Factors affecting microbial growth and death
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Factors affecting microbial growth and death are the result of a combination of microbes and environmental factors. Changes in environmental conditions can cause changes in microbial morphology, physiology, growth, reproduction, etc.; or resistance, adapt to certain changes in environmental conditions; when environmental conditions change beyond a certain limit, it leads to the death of microorganisms. In order to inhibit and eliminate the harmful effects of microorganisms, various physical, chemical or biological methods are often employed to inhibit or kill microorganisms. The following terms are commonly used to indicate the degree of killing of microorganisms. Sterilization: Physical or chemical destruction of all microorganisms on the object (including pathogenic and non-pathogenic microorganisms and bacterial spores, mold spores, etc.) is called sterilization. Disinfection: Physical or chemical methods can only kill pathogenic microorganisms on objects, but not necessarily kill all non-pathogenic microorganisms and spores and spores. The drug used for disinfection is called a disinfectant. Antiseptic: A method of preventing or inhibiting the growth and reproduction of microorganisms is called preservative or bacteriostatic. Chemicals used for preservation are called preservatives. Certain chemicals are preservatives at low concentrations and disinfectants at high concentrations. Sterile: Refers to the absence of living microorganisms. A method of preventing or eliminating all microorganisms from entering an animal's body or object is called a sterile method. When operated aseptically, it is referred to as aseptic processing. Strict aseptic processing is required to prevent microbial contamination during surgical or microbiological experiments. Different microorganisms have different sensitivities to various physical and chemical factors. Different doses have different effects on microorganisms, or they may be sterilized, or may only be disinfected or preserved. The comprehensive effects of multiple factors should also be considered when understanding and applying any physical or chemical factors to inhibit or kill microorganisms. For example, adding another chemical while increasing the temperature accelerates the destruction of microorganisms. E. coli in the presence of phenol, the temperature increased from 30 ° C to 42 ° C significantly accelerated death; the physiological state of microorganisms also affect the role of physical and chemical factors. Nutrient cells are generally less resistant to spores, and younger, metabolically active cells are more susceptible to destruction than older, dormant cells; the medium in which microbes grow and the environment in which they are exposed also have a significant effect on microbial damage. . For example, in acid or alkali, the damage of heat to microorganisms is increased, and the viscosity of the medium also affects the penetration ability of the antibacterial factor; the presence of organic matter also interferes with the effect of the antimicrobial chemical factor, or the combination of organic matter and chemical agents The failure, or the organic matter covering the cell surface, hinders the penetration of chemicals. The common physical and chemical factors affecting microbial growth and death are: 1. Temperature: Temperature is one of the most important factors affecting the growth and survival of organisms. Its impact on the living organism is manifested in two aspects: on the one hand, as the temperature rises, the biochemical reaction rate and growth rate in the cells accelerate. Under normal circumstances, the biochemical reaction rate is doubled for every 10 °C increase in temperature; on the other hand, important components of the body such as proteins and nucleic acids are sensitive to temperature, and may suffer irreversible damage with increasing temperature. Therefore, only within a certain range, the metabolic activity and growth of the body will increase with the increase of temperature. When the temperature rises to a certain extent, it will begin to have an adverse effect on the body. If it continues to rise, the cell function will drop drastically. death. In general, microbial growth has a wide temperature range, and known microorganisms can grow at minus 12-100 °C. Each microorganism can only grow within a certain temperature range. Various microorganisms have the lowest temperature, optimum temperature, maximum temperature and lethal temperature for their growth and reproduction. Minimum growth temperature: refers to the lowest temperature limit at which microorganisms multiply. If it is lower than this temperature, growth stops completely. Optimum growth temperature: The temperature at which microbes can rapidly grow and multiply is called the optimum growth temperature. At this temperature, the microbial population grows at the fastest rate and has the shortest generation time. The optimum growth temperature for different microorganisms is different. Maximum growth temperature: refers to the highest temperature limit for the growth and reproduction of microorganisms. Lethal temperature: If the maximum growth temperature is further increased, the microorganisms can be killed. The lowest temperature limit of the lethal microorganism is the lethal temperature, and the lethal temperature is related to the treatment time. Microorganisms can be classified into three types: low temperature microorganisms, medium temperature microorganisms and high temperature microorganisms according to their growth temperature range. 2. Hydrogen ion concentration (pH): The pH in the environment is usually expressed as the negative logarithm of the hydrogen ion concentration, that is, the pH. The pH value in the environment has a great influence on the life activities of microorganisms. The main role is to cause changes in cell membrane charge, which affects the absorption of nutrients by microorganisms; affects the activity of enzymes in metabolic processes; and changes the nutrients in the growing environment. Toxicity and toxicity of harmful substances. Each microorganism has its optimum pH and a certain pH range. The enzyme activity is highest in the optimum range, and the growth rate of microorganisms is also highest if other conditions are suitable. Most bacteria, algae and protozoa have an optimum pH of 6.5-7.5 and can grow between pH 4-10; actinomycetes are generally most suitable for slightly alkaline, pH 7.5-8; yeasts, molds Suitable for acidic environments with pH 5-6, but the survival range is between pH 1.5-10. Some bacteria can even live in a strong acid or alkaline environment. Microorganisms grow in the matrix, and metabolism changes the concentration of hydrogen ions in the matrix. As the pH of the environment changes, microbial growth is blocked, and when the minimum or maximum pH is exceeded, it will cause the death of microorganisms. In order to maintain the stability of the pH during the growth of the microorganisms, the pH should be adjusted when formulating the medium, and buffers are often added to ensure the relative stability of the pH during the growth and reproduction of the microorganisms. Strong acids and strong bases have bactericidal power. The inorganic acid has strong bactericidal power but is highly corrosive. Certain organic acids such as benzoic acid can be used as preservatives. Strong bases can be used as fungicides, but because of their high toxicity, their use is limited to the disinfection of excreta and warehouses, sheds and other environments. Strong bases have a stronger effect on Gram-negative bacteria and viruses than on Gram-positive bacteria. 3. Oxidation reduction potential: The oxidation-reduction potential (Φ) has a significant effect on the growth of microorganisms. The Φ value in the environment is related to the partial pressure of oxygen and is also affected by pH. When the pH is low, the oxidation-reduction potential is high; when the pH is high, the oxidation-reduction potential is low. The Φ values ​​required for the growth of various microorganisms are different. Generally, aerobic microorganisms can grow above Φ value + 0.1 volts, and the Φ value is +0.3 volts to +0.4 volts. Anaerobic microorganisms can only grow at Φ values ​​below +0.1 volts. Facultative anaerobic microorganisms undergo aerobic respiration at +0.1 volts or higher and fermentation at +0.1 volts or less. 4. Radiation: Radiation is energy that is transmitted or transmitted from one place to another in a fluctuating manner through air or outer space. They are either ions or electromagnetic waves. Electromagnetic radiation includes visible light, infrared light, ultraviolet light, X-rays, xenon rays, and the like. (1) Ultraviolet radiation Ultraviolet radiation is non-ionizing radiation, and the bactericidal power is the strongest at a wavelength of 265-266 nm. Ultraviolet radiation has obvious lethal effect on microorganisms and is a strong fungicide. UV germicidal lamps are widely used in medical and aseptic operations. Due to the poor UV penetration ability and the difficulty of transmitting opaque substances, the UV germicidal lamp is only suitable for the disinfection of air and object surfaces. (2) Ionizing radiation X-rays and alpha rays, beta rays and xenon rays are both ionizing radiation. At sufficient doses, it has a lethal effect on various bacteria. It is commonly used for the disinfection of disposable plastic products and also for the disinfection of food. 5. Drying: Moisture is essential for the normal life activities of microorganisms. Drying can cause cells to lose water and cause metabolism to stop and die. The type of microorganisms, environmental conditions, degree of drying, etc. all affect the effect of drying on microorganisms. Dorsal spores are also very resistant to drying and can not be killed for a long time under dry conditions. This property has been used for the preservation of strains, such as the use of sand pipes to preserve spore-forming strains. In daily life, drying, drying and drying are also commonly used to preserve food. 6. Osmotic pressure: The phenomenon that water or other solvent diffuses through a semipermeable membrane is permeation. The pressure at which the solvent passes through the semipermeable membrane upon permeation is the osmotic pressure. Its size is proportional to the solution concentration. The osmotic pressure suitable for microbial growth is wide, and they tend to have a certain adaptability to osmotic pressure. Sudden changes in osmotic pressure can inactivate microbes, gradually change the osmotic pressure, and microorganisms can often adapt to this change. For general microorganisms, if their cells are placed in a hypertonic solution, water will pass through the cell membrane from the low concentration of cells into the solution around the cells, causing the cells to dehydrate and cause the separation of the plasmolysis, so that the cells cannot grow or even die. Conversely, if the microorganism is placed in a hypotonic solution or water, the water in the external environment will enter the cell from the solution causing the cells to swell and even rupture the cells. Since general microorganisms cannot tolerate high osmotic pressure, high concentrations of salt or sugar are commonly used in daily life to preserve foods such as pickled vegetables, meat and preserves. 7. Ultrasound: Ultrasound has a strong biological effect. The role of ultrasound is to rupture cells, so almost all microorganisms can be destroyed by it. The effect is related to frequency, treatment time, microbial species, cell size, shape and quantity. 8. Heavy metals and their compounds: Some heavy metal ions are components of microbial cells. When the concentration of these heavy metal ions in the medium is low, it promotes the growth of microorganisms, and vice versa. It also has some heavy metal ions, regardless of the concentration. The size and the growth of microorganisms can cause harmful or lethal effects. Therefore, most heavy metals and their compounds are effective fungicides or preservatives. The most powerful ones are Hg, Ag and Cu. For example, mercury dichloride, also known as mercury, is a highly bactericidal disinfectant. 0.1-1% silver nitrate is commonly used for skin disinfection. 9. Organic Compounds: There are many kinds of organic compounds which have harmful effects on microorganisms, and phenols, alcohols, aldehydes and the like can denature proteins, and are commonly used fungicides. Phenol: Phenol is also known as carbolic acid. Their harmful effects on bacteria may be mainly to denature proteins, and at the same time have a surface active effect, destroying the permeability of the cell membrane and causing the contents of the cells to overflow. When the concentration is high, it is a lethal factor, and vice versa. Cresol is a derivative of phenol. The bactericidal power is several times stronger than phenol. The solubility of cresol in water is low, but it is easy to form an emulsion in the soap solution and the alkaline solution. Commercially available disinfectant coal phenol soap liquid (Lesul) is a mixture of cresol and soap, commonly used 3-5% solution to disinfect skin, table and utensils. Alcohol: It is a dehydrating agent, a protein denaturant, and a lipid solvent, which can dehydrate and denature proteins, damage cell membranes and have bactericidal ability. Ethanol is a commonly used disinfectant and is commonly used in the disinfection of glass rods, slides and other utensils in the laboratory. 50-70% ethanol can kill vegetative cells; 70% ethanol has the best bactericidal effect, more than 70% and even anhydrous alcohol is less effective. Formaldehyde: Formaldehyde is also a commonly used bactericide and fungicide with good results. Pure formaldehyde is gaseous and soluble in water. The commercially available formalin solution is a 37-40% aqueous formaldehyde solution. 10. Halogen elements and their compounds: Iodine: is a strong fungicide. 3-7% iodine is dissolved in 70-83% ethanol to form iodine, which is an effective disinfectant for skin and small wounds. Iodine is generally used as a topical medicine. Chlorine or chloride: This is one of the most widely used disinfectants. Chlorine is generally used for disinfection of drinking water, and hypochlorite is often used for disinfection in food processing. The bactericidal mechanism of chlorine and chloride is that chlorine combines with water to produce hypochlorous acid (HClO), which is easily decomposed to produce new ecological oxygen, which is a strong oxidant that destroys microorganisms. 11. Surfactant: A substance having an effect of lowering surface tension is called a surfactant. The addition of such substances to the medium can affect the growth and division of microbial cells. Such as soap, bleach, washing powder, etc. 12. Dyes: Dyes, especially basic dyes, inhibit bacterial growth at low concentrations. Due to the selective bacteriostatic characteristics of these dyes, low concentrations of dyes are often added to the medium to prepare the selection medium. For example: alkaline triphenylmethane dyes, including malachite green, bright green, crystal violet, etc., have a strong inhibitory effect on Gram-positive bacteria. 13. Chemotherapy: A chemotherapeutic agent that directly interferes with the growth and reproduction of pathogenic microorganisms and can be used to treat infectious diseases. It can selectively act on a certain part of the metabolism of pathogenic microorganisms, causing its growth to be inhibited or killed. However, it is less toxic to human cells and is often used for oral or injection.