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Effect s of temperature on microorganisms.

Microorganisms have been found growing in virtually all environments
where there is liquid water, regardless of its temperature. Subsequently,
procaryotes have been detected growing around black smokers and hydrothermal
vents in the deep sea at temperatures at least as high as +115°C. Microorganisms
have been found growing at very low temperatures as well. In supercooled
solutions of H2O as low as -20°C, certain organisms can extract water for growth, and many forms of life flourish in the icy waters of the Antarctic, as well as
household refrigerators, near 0°C.

Considering the total span of temperature where liquid water exists, the
procaryotes may be subdivided into several subclasses on the basis of one or another of their cardinal points for growth. For example, organisms with an optimum temperature (T) near +37°C (the body temperature of warm-blooded animals) are called mesophiles . Organisms with an optimum T between about +45°C and +70°C are thermophiles . Some Archaea with an optimum T of +80°C or higher and a maximum T as high as +115°C, are now referred to as extreme thermophiles or hyperthermophiles . The cold-loving organisms are psychrophiles defined by their ability to grow at 0°C, A variant of a psychrophile (which usually has an optimum T of +10-15°C) is a psychrotroph , which grows at 0°C but displays an optimum T in the mesophile range, nearer room temperature. Psychrotrophs are the scourge of food storage in refrigerators since they are invariably brought in from their mesophilic habitats and continue to grow in the refrigerated environment where they spoil the food. Of course, they grow slower at +2°C than at +25°C.

Favorable action of the optimum temperature is used in the cultivation of microorganisms for the purpose of laboratory diagnosis, preparation of vaccines and other preparations.

Low temperature (refrigeration and freezing): most organisms grow very
little or not at all at 0°C. Store perishable foods at low temperatures to slow rate
of growth and consequent spoilage (e.g. milk). Low temperatures are not
bactericidal. Psychrotrophs, rather than true psychrophiles, are the usual cause
of food spoilage in refrigerated foods. For example, Listeria monocytogenes is of great concern in refrigerated foods.

The mechanism of action of low temperatures - inhibition of metabolic processes, growth and reproduction of microorganisms and transition to a state of suspended animation.

High temperature has a killing effect. The killing effect of high temperature (above the maximum) is used for sterilization. The mechanism of action is the denaturation of the protein (enzymes), damage to the ribosomes, the violation of the osmotic barrier. The psychrophils and mesophiles are the most sensitive to the action of high temperature. Specific resistance is shown by bacterial spores.

The lethal temperature varies in microorganisms. The time required to kill
depends on the number of organisms, species, nature of the product being
heated, pH, and temperature. Whenever heat is used to control microbial growth
inevitably both time and temperature are considered.


Electromagnetic radiation of various types bombards our world. As the wavelength of electromagnetic radiation decreases, the energy of the radiation increases – gamma rays and X rays are much more energetic than visible light or infrared waves. Sunlight is the major source of radiation on the earth. It includes visible light, ultraviolet radiation, infrared rays and radio waves. Most life is dependent on the ability of photosynthetic organisms to trap the light energy of the sun as visible light. Many forms of electromagnetic radiation are very harmful to microorganisms. Ionizing radiation, radiation of very short wavelength or high energy can cause atoms to lose electrons or ionize. The two major forms of ionizing radiation, X rays which are artificially produced and gamma rays which are emitted during radioisotope decay. Low levels of ionizing radiation will produce mutations, higher levels are directly lethal. Some prokaryotes like Deinococcus radiodurans and bacterial endospores are resistant.

Effect s of irradiation on microorganisms : usually destroys or distorts nucleic acids; it breaks hydrogen bonds, oxidises double bonds, destroys ring structures and polymerizes some molecules.

The mechanism of the damaging effect of UV rays: the formation of dimers of thymine in the DNA molecule, which stops cell division and is the main cause of their death. The damaging effect of UV rays is more pronounced for microorganisms than for animals and plants.

The mechanism of ionizing radiation ( X-ray):  has a powerful penetrating effect and damages the cellular genome. The mechanism of the damaging action: the ionization of macromolecules, which is accompanied by the development of mutations or cell death. At the same time, lethal doses for microorganisms are several orders of magnitude higher than for animals and plants.

Ultraviolet light is usually used f or sterilization  (commonly used to sterilize the surfaces of objects), although X-rays and microwaves are possibly useful. Many spoilage organisms are easily killed by irradiation. In some parts of Europe, fruits and vegetables are irradiated to increase their shelf life up to 500 percent.

Drying (removal of H2O - Desiccation): most microorganisms cannot grow at reduced
water activity (Aw < 0.90). Often used to preserve foods (e.g. fruits, grains,
etc. ). Methods involve removal of water from product by heat, evaporation,
freeze-drying, and addition of salt or sugar.

Drying from the frozen state under vacuum is lyophilization or freeze-drying. It is used to preserve cultures of microorganisms that in this state for years (10-20 years) do not lose their viability and do not change properties. Microorganisms are in this state in anabiosis. Lyophilization is used in the production of bacterial preparations from living microorganisms: eubiotics, phages, live vaccines

Filtration: involves the physical removal (exclusion) of all cells in a liquid or
gas, especially important to sterilize solutions which would be denatured by heat
(e.g. antibiotics, injectable drugs, amino acids, vitamins, etc. )

Ultrasound (sound waves of 20 thousand Hz) has a bactericidal effect. Mechanism: the formation of cavitation cavities in the cytoplasm of the cell, which are filled with liquid vapors and in them pressures of up to 10,000 atm occur, which leads to the formation of highly reactive hydroxyl radicals, to the disintegration of cellular structures and the depolymerization of organelles, and the denaturation of molecules.


Most organisms on land or on the surface of water is always subjected to a pressure of 1 atm. The hydrostatic pressure can reach 600 to 1100 atm in the deep sea. Despite these extremes, bacteria survive and adapt. Many are barotolerant. Some bacteria in the gut of deep sea invertebrates such as amphipods and holothurians are truly barophilic and grow more rapidly at high pressures (Ex. Photobacterium , Shewanella, Colwellia ).




Depending on the nature, concentration and duration of the action, chemicals stimulate growth (they are used as energy sources), have a microbicidal, microbostatic, mutagenic effect or may be indifferent to vital processes.

For example, a 0.5-2% glucose solution is a food source for microbes, and a 20-40% solution has a depressant effect.

For microorganisms, the optimal pH of the medium is required. pH refers to the acidity or alkalinity of a solution. It is a measure of the hydrogen ion activity of a solution and is defined as the negative logarithm of the hydrogen ion concentration.

pH = -log [H+] = log (1/H+)

The pH scale ranges from 1.0 to 14.0 and most microorganisms grow vary widely from pH 0 to 2.0 at the acid end to alkaline lakes and soil that may have pH values between 9.0 and 10. The pH can affect the growth of microorganisms and each species has a definite pH growth range and pH growth optimum.

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