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Of the physical factors, microorganisms have the greatest impact: temperature, drying, radiant energy, ultrasound, pressure.
Temperature: the vital activity of each microorganism is limited by certain temperature boundaries. This temperature dependence is usually expressed by three points: the minimum (min) temperature - below which reproduction ceases, the optimal (opt) temperature - the best temperature for the growth and development of microorganisms and the maximum (max) temperature - the temperature at which cell growth either slows down or stops at all. For the first time in the history of science, Pasteur developed methods for destroying microorganisms when exposed to high temperatures.
Optimum temperature is usually equated to ambient temperature.
All microorganisms in relation to temperature can conditionally be divided into 3 groups:
The first group: psychrophiles are cold-loving microorganisms, grow at low temperatures: min t - 0 ° С, opt t - from 10-20 ° С, max t - up to 40 ° С. Such microorganisms include inhabitants of the northern seas and bodies of water. Many microorganisms are very resistant to low temperatures. For example, a cholera vibrio can be stored in ice for a long time without losing its viability. Some microorganisms withstand temperatures up to -190 ° C, and bacterial spores can withstand up to -250 ° C. The action of low temperatures suspends putrefactive and fermentation processes, so we use refrigerators in everyday life. At low temperatures, microorganisms fall into a state of suspended animation, in which all the vital processes in the cell are slowed down.
The second group includes mesophiles - this is the most extensive group of bacteria, which includes saprophytes and almost all pathogenic microorganisms, since opt temperature for them is 37 ° C (body temperature), min t = 10 ° C, maxt = 45 ° C.
The third group includes thermophiles - thermophilic bacteria, develop at t above 55 ° С, min t = 30 ° С for them, max t = 70–76 ° С. These microorganisms live in hot springs. Among thermophiles there are many spore forms. Bacterial spores are much more resistant to high temperatures than vegetative forms of bacteria. For example, spores of anthrax bacilli can withstand boiling for 10–20 s. All microorganisms, including spore ones, die at a temperature of 165-170 ° C within an hour. The effect of high temperatures on microorganisms is the basis of sterilization. ,
Drying. For the normal functioning of microorganisms, water is needed. Drying leads to dehydration of the cytoplasm, the integrity of the cytoplasmic membrane is violated, which leads to cell death. Some microorganisms die under the influence of drying within a few minutes: these are meningococci, gonococci. More resistant to drying are tuberculosis pathogens, which can remain viable for up to 9 months, as well as capsular forms of bacteria.
Spores are particularly resistant to drying. For example, mold spores can retain germination capacity for 20 years, and anthrax spores can persist in the soil for up to 100 years.
For the storage of microorganisms and the manufacture of drugs from bacteria, the method of freeze drying is used. The essence of the method is that microorganisms are first frozen at -273 ° C, and then dried under vacuum. In this case, microbial cells go into a state of suspended animation and retain their biological properties for several years. In this way, for example, the preparation “colibacterin” is prepared containing strains of E. coli.
Radiant energy. In nature, bacterial cells are constantly exposed to solar radiation. Direct sunlight has a detrimental effect on microorganisms. This refers to the ultraviolet spectrum of sunlight (UV rays), they inactivate cell enzymes and destroy DNA. Pathogenic bacteria are more sensitive to UV rays than saprophytes. Therefore, in a bacteriological laboratory, microorganisms are grown and stored in the dark.
Buchner's experience shows how UV rays are detrimental to cells: a Petri dish with a dense medium is seeded with a continuous lawn. Part of the sowing is covered with paper, and a Petri dish is placed in the sun, and then after a while it is placed in a thermostat. Only those microorganisms that were under the paper sprout. Therefore, the importance of sunlight to improve the environment is very high.
The bactericidal effect of UV rays is used to sterilize closed rooms: operating rooms, maternity wards, dressing rooms, in kindergartens, etc. For this, bactericidal ultraviolet radiation lamps with a wavelength of 200-400 nm are used.
Microorganisms are also affected by other types of radiant energy - this is x-ray radiation, a-, p- and y-rays have a detrimental effect on microorganisms only in large doses. These rays destroy the nuclear structure of the cell. In recent years, radiation products have been used to sterilize disposable products - syringes, suture material, Petri dishes.
Small doses of radiation, on the contrary, can stimulate the growth of microorganisms.
Ultrasound causes damage to the cell. Under the action of ultrasound, a very high pressure arises inside the cell. This leads to rupture of the cell wall and cell death. Ultrasound is used to sterilize products: milk, fruit juices.
High pressure. Bacteria, and especially spores, are very resistant to atmospheric pressure. In nature, bacteria are found that live in the seas and oceans at a depth of 1000-10,000 m under pressure from 100 to 900 atm. The combined effect of elevated temperatures and high pressures is used in steam sterilizers for steam sterilization under pressure.
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