect Nitrogenase from atmospheric oxygen.