ADAPTATION OF MICROORGANISMS TO HEAVY METALS

Many heavy metals (HM) (Zn, Cu, Mn, Co, etc.) take an active part in the most important processes of vital activity of microorganisms as microelements. However, at high concentrations they become toxic, and a number of metals (Pb, Hg, Cd, etc.) are highly toxic even at low concentrations. Microorganisms are able to resist the toxic effects of HMs due to the presence of various resistance mechanisms that are aimed at converting cations to a less toxic form or oxidation state, which makes them less mobile and bioavailable. The very first reaction of microorganisms to the toxic effects of metals is a change in cell morphology, their agglomeration, which leads to a decrease in the availability of binding sites for toxic metals. The mechanisms used by bacteria can be divided into biochemical and molecular. Bacterial cells have the ability to sorb metal cations with the help of metal-binding functional groups (carboxylic, sulfhydryl, hydroxyl, sulfate, phosphate, and amino groups) of the cell membrane,preventing their penetration into the cell. Bacteria have a variety of efflux systems for HM outflow from cells with the help of carrier proteins belonging to different families, which maintain a low concentration of HM inside the cell, protecting cellular components. Polysaccharides, biosurfactants, inorganic anions (phosphate, carbonate, and sulfide ions) and other metabolic products of microorganisms participate in extracellular detoxification, and gluta- thione, metal-binding proteins, intracellular polyphosphate granules, which bind HM cations into poorly soluble compounds, participate in intracellular sequestration. The reduction of HM ions with the help of enzymes leads to the formation of their less toxic forms. The genes responsible for bacterial resistance to toxic metals are localized on chromosomes or plasmids and can be transferred to closely related bacterial species, which plays an important role in the spread of HM resistance in nature. Microorganisms also demonstrate indirect mechanisms of HM tolerance aimed at maintaining cell integrity by protecting them from oxidative stress.