GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

6. MICROBIAL GROWTH

6.2. ADAPTIVE RESPONSES OF MICROORGANISMS TO STRESS FACTORS

6.2.1. Changes in the Lipid Composition of Membranes

According to current concepts, the primary stress effect involves altering the fluidity of the membrane lipid bilayer, which leads to metabolic imbalance in the functioning of membrane-bound Enzymes. The hypothesis that Organism resistance is related to membrane composition was first proposed back in 1978 while studying The Effect of ethanol on the membrane Lipid Composition of Yeasts. Eukaryotes differ significantly from prokaryotes in their lipid composition. In particular, the latter generally lack sterols and long-chain Fatty acids, as well as sphingomyelin; some eubacteria contain hopanoids (potential analogues of sterols), whereas archaebacteria contain unique Lipids esterified with a C₂₀-alcohol linked to glycerol.

However, despite differences in lipid composition, both eukaryotic and Prokaryotic Cells possess mechanisms that ensure adaptive changes in membrane lipid composition in response to environmental Temperature—the so-called homeoviscous adaptation. Specifically, a decrease in temperature leads to an increased content of Unsaturated fatty acids in Membrane Lipids. This prevents lipid solidification and excessive membrane rigidity. In response to an increase in ambient temperature, cells react by decreasing the content of unsaturated fatty acids, which prevents excessive lipid fluidization and ensures membrane stabilization.

The capacity for homeoviscous adaptation has been found in both mesophilic Bacteria and psychrophiles; however, at the same temperature, the content of unsaturated fatty acids in psychrophiles is higher than in mesophiles. In the psychrophilic Gram-positive bacterium Micrococcus cryophilus, a decrease in temperature does not increase the content of unsaturated fatty acids, but instead decreases the length of their carbon chains.

High-temperature stress in the fungus Cunninghamella japonica is accompanied by an increase in the unsaturation of Glycolipids, the levels of sterols and phosphatidylethanolamine, and A change in The ratio of glyco- and Phospholipids. It should be noted that during prolonged high-temperature stress in Fungi, alterations occur not only in the lipid but also in the carbohydrate COMPOSITION OF THE mycelium, notably an intensification of trehalose synthesis.

Changes in the composition of microbial membrane lipids can be triggered not only by temperature stresses, but also by the action of membranotropic agents (Antibiotics, ethanol) and toxic compounds. For instance, the action of ethanol on Saccharomyces cerevisiae Yeast cells leads to an increased lipid saturation due to a rise in their oleic acid content. The addition of toluene led to a significant increase in the Cholesterol/phospholipid molar ratio of The Cell membrane of Acholeplasma laidlawii During the first hours of cell adaptation to toluene, and subsequently to an increase in the fatty acid unsaturation index.

The Development of research into the Mechanisms of microbial resistance to freezing and drying is associated with Structure/149.html">The problem of increasing cell viability during conservation. It has been shown that under certain pre-cultivation conditions, it is possible to achieve changes in The chemical composition of the membrane towards an increased fatty acid content, thereby enhancing cell survival during lyophilization and subsequent low-temperature storage. For example, growing Serratia marcescens and Erwinia uroideae on a nutrient medium supplemented with 0.2% oleic acid and 0.5–1.0% Tween-80 can increase cell survival by 20–30% during lyophilization and ensure the maintenance of their stability for 1.5 years. During low-temperature storage (-70 C), high cell survival (88–98%) is ensured by the addition of 0.03% oleic acid and 1.0% Tween-80. An increase in the content of unsaturated fatty acids in membrane lipids can also be achieved by adding acetone, cyclohexane, dioxane, Ethylene glycol, sodium deoxycholate, as well as sodium salts of oleic, linoleic, linolenic, and palmitic acids to the bacterial culture medium.