Biologists call bacteria an evolutionary recipe for success - they are so resistant to any environmental conditions. Some of them thrive even with lethal doses of radiation.
Microbiologist John Bautista of the University of Louisiana has seen a lot. However, of his first encounter with the microbe, jokingly nicknamed "Conan the superbug", he said: "Honestly, I had a hard time believing that such an organism could actually exist."
In the early 1960s, Thomas Brock discovered bacteria in Yellowstone National Park that could withstand temperatures close to the boiling point. After this, microbiologists began to find more and more new types of extreme microbes. However, Conan surpassed everyone: the most resistant microorganism, it can withstand biting frost, sizzling heat, acid baths and poisons. But most striking of all was his reaction to high doses of radiation. Even a 1500-fold excess of the dose, lethal for other organisms, did not bring any harm to the bacteria.
Conan was first discovered in the 1950s in spoiled canned meat intended for the army. To protect against bacterial contamination, canned foods in the United States are typically sterilized using radiation. The scientists were even 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 an inconspicuous baby from death in any situation?
Confused microbiologists began to unravel 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 was able to overcome its disastrous consequences.
While some poisons or ionizing radiation cause relatively minor damage to only one of the body's two DNA strands, radioactive radiation causes damage to both DNA strands, and their restoration is often beyond the body's ability to repair. So, for the death of E. coli living in the human intestine, two or three such DNA damage is enough.
Conan, on the contrary, quickly repaired two hundred such “breakdowns.” The fact is that in the process of evolution, it has developed effective mechanisms for restoring gene damage - including the appearance of a special enzyme that finds suitable “spare parts” in the hereditary material, copies them and inserts them into the damaged areas.
Another circumstance contributes to the restoration of DNA in Conan: 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 moment 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 becomes damaged under the influence of radiation, the spare chromosomes serve as templates from which the body copies the correct gene sequences.
In 2007, microbiologist Michael J. Daly discovered another reason for Conan's hyperdurability: the bacterium has incredibly high intracellular concentrations 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 Conan to cleanse soil contaminated by radiation.
Temperature - one of the main factors determining the possibility and intensity of microorganism reproduction.
Microorganisms can grow and exhibit their vital functions in a certain temperature range and depending on the relationship to temperature are divided into psychrophiles, mesophiles and thermophiles. Temperature ranges for the growth and development of microorganisms of these groups are given in Table 9.1.
Table 9.1 Division of microorganisms into groups depending on
from relation to temperature
|
microorganisms |
T(°C) max. |
Separate representatives |
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|
1. Psychrophiles (cold-loving) |
Bacteria living in refrigerators, marine bacteria |
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2. Mesophiles |
Most fungi, yeasts, bacteria |
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3. Thermophiles (heat-loving) |
Bacteria living in hot springs. Most form persistent spores |
The division of microorganisms into 3 groups is very arbitrary, since microorganisms can adapt to temperatures that are unusual for them.
Temperature limits for growth are determined by the thermoresistance of enzymes and cellular structures containing proteins.
Among mesophiles, there are forms with a high temperature maximum and a low minimum. Such microorganisms are called thermotolerant.
The effect of 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 when heated faster 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. Thus, when heated in a humid environment for 15 minutes at a temperature of 50–60 °C, most fungi and yeasts die; at 60–70 °C – vegetative cells of most bacteria, fungal and yeast spores are destroyed at 65–80 °C. The vegetative cells of thermophiles (90–100 °C) and bacterial spores (120 °C) have the greatest heat resistance.
The high heat resistance 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 their low content of free moisture and multilayer shell, which includes calcium salt-dipicolinic acid.
Various methods of destroying microorganisms in food products are based on the destructive effects of high temperatures. These include boiling, cooking, blanching, frying, as well as sterilization and pasteurization. Pasteurization – the process of heating to 100˚C during which the vegetative cells of microorganisms are destroyed. Sterilization – complete destruction of vegetative cells and microorganism spores. The sterilization process is carried out at temperatures above 100 °C.
The influence of low temperatures on microorganisms. Microorganisms are more resistant to low temperatures than to high temperatures. Despite the fact that the reproduction and biochemical activity of microorganisms stop at temperatures below the minimum, cell death does not occur, because microorganisms become suspended animation(hidden life) and remain viable for a long time. As the temperature rises, cells begin to multiply intensively.
Reasons death of microorganisms when exposed to low temperatures are:
Metabolic disorders;
An increase in the osmotic pressure of the environment due to freezing of water;
Ice crystals may form in the cells, destroying the cell wall.
Low temperature is used when storing food in a refrigerated 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 grow in the dark, and with prolonged exposure to solar radiation, these microorganisms can die.
The effect of radiant energy is subject to laws of photochemistry: changes in cells can only be caused by absorbed rays. Consequently, the penetrating ability of the rays, which depends on the wavelength and dose, is important for the effectiveness of irradiation.
The radiation dose, in turn, is determined by the intensity and time of exposure. In addition, the effect of radiant energy depends on the type of microorganism, the nature of the irradiated substrate, the degree of its contamination with microorganisms, as well as 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 changes in the hereditary properties of microorganisms - mutations which is widely used to obtain highly productive strains.
Death of microorganisms under the influence of ultraviolet rays linked:
With inactivation of cellular enzymes;
With the destruction of nucleic acids;
With the formation of hydrogen peroxide, ozone, etc. in the irradiated environment.
It should be noted that the most resistant to ultraviolet rays are bacterial spores, then fungal and yeast spores, then colored (pigmented) bacterial cells. The least resistant are vegetative bacterial cells.
Death of microorganisms under the influence of ionizing radiation called:
Radiolysis of water in cells and substrate. In this case, free radicals, atomic hydrogen, and peroxides are formed, which, when interacting with other cell substances, cause a large number of reactions that are not characteristic of a normally living cell;
Inactivation of enzymes, destruction of membrane structures, nuclear apparatus.
The radiostability of various microorganisms varies widely, 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 are used to disinfect the air of refrigeration chambers, medical and industrial premises, and the bactericidal properties of ultraviolet rays are used to disinfect water.
Processing food products with low doses of gamma radiation is called radurization.
Electromagnetic vibrations and ultrasound. Radio waves- these are electromagnetic waves characterized by a relatively long length (from millimeters to kilometers) and frequencies from 3·10 4 to 3·10 11 hertz.
The passage of short and ultra-radio waves through a medium causes the appearance of high-frequency (HF) and ultra-high-frequency (microwave) alternating currents in it. In an electromagnetic field, electrical energy is converted into thermal energy.
The death of microorganisms in a high-intensity electromagnetic field occurs as a result of the thermal effect, but the mechanism of action of microwave energy on microorganisms has not been fully disclosed.
In recent years, ultra-high frequency electromagnetic processing of food products has been increasingly used in the food industry (for cooking, drying, baking, reheating, defrosting, pasteurization and sterilization of food products). Compared to the traditional method of heat treatment, the time of heating with 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 refers to mechanical vibrations with frequencies greater than 20,000 vibrations per second (20 kHz).
The nature of the destructive effect of ultrasound on microorganisms is associated with:
WITH cavitation effect. When ultrasonic waves propagate in a liquid, rapidly alternating rarefaction and compression of liquid particles occurs. When the medium is discharged, tiny hollow spaces are formed - “bubbles”, which are filled with environmental vapors and gases. During compression, at the moment the cavitation “bubbles” collapse, a powerful hydraulic shock wave arises, causing a destructive effect;
With electrochemical action of ultrasonic energy. In an 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 used in various fields of engineering and technology in many sectors of the national economy. Research is being conducted on the use of ultrasonic energy for the sterilization of drinking water, food products (milk, fruit juices, wines), washing and sterilization of glass containers.
Changing environmental conditions affects the life activity of microorganisms. Physical, chemical, and biological environmental factors can accelerate or suppress the development of microbes, can change their properties or even cause death.
The environmental factors that have the most noticeable effect on the environment include humidity, temperature, acidity and chemical composition of the environment, the effect of light and other physical factors.
Humidity
Microorganisms can live and develop only in an environment with a certain moisture content. Water is necessary for all metabolic processes of microorganisms, for normal osmotic pressure in the microbial cell, to maintain its viability. Different microorganisms have different needs for water. Bacteria are mainly moisture-loving; when the environmental humidity is below 20%, their growth stops. For molds, the lower limit of environmental humidity is 15%, and with significant air humidity it is lower. The settling of water vapor from the air onto the surface of the product promotes the proliferation of microorganisms.
When the water content in the medium decreases, the growth of microorganisms slows down and may stop completely. Therefore, dry foods can be stored much longer than foods with high humidity. Drying food allows you to keep food at room temperature without refrigeration.
Some microbes are very resistant to drying; some bacteria and yeast can survive in a dried state for up to a month or more. Spores of bacteria and molds remain viable in the absence of moisture for tens and sometimes hundreds of years.
Temperature
Temperature is the most important factor for the development of microorganisms. For each microorganism there is a minimum, optimum and maximum temperature regime for growth. Based on this property, microbes are divided into three groups:
- psychrophiles - microorganisms that grow well at low temperatures with a minimum at -10-0 °C, optimum at 10-15 °C;
- mesophiles - microorganisms for which optimum growth is observed at 25-35 °C, minimum at 5-10 °C, maximum at 50-60 °C;
- thermophiles - microorganisms that grow well at relatively high temperatures with optimum growth at 50-65 °C, maximum at temperatures above 70 °C.
Most microorganisms are mesophiles, for which the optimal temperature is 25-35 °C. Therefore, storing food products at this temperature leads to the rapid proliferation of microorganisms in them and food spoilage. Some microbes, when significantly accumulated in foods, can lead to food poisoning in humans. Pathogenic microorganisms, i.e. causing infectious diseases in humans are also classified as mesophiles.
Low temperatures slow down the growth of microorganisms, but do not kill them. In refrigerated foods, microbial growth is slow but continues. At temperatures below 0 °C, most microbes stop reproducing, i.e. When food is frozen, the growth of microbes stops, some of them gradually die off. It has been established that at temperatures below 0 °C, most microorganisms enter a state similar to anabiosis, retain their viability, and continue their development as the temperature rises. This property of microorganisms should be taken into account during storage and further culinary processing of food products. For example, salmonella can persist in frozen meat for a long time, and after defrosting the meat, under favorable conditions, they quickly accumulate to an amount dangerous to humans.
When exposed to high temperatures exceeding the maximum endurance of microorganisms, they die. Bacteria that do not have the ability to form spores die when heated in a humid environment to 60-70 ° C in 15-30 minutes, to 80-100 ° C in a few seconds or minutes. Bacterial spores have much higher heat resistance. They are able to withstand 100 °C for 1-6 hours; at a temperature of 120-130 °C, bacterial spores in a humid environment die after 20-30 minutes. Mold spores are less heat resistant.
Thermal culinary processing of food products in public catering, pasteurization and sterilization of products in the food industry lead to partial or complete (sterilization) death of vegetative cells of microorganisms.
During pasteurization, the food product is exposed to minimal temperature effects. Depending on the temperature regime, low and high pasteurization are distinguished.
Low pasteurization is carried out at a temperature not exceeding 65-80 ° C, for at least 20 minutes to better guarantee the safety of the product.
High pasteurization is a short-term (no more than 1 minute) exposure of the pasteurized product to a temperature above 90 °C, which leads to the death of pathogenic non-spore-bearing microflora and at the same time does not entail significant changes in the natural properties of the pasteurized products. Pasteurized foods cannot be stored without refrigeration.
Sterilization involves freeing the product from all forms of microorganisms, including spores. Sterilization of canned food is carried out in special devices - autoclaves (under steam pressure) at a temperature of 110-125 ° C for 20-60 minutes. Sterilization provides the possibility of long-term storage of canned food. Milk is sterilized using ultra-high-temperature treatment (at temperatures above 130 ° C) for a few seconds, which allows you to preserve all the beneficial properties of milk.
Environment reaction
The vital activity of microorganisms depends on the concentration of hydrogen (H +) or hydroxyl (OH -) ions in the substrate on which they develop. For most bacteria, a neutral (pH about 7) or slightly alkaline environment is most favorable. Molds and yeasts grow well in a slightly acidic environment. A highly acidic environment (pH below 4.0) inhibits the growth of bacteria, but mold can continue to grow in a more acidic environment. Suppressing the growth of putrefactive microorganisms when the environment is acidified has practical applications. The addition of acetic acid is used when pickling foods, which prevents rotting processes and allows food to be preserved. The lactic acid formed during fermentation also inhibits the growth of putrefactive bacteria.
Salt and sugar concentration
Table salt and sugar have long been used to increase the resistance of foods to microbial spoilage and to better preserve food products.
Some microorganisms require high salt concentrations (20% or higher) for their development. They are called salt-loving, or halophiles. They can cause spoilage of salty foods.
High concentrations of sugar (above 55-65%) stop the proliferation of most microorganisms; this is used when preparing jam, jam or marmalade from fruits and berries. However, these products can also be spoiled by the growth of osmophilic molds or yeasts.
Light
Some microorganisms require light for normal development, but for most of them it is harmful. The ultraviolet rays of the sun have a bactericidal effect, that is, at certain doses of radiation they lead to the death of microorganisms. The bactericidal properties of ultraviolet rays of mercury-quartz lamps are used to disinfect air, water, and some food products. Infrared rays can also cause the death of microbes due to thermal effects. Exposure to these rays is used in the heat treatment of products. Electromagnetic fields, ionizing radiation and other physical environmental factors can have a negative impact on microorganisms.
Chemical factors
Some chemicals can have a detrimental effect on microorganisms. Chemicals that have a bactericidal effect are called antiseptics. These include disinfectants (bleach, hypochlorites, etc.) used in medicine, in the food industry and public catering.
Some antiseptics are used as food additives (sorbic and benzoic acids, etc.) in the production of juices, caviar, creams, salads and other products.
Biological factors
The antagonistic properties of some are explained by their ability to release substances into the environment that have an antimicrobial (bacteriostatic, bactericidal or fungicidal) effect - antibiotics. Antibiotics are produced mainly by fungi, less often by bacteria, they exert their specific effect on certain types of bacteria or fungi (fungicidal effect). Antibiotics are used in medicine (penicillin, chloramphenicol, streptomycin, etc.), in animal husbandry as a feed additive, in the food industry for food preservation (nisin).
Phytoncides, substances found in many plants and foods (onions, garlic, radishes, horseradish, spices, etc.), have antibiotic properties. Phytoncides include essential oils, anthocyanins and other substances. They are capable of causing the death of pathogenic microorganisms and putrefactive bacteria.
Egg whites, fish roe, tears, and saliva contain lysozyme, an antibiotic substance of animal origin.
Microorganisms, according to their sensitivity to radiation effects, are usually ranked in this order: - bacteria are the most sensitive, then molds, yeasts, bacterial spores, viruses. However, this division is not absolute, since among bacteria there are species that are more radioresistant than viruses.
The radiosensitivity of microorganisms is modified by various factors, both internal: the genetic nature of the cell itself, the life phase of the cell and others, and external: temperature, concentration of oxygen and other gases, the composition and properties of the environment in which irradiation is performed, as well as the type of radiation exposure and its power and other factors. The radiosensitivity of microorganisms is significantly lower than that of plants and animals by 1-2 orders of magnitude; in some cases, the bactericidal effect for some species can only be achieved at significant doses: 1-2 Mrad.
Already at the first stages of studying the radiation sensitivity of microorganisms, it was shown that at a dose of 5000 R the survival rate of E. coli was significantly reduced, and at a dose of 20 kR 95% of bacteria died. The culture of microorganisms of each type contains a mixture of cells that differ in sensitivity to radiation. For example, for a culture of Escherichia coli, 66% LD50 corresponded to a dose of 1.2 krad, and for 34% of bacteria - 3.5 krad. When intestinal bacteria are irradiated with gamma rays, their inactivation occurs in the range from 24 to 168 krad, and the death of all cells at doses of about 300 krad.
To obtain the same biological effect, different types of microorganisms require different doses of radiation. These differences depend on a number of biological characteristics of the irradiated bacteria, irradiation conditions, environmental influences and other factors. Particular importance is attached to the unequal sensitivity of nucleic acid metabolism and DNA of different organisms to radiation exposure.
The sensitivity of bacteria to radiation varies significantly within the same species and even within a population of bacterial cells. The population of cells consists of bacteria, arranged according to their resistance to radiation in a variational series, as well as according to other biological characteristics. Therefore, particularly radioresistant cells are always present in the population; in order to kill them, it is necessary to irradiate with more powerful doses than those that kill the bulk of the more radiosensitive cells. Gram-positive bacteria are less sensitive to radiation than gram-negative bacteria.
Bacterial spores have very low radiosensitivity, but even among non-spore-forming microorganisms there are organisms whose radioresistance may exceed that of spores. Most often they belong to the cocas or sarcins. Micrococci are known to have a semi-lethal dose of 400 krad (4 kGy). During radiation sterilization of meat, fish and other products, cocci were most often found after irradiation in doses from 600 to 1500 krad. An example of high radio resistance can also be bacteria isolated from the waters of nuclear reactors.
In addition to spores, which are highly resistant to ionizing radiation, highly radioresistant bacteria are known that do not form spores. Highly radioresistant bacteria are most often found among cocci. The surface of various medical products, as well as the air in the premises where these products are manufactured, can be contaminated with various bacteria, including Sarcins, which are particularly resistant to ionizing radiation. The well-known Micrococcus radiodurans, isolated from irradiated meat by Anderson et al., also belongs to cocci. Spectrophotometric analysis of the pigment of radioresistant micrococci isolated by Anderson showed that most of the pigments are carotenoids. Pigments isolated from radioresistant cells were sensitive to radiation. However, non-pigmented micrococcus variants also exhibited high radioresistance. Subsequently, the micrococcus isolated by Anderson attracted the attention of radiobiologists and was named Micrococcus radiodurance. It was more resistant not only to x-rays or gamma radiation, but also to ultraviolet irradiation. The micrococcus turned out to be 3 times more resistant to ultraviolet rays than E. coli. To delay DNA synthesis in micrococcus cells, fractions are required that are 20 times higher than those that cause a similar effect in Escherichia coli.
It can be assumed that the high radioresistance of the micrococcus is associated with a special system for repairing damage caused by radiation. The different nature of repairs of damage to Micrococcus radiodurnence resulting from ultraviolet irradiation and the action of ionizing radiation has been noted.
Highly radioresistant bacteria were isolated from dust from factories producing plastic medical devices in Denmark by Christensen et al. They were Streptococcus Faccium. It turned out that the radioresistance of different strains of the same type of microorganisms varies significantly. Thus, for most strains of Sir, faecium, a dose of 20 - 30 kGy is bactericidal, and only a few strains can withstand irradiation at a dose of 40 kGy. Strains Str. faecium isolated from dust turned out to be more radioresistant. Although most strains died when irradiated at doses from 20 to 30 kGy, some strains (4 out of 28 studied) withstood irradiation at doses up to 45 kGy.
Concentration of microbial cells in the irradiated object
One of the reasons that plays a significant role in the effectiveness of radiation sterilization is the concentration of microbial cells in the irradiated object.
In 1951, Hollander et al. established that the sensitivity of bacteria to irradiation is a function of cell concentration. As the concentration in the irradiated suspension decreases, its radiosensitivity increases. 10 7 cells were the optimal concentration of bacteria at which the effect of ionizing radiation was most effective. Many researchers noted that the sterilizing effect of irradiation depends both on the fraction of irradiation and on the density and volume of the irradiated suspension (7 , 36, 75 , 141 - 143). When E. coli is irradiated with beta rays from a Van de Graaff accelerator (2 MeV ) It was found that the absolutely sterilizing dose depends only on the concentration of the irradiated suspension. There is a direct proportional relationship between the concentration of microbes and the dose that kills 100% of cells: the lower the density of the irradiated suspension, the lower the radiation dose that gives the full bactericidal effect.
Figure 2.1 - Inactivation curves of various microorganisms.
1 - M. radiodurans R; 2 - Staphylococci; 3 - Micrococci; 4 - Coryneform rod; 5 - Spores; 6 - Str. faecium.
When irradiating a culture of Escherichia coli bacteria, the sterilizing effect of gamma radiation for relatively thin suspensions (8 * 10 5 -10 8 microbial bodies per 1 ml) was achieved at a dose of 2 kGy. Irradiation of a denser microbial suspension containing 10 10 microbial bodies per 1 ml at a dose of 2 kGy did not produce a bactericidal effect. Even with irradiation at doses of 4 and 5 kGy, growth of single colonies was sometimes observed. Complete sterilization of suspensions containing 10 10 and 2 * 10 10 microbial bodies per 1 ml was achieved only with irradiation at a dose of 6 kGy. A further increase in the number of microbial bodies in 1 ml of the irradiated medium did not require an increase in the irradiation dose for a full bactericidal effect. So. a suspension of Flexner dysentery bacteria at a concentration of 7*10 10 microbial bodies in 1 ml was completely inactivated by a dose of 6 kGy. Sarcina is one of the most radioresistant microorganisms. When thick suspensions of various microorganisms, both more radioresistant and less radioresistant, were irradiated at doses of 1, 2, 4, 8 kGy and 15 kGy, a relationship was observed between a decrease in the number of surviving microorganisms and an increase in the radiation dose. The higher the radiation dose, the fewer microorganisms survived after irradiation. A complete sterilizing effect was achieved by irradiating microorganisms at a concentration of 4 * 10 10 billion microbial bodies per 1 ml at a dose of 15 kGy. This proportion also killed the most resistant microorganisms - sarcin and Bacillus subtilis.
Thus, an increase in the concentration of microorganisms in an irradiated object increases their radioresistance. This situation is true for microorganisms with different radiosensitivities.
However, the increase in radioresistance of the irradiated suspension is not a consequence of the formation of radioresistance in irradiated cells. After irradiation of thick suspensions in bactericidal doses, single individuals survive, forming colonies of microbes when sown on agar. A study of the radiosensitivity of these surviving bacteria showed that they did not become more resistant to radiation compared to the original bacterial culture. This phenomenon can occur when suspensions of microorganisms of significantly less density are irradiated. It is known in the literature under the name "tail". The study of the tails also showed that bacteria that survived irradiation at lethal doses do not have increased radiosensitivity. An explanation for the observed phenomena should be sought among the reasons causing the death of microorganisms from ionizing radiation. The most likely reason for the increase in radioresistance of microorganisms with increasing concentration is a decrease in the partial pressure of dividing cells. During cell division, the nucleus becomes more vulnerable to irradiation