GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

6. MICROBIAL GROWTH

6.2. ADAPTIVE RESPONSES OF MICROORGANISMS TO STRESS FACTORS

6.2.5. Regulatory systems of response to stress factors

Cellular processes occurring under stress conditions have been studied primarily using enteric Bacteria as a model, predominantly Escherichia coli and Salmonella typhimurium. In recent years, other bacterial and Yeast cultures have also become subjects of research in the genetics and molecular biology of adaptation mechanisms.

Depending on The Nature of the stressor and the type of damage inflicted, the cellular response can vary. Five Regulatory Systems of response to stress factors have been identified in enteric bacteria: stringent control; SOS Response; adaptive response; heat Shock Protein Synthesis; and oxidative stress response. In all these cases, profound metabolic reorganizations take place, associated with the slowing down or cessation of reproduction and the synthesis of Proteins necessary for survival. In some instances, special compounds known as alarmones act as regulatory mediators.

The oxidative stress response system controls the synthesis of A number of proteins, among which antioxidant Enzymes (catalase, peroxidase, superoxide dismutase), heat shock proteins, and DNA repair enzymes have been identified. DNA repair enzymes eliminate DNA Damage caused by UV and ionizing radiation, dehydration, peroxide compounds, and other DNA-damaging agents. For example, γ-irradiation causes single- and double-stranded DNA breaks. Under METABOLISM/18.html">The Influence of UV irradiation, thymine dimerization occurs in the DNA molecule, As a result of which DNA Replication is inhibited, and The Cell loses its ability to divide. Many microorganisms possess specific enzymes that repair UV-induced damage by splitting the thymine dimer. These enzymes are activated by visible light, a process known as photoreactivation. Photoreactivation is one of the Components of the DNA repair system. It should be noted that the DNA repair system operates most efficiently in response to UV irradiation, followed by X-ray and γ-irradiation. α-Irradiation causes virtually irreversible DNA damage. DNA repair systems are present in all microorganisms, but their efficiency varies among different organisms.

The synthesis of heat shock proteins (HSPs) is induced by sublethal Temperature stress, UV irradiation, ethanol, nalidixic acid, and is also observed during oxidative stress, carbon and nitrogen starvation, and the transition of the culture to the stationary growth phase. According to current concepts, HSPs act as mediators of Conformational Changes in protein molecules within the cell under the influence of stress factors.

The SOS response system is triggered in microbial Cells under conditions of irradiation, exposure to ionizing radiation, and the action of Chemical Mutagens.

The adaptive response system Functions upon exposure to methylating, ethylating, and alkylating agents.

The stringent control system is activated in response to the depletion of carbon and nitrogen sources in the medium, salt stress, and temperature drop.

Thus, microorganisms possess a whole network of diverse adaptive mechanisms that enable them to withstand stress and survive in adverse environmental conditions.

It should be noted that certain anti-stress adaptation mechanisms (Changes in membrane Lipid Composition caused by stress factors, synthesis of heat shock proteins, protective compounds, and the Organism's anti-radical defense) are quite similar in eukaryotes and prokaryotes, meaning they do not depend on Cellular Organization.

Furthermore, the action of a specific stress factor is not limited to the functioning of just one corresponding adaptation mechanism. Under adverse conditions, microbial cells activate a complex of induced responses controlled by intricate regulatory pathways; in other words, an integrated system of stress-resistance mechanisms exists.

There is no doubt that studying biochemical and genetic changes in cells under stress conditions provides a fresh perspective on solving such key biological problems as Aging, anabiosis, cytodifferentiation, and resistance to external influences.

In addition, these studies are crucial for The Development of biotechnology. Producer microorganisms should be viewed as entities whose properties change depending on environmental conditions. No modern cultivation technique will yield expected process improvements without a basic understanding of producer physiology. No deep knowledge of biochemistry will provide information on how to increase process rates or which limitation or inhibition Methods to apply. Metabolic overproducing mutants obtained by advanced Introduction/32.html">Genetic Engineering tools will not thrive unless optimal conditions are found that provide the cell itself with an advantage for the overproduction of a given metabolite.

Therefore, microbial physiology, which investigates the relationship between metabolic activity and environmental changes, must play a central role in the development of biotechnology.