Influence of environmental factors on microbes. Influence of physical and chemical environmental factors on bacteria Cause of death of microorganisms under the influence of ionizing radiation

Radiation microbiology is the branch of microbiology that studies the effects of ultraviolet and ionizing radiation on microorganisms. Research in the field of radiation microbiology is aimed at: 1) studying the mechanisms biological action ultraviolet and ionizing radiation on microorganisms; 2) the use of radiation as a factor causing hereditary variability or death of bacteria.

Microorganisms serve as a widely used object of radiobiological experiments for research general patterns effects of radiation on the cell. In this area radiation microbiology is directly connected with radiobiology (see). At the same time, radiation microbiology solves important practical problems of national economic importance, for example, the use of radiation as a factor in altering the nature of microorganisms in order to obtain high yields of biologically valuable substances (antibiotics, vitamins, hormones, amino acids). The method of "cold" sterilization is based on the sterilizing effect of radiation (see), which often has advantages over sterilization with heat or antiseptics, and sometimes it turns out to be the only possible one.

The effect of ionizing radiation on heredity was first discovered in experiments on microorganisms. In 1925, G. A. Nadson and G. S. Filippov discovered that under the influence of X-rays, changes occur in microorganisms that persist in subsequent generations (mutations). This observation marked the beginning of the development of a new branch of knowledge - radiation genetics (see). Radiation microbiology takes into account the laws revealed by this science, in particular, the fact that in a certain range of radiation doses the number of mutant forms increases in proportion to the dose. With the help of ionizing radiation, the natural frequency of the mutation process can be increased tenfold. This, of course, increases the yield of a wide variety of hereditarily altered variants affecting various inherited traits of microorganisms. That is why irradiation in itself without subsequent selection cannot serve as a way to obtain forms of microorganisms changed in the desired direction. Irradiation only ensures the appearance in the microbial population more hereditary variants. Subsequent selection according to the trait of interest allows you to quickly and with a greater probability of success select the variant necessary for certain needs. For example, the selection of penicillin-producing strains Penicillium chrysogenum with preliminary exposure to X-ray and ultraviolet radiation allowed American microbiologists to select variants with a productivity more than 100 times higher than the production of penicillin by the original strain. The use of neutron-induced mutants - X-ray and ultraviolet radiation or chemical mutagens, increased the productivity of strains-producers of streptomycin, chlortetracycline, oxytetracycline by 15-30 times. Work is underway on radiation selection other industrially important strains of microorganisms (vaccine, toxigenic, amino acid producers, etc.).

The problems of radiation microbiology related to the use of the sterilizing effect of radiation are primarily associated with the determination of radiation doses and irradiation conditions that ensure the death of microorganisms. The bactericidal effect of X-rays was already known at the end of the last century. However practical use ionizing radiation for the purposes of sterilization became possible only in last years thanks to the creation of powerful irradiators, in particular gamma irradiators charged with radioactive cobalt. Modern gamma irradiators make it possible to provide huge doses of radiation in a short time and in large volumes of the irradiated object. The need to create high-power plants for sterilization purposes is explained by the relatively high radioresistance of microorganisms. While lethal radiation doses for mammals range from 400-1000 rads, microbial inactivation, depending on the irradiation conditions, occurs only when doses of the order of hundreds of thousands or millions of rads are used.

The bactericidal effect of ionizing radiation depends on a number of factors. Drying of microorganisms leads to an increase in radioresistance. A similar effect is exerted by a decrease partial pressure oxygen in the irradiated object, the decrease in temperature during irradiation, as well as the conditions created after irradiation. In cases of irradiation of microbial cultures, the sensitivity of microorganisms varies depending on the culture development cycle.

Different microorganisms have different radioresistance. So, for example, to achieve a sterilizing effect when irradiating suspensions of non-spore-forming bacteria (Bact. coli, Proteus vulgaris), irradiation in doses of 100,000-500,000 rads is necessary. To inactivate spores of spore-forming microorganisms, large doses are needed - 1,500,000-2,500,000 rad. - Viruses are even more resistant: the sterilizing effect occurs only with irradiation at doses of 3,000,000-5,000,000 rad.

Near ultraviolet (UV)- radiation with a wavelength of 400 - 320 nm - even in low doses has a certain effect on bacteria. For example, when moving cells of E. coli or Salmonella typhimurium are illuminated with near UV light, an increase in the frequency of cell tumbling is first observed; repellent effect, then somersaults completely stop and paralysis of the flagella sets in, i.e. light disrupts the mechanisms of movement and taxis. The chromophore is a flavoprotein.

In sublethal doses, near UV causes a slowdown in crop growth, mainly due to the lengthening of the lag phase. The rate of cell division also somewhat decreases, the ability of bacteria to support the development of the phage is suppressed, and the induction of enzymes is inhibited. These effects are determined mainly by the absorption of UV rays by 4-thiouridine, an unusual base present in the 8th position in many prokaryotic (but not eukaryotic) tRNAs. The greatest effect is exerted by light with a wavelength of about 340 nm. Excited by light, 4-hyouredin forms cross-links with cytosine located in the 13th position in tRNA, which prevents the binding of tRNA to amino acids and leads to an increase in the formation of guanosine triphosphate on ribosomes and to the suspension of RNA and protein synthesis, respectively. In Bacillus subtillis, another near-UV-sensitive system has also been found, in which menaquinone is the light-receiving chromophore.

At relatively high doses of near UV irradiation, mutagenic and lethal effects are observed. Violation of DNA is caused not so much by UV rays themselves, but by various other molecules excited by light. And in these effects, the absorption of the near UV by 4-thiouredine plays a role. The mutagenic and lethal effects of near UV are largely dependent on the presence of oxygen.

The lethal effect of near UV irradiation may be associated with damage not only to DNA, but also to membranes, in particular, their transport systems. The sensitivity to near UV of a bacterium can strongly depend on the stage of growth of the culture, which is not observed under the action of far UV.

The effect of near UV can be mediated by a photosensitizer. Thus, in the presence of acridine in E. coli, near UV damages both DNA and outer cytoplasmic membranes, as a result of which cells become sensitive to lysozyme, detergents, and osmotic shock.

Near UV can cause photoprojection at low radiation doses, i.e. reduce the biological effect of subsequent exposure to far UV. The idea of ​​the mechanism of this effect is contradictory. At relatively high doses of near-UV irradiation, the opposite effect can also be observed, i.e. enhancing the effect of subsequent exposure to far UV.

Medium UV is radiation with a wavelength of 320 - 290 nm, and far UV- with a wavelength of 290 - 200 nm. The biological effects of medium and far UV are similar. As already mentioned, under irradiation with sunlight, the death of bacteria is associated mainly with the action of UV. lower limit wavelength of light that hits earth's surface, is about 290 nm, while in research light sources with a shorter wavelength are used. It is believed that the body's resistance to solar radiation, as a rule, corresponds to its resistance to non-ionizing radiation from artificial sources.

DNA intensely absorbs UV in the region of 240 - 300 nm, i.e. in the region of middle and far UV, with an absorption peak in the region of 254 nm. This explains the high mutagenic and lethal efficiency of irradiation with medium and far UV. The formation of pyrimidine dimers in DNA is the main mechanism responsible for the lethal and mutagenic effects. Dimers can include 2 adjacent thymine or cytosine residues or 1 thymine and 1 cytosine residues. Under the influence of UV radiation, hydroxylation of cytosine and uracil, the formation of cytosine-thymine adducts, DNA-protein cross-links, the formation of DNA cross-links, chain breaks, and DNA denaturation also occur. Such damage increases with increasing irradiation intensity.

ionizing radiation constitutes a certain component of natural radiation, determined by unstable isotopes that are constantly in the soil and precipitation. In the areas of occurrence of radioactive minerals, the natural background of radiation is increased. Isotopes can get into living organisms and then they are subjected to internal radiation. Bacteria are sometimes able to accumulate certain elements in very large quantities.

Ionizing radiation also arises under the influence of cosmic rays. Space serves as a source of primary cosmic rays, which give rise to secondary ones that affect living organisms. The intensity of such radiation depends on geographical latitude, especially on altitude, and approximately doubles every 1500 m. During the period solar flares the background of cosmic radiation is increased. Artificial ionizing radiation comes from testing nuclear weapons, the operation of nuclear power plants, the use of radioisotopes for medical, scientific and other purposes. The presence of such sources is the reason that microorganisms today are subjected to high doses of radiation.

Ionizing radiation also causes DNA damage, which is usually divided into direct and indirect, arising in connection with the formation of free radicals. Damage is predominantly a single-strand or double-strand break in the DNA molecule.

The radioresistance of various bacteria varies over a very wide range and is controlled by many genes. Mutants that are more radioresistant or radiosensitive can be obtained relatively easily. Radioresistance depends primarily on the work of various repair and regulation systems. At the same time, the degree of resistance of the body to various types of radiation, especially UV and ionizing radiation, may not coincide. Various bacterial repair systems will be discussed below.

A connection was established between the radioresistance of bacteria and the characteristics of its habitat. Thus, microorganisms isolated from radon mineral springs are 3-10 times more resistant to radiation than organisms of the same species isolated from non-radioactive water. In the cooling systems of nuclear reactors, where the average dose of radiation exceeds 10 6 FER (the physical equivalent of X-rays), various bacteria live, in particular representatives of the genus Pseudomonas. However, it is generally difficult to find a reasonable explanation for the adaptive significance of the high radioresistance of some bacteria. The radioresistance of some cocci isolated from irradiated products is especially high. In this case, it is obvious that irradiation could serve as a selection factor, but not as a factor that caused adaptation. Thus, the UV dose required to inactivate 90% of the cells of a UV-resistant strain of E. coli is about 1000 erg/mm2, while to achieve the same effect in Deinococcus radiodurans, a dose of 10,000 - 15,000 erg/mm is required" 2 or 5 x 10 5 rad in the case of radioactive irradiation. The coccus Deinococcus radiophilus is even more resistant to UV and γ radiation. As already mentioned, the level of radioresistance is determined mainly by the degree of development of repair systems. Deinococcus radiodurans, apparently, is able to repair even double-stranded DNA breaks are lethal to most microorganisms.

The degree of radioresistance of some bacteria far exceeds the limiting level of radiation that organisms can encounter in nature. The most likely explanation for this discrepancy may be the assumption that radio resistance is only one of the many manifestations of the action of general-purpose systems. It would be more correct to speak about the degree of resistance of bacteria to certain disturbances in the structure of their cells than about resistance to the action of certain environmental factors, since the same disturbances can be caused by different reasons. This applies primarily to DNA damage repair systems.

Biologists call bacteria an evolutionary recipe for success - they are so resistant to any environmental conditions. Some of them feel great even with lethal doses of radiation.

Microbiologist John Batista of the University of Louisiana has seen a lot. However, about his first encounter with a microbe, jokingly nicknamed "Superbug Conan", he said: Honestly, it was not easy for me to believe in the reality of the existence of such an organism.

In the early 1960s, Thomas Brock discovered bacteria in Yellowstone National Park that could withstand temperatures close to boiling point. After that, microbiologists began to find more and more new types of extreme microbes. However, Conan has surpassed all: the most resistant microorganism, it withstands harsh frost, sizzling heat, acid baths and poisons. But most striking of all was his reaction to high doses of radiation exposure. Even a 1500-fold excess of a dose that is lethal to other organisms did no harm to the bacteria.

Conan was first discovered in the 1950s in spoiled canned meat destined for the army. To protect against bacterial contamination, canned food in the United States is usually sterilized using radioactive radiation. Scientists were all the more surprised when they saw pink mold in the jars with the smell of rotten cabbage, clearly of bacterial origin. They were puzzled. After all, radiation usually causes deep damage to the genetic material in living organisms. If the amount of such damage exceeds a certain critical level, the microorganism dies. But for Conan the law is not written. What mechanisms save a nondescript crumb from death in any situation?

The baffled microbiologists set about unraveling the mystery of Conan. They examined his genetic material before and after exposure to radiation and analyzed metabolic processes. To their surprise, the results showed that Conan also suffered greatly from radiation, but at the same time knew how to overcome its disastrous effects.

If some poisons or ionizing radiation cause relatively minor damage to only one of the two strands of an organism's DNA, then radiation causes damage to both strands of DNA, and their restoration is often unbearable for the body. So, for the death of E. coli living in the human intestine, two or three such DNA damages are enough.

Conan, on the contrary, quickly restored two hundred such "breakdowns". The fact is that in the process of evolution, he developed effective mechanisms for restoring gene damage - including a special enzyme that looks for suitable "spare parts" in the hereditary material, copies them and pastes them into the damaged areas.

Conan's DNA recovery is facilitated by another circumstance: Conan's genome consists of four circular DNA molecules, and in each cell the genome is present not in one, as in most bacteria, but in several copies. It is thanks to these copies that the damaged areas are restored. Since the cell is most vulnerable to radiation at the time of division, when the circular DNA molecule must open, Conan developed another method of protection: the bacterium leaves three molecules folded into a ring, and uses the fourth for reproduction needs. If this chromosome is damaged by radiation, the spare chromosomes serve as templates from which the body copies the correct gene sequences.

In 2007, microbiologist Michael J. Daley discovered another reason for Conan's hypertolerance: the bacterium has an incredibly high intracellular concentration of manganese, an element that also helps repair DNA damage.

And yet, despite the discoveries made, the mystery of Conan's super-resistance to radiation has not yet been fully solved. Research is in full swing: scientists hope to effectively use Konan to clean up soils contaminated with radiation.

Solar radiation passing through the upper atmosphere and reaching the Earth's surface consists of electromagnetic waves with a length 300-10.000 nm.

75 % of light falling on the Earth - this is the visible part of the spectrum - covers the range 390-760 nm. This part is perceived by the human eye.

20 % - infrared radiation (near) with λ waves from 790 nm and further (790-1100).

5 % - UV with λ waves 300-380 nm.

The ozone layer absorbs wavelengths 220-300 nm.

Effect of visible light on microorganisms

Visible light is used by photosynthetic microorganisms. The spectral composition of PAR is different for different groups of microorganisms and depends on the set of pigments. Oxygenic photosynthesis (cyanobacteria, prochlorophytes) is possible in the range from 300 to 750 nm. These bacteria have chlorophyll a and b, with an absorption maximum of 680-685 and 650-660 nm, respectively. In cyanobacteria, phycobiliproteins (red and blue pigments) absorb light with length 450-700 nm.

Anoxygenic photosynthesis (purple, green bacteria) - ranging from 300 to 1100 nm. bacteriochlorophyll b absorbs light with a wavelength 1020-1040 up to 1100 nm.

All photosynthetic prokaryotes have additional light-harvesting pigments - carotenoids, which absorb light in the blue and blue-green parts of the spectrum ( 450-550 nm).

Phototrophic bacteria live in the anaerobic zone of water bodies, where there is H 2 S. Infrared radiation does not penetrate to a depth of 10-30 m, the maximum energy falls on the light of λ waves of 450-500 nm.

Visible light influences the behavior of phototrophic bacteria. There is a phenomenon phototaxis. F. is the reaction of bacteria to a change in the spectral composition of light or illumination. In eubacteria, bacteriochlorophylls and carotenoids serve as photoreceptors. Special sensory pigments have been found in archaea (sensor rhodopsins in halobacteria). Positive phototaxis- the movement of bacteria towards the light, negative- the movement of cells in the direction of decreasing illumination.

For some bacteria that do not use light energy, it serves as a regulator of certain metabolic processes. For example, in an aquatic bacterium P. putida observed the activation of some enzymes by light, which can be considered as an adaptation, since it is under illumination that the synthesis of phytoplankton begins, the products of which are used by this heterotrophic bacterium.

Some non-photosynthetic bacteria have photochromism. Photochromism is the dependence of pigment formation on illumination. It is typical for myxobacteria, many actinomycetes and related microorganisms. For example, the synthesis of carotenoids by some mycobacteria is stimulated by blue light. Photochromism can be controlled by both chromosomal and plasmid genes. Pigments are able to protect these microorganisms from the action of visible color.

Sunlight has a strong antimicrobial effect. Visible light exposure is responsible for less than 1% of lethal injuries (80% of lethal injuries are associated with light wavelengths less than 312 nm). Visible light at 450 nm induces base pair substitutions and frameshift mutations in E. coli. Light waves of 550 nm, and especially 410 nm, cause photolysis Myxococcus xanthus. The effect is determined by the absorption of light by iron porphyrins.

There are substances photosensitizers, in the molecule of which there is a chromophore that absorbs light and transfers its energy to other molecules that are not able to absorb light. Light passes through colorless cells without consequences. But if you introduce a photosensitizer into such a cell, it is damaged. Natural photosensitizers - chlorophyll, phycobilins, porphyrins, etc.

Influence of infrared radiation on microorganisms

To date, no biological effects have been registered for radiation with a wavelength of more than 1100 nm. The main effect of infrared radiation is heating.

The effect of ultraviolet rays on microorganisms

For microorganisms, UV radiation is most dangerous. Distinguish near, middle and far UV.

Near UV is radiation with a wavelength 400-320 nm.

Medium UV – λ= 320-290 nm.

Far UV– λ= 290-200 nm.

Near UV in small doses, it disrupts the mechanisms of movement and taxis. The chromophore is a flavoprotein.

In sublethal doses, it causes a slowdown in growth, the rate of cell division, the induction of enzymes, and the ability of bacteria to support the development of the phage are inhibited.

These effects are determined by the fact that bacteria have an unusual base in the 8th position in tRNA 4-thiouridine(absent in eukaryotes). This base strongly absorbs UV, the greatest effect is light with a wavelength of 340 nm. Light-excited 4-thiouridine binds to cytosine located at position 13 in tRNA, which prevents tRNA from binding to amino acids and, therefore, leads to the suspension of protein synthesis.

At relatively high doses of near UV - mutagenic and lethal effects. Violations of DNA in this case are not so much the UV rays themselves, but other molecules excited by the light. Also of importance in these effects is the near UV absorption of 4-thiouridine. The mutagenic and lethal effect depends on the presence of oxygen.

The lethal effect is associated not only with damage to DNA, but also to membranes (their transport systems).

The biological effects of mid and far UV are similar. DNA intensively absorbs UV in the region of 240-300 nm. in the region of middle and far UV with an absorption peak in the region of 254 nm. In the lab. UV lamps are dominated by radiation in the region of 260 nm (the lower limit of the wavelength of light incident on the earth's surface is about 290 nm).

Medium and far UV causes mutagenic and lethal effects. The main mechanism of the damaging action is the formation pyrimidine dimers. The composition of dimers can include two neighboring thymine (T-T) or cytosine (C-C) or thymine and cytosine (T-C). The formation of dimers occurs due to covalent interactions between DNA bases. In addition, hydrogen bonds in DNA are broken. This (both 1 and 2) leads to the appearance of non-viable mutants. Also, under the action of UV, hydroxylation of cytosine and uracil, the formation of DNA-protein cross-links, the formation of DNA cross-links, and DNA denaturation occur.

Due to the damaging and lethal effects of UV rays, despite the fact that these are the most energy-rich rays, they are not used in the process of photosynthesis. The lower limit of photosynthesis is the use of waves with a wavelength of 450 nm.

Effect of ionizing radiation on microorganisms

Ionizing radiation is very high energy radiation capable of knocking electrons out of atoms and attaching them to other atoms to form positive and negative ions. It is believed that ionization is the main cause of radiation damage to the cytoplasm, and the degree of damage is proportional to the number of pairs of ions.

Light and most solar radiation do not have this ability.

The source of ionizing radiation is radioactive substances contained in rocks. Also comes from space. During solar flares, the background radiation increases.

Artificial ionizing radiation occurs as a result of nuclear weapons testing, the operation of nuclear power plants, the use of radioisotopes in medicine, science, etc.

are of great ecological importance the following types ionizing radiation:

1. α-radiation - corpuscular radiation - these are the nuclei of helium atoms. The length of the run in the air is several cm. They are stopped by a sheet of paper or the stratum corneum of human skin. However, when stopped they cause strong local ionization.

2. β-radiation - corpuscular radiation - these are fast electrons. The length of the run in the air is several meters, and in the fabric a few cm.

α-radiation and β-radiation have the greatest effect when absorbed by living tissue.

3. γ-radiation - ionizing electromagnetic radiation. Has a high penetrating power. Easily penetrates into living tissues. May have an effect when the source of radiation is outside the body.

4. x-ray radiation- electromagnetic radiation, very close to γ-radiation.

Microorganisms are the most resistant to ionizing radiation (more than 10 6 Rad). 1 Rad is such a dose of radiation at which 100 erg of energy falls on 1 g of tissue. 1 X-ray \u003d 1 Rad. Mammals are sensitive to a dose of 100 Rad.

Damage mechanism

The main target for ionizing radiation is DNA. DNA damage is direct and mediated. Straight lines are single-stranded or double-stranded DNA breaks. There are rarely.

More frequent mediated damage. They arise in connection with the formation of free radicals, which cause single- and double-strand breaks (modify pyrimidine bases), which leads to DNA denaturation. In addition, the resulting free radicals cause protein denaturation. All this leads to the death of microorganisms, incl. viruses.

Mechanisms of radioresistance

1. The main mechanism of radioresistance (both to UV and ionizing radiation) is a well-functioning DNA repair system.

2. Pigments (carotenoids) have radioprotective properties, but provide effective protection against UV.

3. The presence of radioprotective substances in cells (for example, sulfur-containing amino acids in D. radiophilus), protects the cell from radiation, but this mechanism is insufficient.

4. The cell wall may play a role in DNA repair systems. At D. radiophilus under the influence of radiation, the enzyme exonuclease is released, which is involved in DNA repair.

5. Increased DNA content.

The radioresistance of microorganisms varies widely. The degree of resistance of the organism to various types of radiation, especially UV and ionizing radiation, may not coincide.

One of the most resistant to UV radiation is the marine flagellate. Bodo marina. Resilience may be related to habitat characteristics. Thus, microorganisms isolated from radon sources are 3-10 times more resistant to radiation than their relatives from ordinary habitats.

in cooling systems nuclear reactors, where the radiation dose exceeds 10 6 FER (the physical equivalent of X-rays), various bacteria live, incl. kind Pseudomonas.

One of the most resistant bacteria to both UV and γ-radiation belongs to deinococci (p. Deinococcus) – D. radiophilus. This bacterium is apparently capable of repairing even double-strand DNA breaks, which are lethal for most microorganisms.

Temperature - one of the main factors determining the possibility and intensity of reproduction of microorganisms.

Microorganisms can grow and show their vital activity in a certain temperature range and depending on temperature are divided into psychrophiles, mesophiles and thermophiles. Temperature ranges of growth and development of microorganisms of these groups are shown in Table 9.1.

Table 9.1 Division of microorganisms into groups depending on

from relation to temperature

microorganisms		T(°C) max.		Separate representatives
1. Psychrophiles (cold-loving)				Bacteria living in refrigerators, marine bacteria
2. Mesophiles				Most fungi, yeasts, bacteria
3. Thermophiles (heat-loving)				Bacteria living in hot springs. Most form persistent spores

The division of microorganisms into 3 groups is very conditional, since microorganisms can adapt to unusual temperatures.

The temperature limits of growth are determined by the thermal resistance of enzymes and cell structures containing proteins.

Among mesophiles, there are forms with a high temperature maximum and a low minimum. Such microorganisms are called thermotolerant.

Action high temperatures on microorganisms. Increasing the temperature above the maximum can lead to cell death. The death of microorganisms does not occur instantly, but over time. With a slight increase in temperature above the maximum, microorganisms may experience "heat shock" and after a short stay in this state, they can be reactivated.

The mechanism of the destructive effect of high temperatures is associated with the denaturation of cellular proteins. The denaturation temperature of proteins is affected by their water content (the less water in the protein, the higher the denaturation temperature). Young vegetative cells, rich in free water, die faster when heated than old, dehydrated ones.

Heat resistance - the ability of microorganisms to withstand prolonged heating at temperatures exceeding the temperature maximum of their development.

The death of microorganisms occurs at different temperatures and depends on the type of microorganism. So, when heated in a humid environment for 15 minutes at a temperature of 50-60 ° C, most fungi and yeast die; at 60–70 °С, vegetative cells of most bacteria, fungal and yeast spores are destroyed at 65–80 °С.

The high thermal stability of thermophiles is due to the fact that, firstly, the proteins and enzymes of their cells are more resistant to temperature, and secondly, they contain less moisture. In addition, the rate of synthesis of various cellular structures in thermophiles is higher than the rate of their destruction.

The heat resistance of bacterial spores is associated with a low content of free moisture in them, a multilayered shell, which includes calcium salt of dipicolinic acid.

Based on the destructive effect of high temperatures various methods destruction of microorganisms in food products. These are boiling, boiling, blanching, roasting, as well as sterilization and pasteurization. Pasteurization - the process of heating up to 100˚С, during which the vegetative cells of microorganisms are destroyed. Sterilization - complete destruction of vegetative cells and spores of microorganisms. The sterilization process is carried out at a temperature above 100 °C.

Effect of low temperatures on microorganisms. Microorganisms are more resistant to low temperatures than to high ones. Despite the fact that the reproduction and biochemical activity of microorganisms stop at temperatures below the minimum, cell death does not occur, because. microorganisms are in a state suspended animation(hidden life) and remain viable for a long time. As the temperature rises, cells begin to multiply rapidly.

Causes death of microorganisms under the influence of low temperatures are:

Metabolic disease;

An increase in the osmotic pressure of the medium due to freezing of water;

Ice crystals can form in the cells, destroying the cell wall.

Low temperature is used when storing food in a chilled state (at a temperature of 10 to -2 ° C) or frozen (from -12 to -30 ° C).

Radiant energy. In nature, microorganisms are constantly exposed to solar radiation. Light is necessary for the life of phototrophs. Chemotrophs can also grow in the dark, and with prolonged exposure to solar radiation, these microorganisms can die.

The impact of radiant energy obeys laws of photochemistry: changes in cells can only be caused by absorbed rays. Therefore, for the effectiveness of irradiation, the penetrating power of the rays, which depends on the wavelength and dose, is important.

The radiation dose, in turn, is determined by the intensity and time of exposure. In addition, the effect of exposure to radiant energy depends on the type of microorganism, the nature of the irradiated substrate, the degree of its contamination with microorganisms, and also on temperature.

Low intensities of visible light (350–750 nm) and ultraviolet rays (150–300 nm), as well as low doses of ionizing radiation, either do not affect the vital activity of microorganisms, or lead to an acceleration of their growth and stimulation of metabolic processes, which is associated with the absorption of light quanta certain components or substances of cells and their transition to an electronically excited state.

Higher doses of radiation cause inhibition of certain metabolic processes, and the action of ultraviolet and X-rays can lead to a change in the hereditary properties of microorganisms - mutations which is widely used to obtain highly productive strains.

The death of microorganisms under the influence of ultraviolet rays related:

With inactivation of cellular enzymes;

With the destruction of nucleic acids;

With the formation of hydrogen peroxide, ozone, etc. in the irradiated medium.

It should be noted that bacterial spores are the most resistant to ultraviolet rays, then fungal and yeast spores, then stained (pigmented) bacterial cells. Vegetative bacterial cells are the least resistant.

Death of microorganisms under the action of ionizing radiation called:

Radiolysis of water in cells and substrate. In this case, free radicals, atomic hydrogen, peroxides are formed, which, interacting with other substances of the cell, cause a large number of reactions that are not characteristic of a normally living cell;

Enzyme inactivation, degradation membrane structures, nuclear apparatus.

The radioresistance of various microorganisms varies over a wide range, and microorganisms are much more radioresistant than higher organisms (hundreds and thousands of times). The most resistant to ionizing radiation are bacterial spores, then fungi and yeast, and then bacteria.

The destructive effect of ultraviolet and X-ray γ-rays is used in practice.

Ultraviolet rays disinfect the air of refrigeration chambers, medical and industrial premises, use the bactericidal properties of ultraviolet rays to disinfect water.

Treatment food products low doses of gamma radiation is called radurization.

Electromagnetic vibrations and ultrasound. radio waves- this is electromagnetic waves, characterized by a relatively large length (from millimeters to kilometers) and frequencies from 3·10 4 to 3·10 11 hertz.

The passage of short and ultraradio waves through the medium causes the occurrence of alternating currents of high (HF) and ultrahigh frequency (SHF) in it. In an electromagnetic field, electrical energy is converted into heat.

The death of microorganisms in a high-intensity electromagnetic field occurs as a result of a thermal effect, but the mechanism of action of microwave energy on microorganisms has not been fully disclosed.

In recent years, microwave electromagnetic food processing has been increasingly used in the food industry (for cooking, drying, baking, heating, defrosting, pasteurization and sterilization of food products). Compared with the traditional method of heat treatment, the time of heating by microwave energy to the same temperature is reduced many times, and therefore the taste and nutritional properties of the product are more fully preserved.

Ultrasound. Ultrasound is called mechanical vibrations with frequencies of more than 20,000 vibrations per second (20 kHz).

The nature of the destructive effect of ultrasound on microorganisms is related to:

FROM cavitation effect. When ultrasonic waves propagate in a liquid, rapidly alternating rarefaction and compression of liquid particles occur. When rarefied, the smallest hollow spaces are formed in the medium - “bubbles” filled with vapors environment and gases. During compression, at the moment of collapse of the cavitation "bubbles", a powerful hydraulic shock wave occurs, causing a destructive effect;

With electrochemical action of ultrasonic energy. In the aquatic environment, water molecules are ionized and oxygen dissolved in it is activated. In this case, highly reactive substances are formed, which cause a number of chemical processes that adversely affect living organisms.

Due to its specific properties, ultrasound is increasingly being used in various fields of engineering and technology in many sectors of the national economy. Research is underway on the use of ultrasonic energy for the sterilization of drinking water, food products (milk, fruit juices, wines), washing and sterilization of glass containers.