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Physical Agents for Control of Microorganisms

Microorganisms, including bacteria, viruses, and fungi, can cause infections and diseases in humans and animals. While there are many ways to control the growth and spread of microorganisms, physical agents are one effective method. Physical agents include heat, radiation, filtration, and other physical methods that can kill or remove microorganisms. In this blog post, we will explore the different physical agents used for the control of microorganisms.

a) Temperature:

Microorganisms, such as bacteria and fungi, have a preferred range of temperature for growth, survival, and reproduction. This temperature range includes a minimum, optimum, and maximum, beyond which the microorganisms may become inactive, damaged, or even die. If the temperature falls below the minimum, microorganisms experience a static effect that slows down their metabolic processes. However, they can still survive in this state. On the other hand, temperatures above the maximum can have a cidal effect, denaturing the proteins and enzymes of the microorganisms and causing their death.

Different types of microorganisms have different temperature ranges that are best suited for their growth and survival. By controlling the temperature, we can manipulate the growth rate and viability of microorganisms, making it a useful tool for various applications such as food preservation, disease control, and biotechnology. Therefore, the Temperature is a powerful physical agent of microbial control.

Temperature is an important factor in controlling the growth and proliferation of microorganisms. Here are some ways temperature is used to control microorganisms:

  1. Refrigeration: Lowering the temperature of food, drugs, and other perishable items to 4°C or below slows down the growth of bacteria and other microorganisms, increasing the shelf life of the products.
  2. Freezing: Freezing at -18°C or below stops the growth of microorganisms completely. Frozen food, tissue samples, and other materials can be stored for long periods of time without the risk of contamination.

Heat in the form of dry heat and moist heat also powerful physical methods for control of microorganisms.

i) Moist heat

Moist heat is more effective than dry heat for killing microorganisms because it can penetrate microbial cells. Moist heat works by denaturing microbial proteins and enzymes, causing them to lose their three-dimensional functional shape. It can also melt lipids in cytoplasmic membranes.

Moist heat sterilization is a process that uses heat and moisture to kill microorganisms on objects and surfaces. Here are some examples of moist heat sterilization methods:

  • Autoclaving: This is a common method used in laboratories and hospitals to sterilize medical equipment, surgical instruments, and laboratory glassware. It involves exposing the objects to high-pressure steam at a temperature of 121°C for at least 15 minutes.
  • Boiling: This is a simple method of sterilizing objects that are heat-resistant but not delicate. For example, baby bottles, kitchen utensils, and surgical dressings can be sterilized by boiling them in water for 10-20 minutes.
  • Pasteurization: This is a method used in the food industry to sterilize liquids such as milk, fruit juices, and beer. The liquid is heated to a temperature of 72-85°C for a short period of time, usually 15-30 seconds, to kill any harmful bacteria.
  • Tyndallization: This is a process used for sterilizing heat-sensitive materials that cannot withstand the high temperatures of autoclaving. The material is exposed to steam for 15-30 minutes, then left to cool for 24 hours. This cycle is repeated three times to ensure complete sterilization.
  • UHT sterilization: This is a method used to sterilize liquid products that are shelf-stable, such as milk and cream. The liquid is heated to a temperature of 135-150°C for a few seconds, then rapidly cooled to prevent any bacterial growth.

ii) Dry Heat:

Dry heat sterilization is a process used to kill microorganisms on objects by exposing them to high temperatures without any moisture. This method is used for materials that cannot be sterilized by other methods. It requires higher temperatures and longer exposure times than moist heat sterilization. The ideal temperature for dry heat sterilization is typically between 160°C to 180°C. It works by oxidizing the cellular components of microorganisms, destroying them. Objects that are sterilized with dry heat must be completely dry. Dry heat sterilization is often used for glassware, metal instruments, and powders that are sensitive to moisture. However, it has some disadvantages, such as longer exposure times, potential damage to some materials, and higher energy costs

Dry heat sterilization is a process that uses hot air to kill microorganisms on objects and surfaces.

Examples of dry heat sterilization methods :

  • Incineration: This is a method used to sterilize laboratory instruments that cannot be sterilized by other methods, such as inoculating loops and needles. The instruments are exposed to a flame until they are red-hot, which destroys any microorganisms present.
  • Hot air oven: This is a common method used to sterilize glassware, metal instruments, and other heat-resistant materials in the laboratory. The materials are placed in a hot air oven and heated to a temperature of 160-180°C for 1-2 hours, depending on the type of material.
  • Flaming: This is a quick and easy method used to sterilize the mouth of test tubes and flasks. The mouth of the tube or flask is passed through a flame for a few seconds, which kills any microorganisms present.

b) Desiccation

  • Desiccation removes water from the environment of microorganisms and prevents their growth and reproduction.
  • This method exposes microorganisms to a dry environment that lacks sufficient moisture for their survival.
  • Microorganisms become unable to cause contamination as their water content decreases, ultimately leading to their death.
  • Desiccation is effective in controlling bacterial growth on surfaces such as laboratory equipment and medical devices.
  • You can achieve desiccation by air-drying or using drying agents such as silica gel to control microorganisms.

c) Osmotic Pressure

Osmotic pressure is a physical method that can be used to control microbial growth by manipulating the solute concentration in the surrounding environment of the microorganisms.

When a microbe is placed in a hypotonic environment, where the solute concentration is lower outside the cell than inside, water flows into the cell and causes it to swell or become turgid. However, when placed in a hypertonic environment, where the solute concentration is higher outside the cell than inside, water flows out of the cell, and the cell loses water, resulting in plasmolysis, a state in which the cell membrane separates from the cell wall. This process kills the microorganisms as it changes the osmotic pressure, which the microbes need for survival.

By controlling the solute concentration, food can be preserved. For example, high sugar concentration or salt brine can be used to create hypertonicity, thereby inhibiting bacterial growth. This method works because water moves from the low solute concentration to the high solute concentration, causing the bacterial cells to lose water and undergo plasmolysis.

Molds are more tolerant of hypertonicity and can overgrow on food unless sealed to exclude oxygen. This property of molds, along with their ability to grow under acidic conditions, is the reason fruits and grains are often spoiled by molds rather than bacteria.

Overall, osmotic pressure is an effective method of controlling microbial growth, and it is widely used in food preservation and other industries.

d) Radiation

Radiation, including ionizing radiation and ultraviolet (UV) radiation, can be used as agents for controlling microorganisms.

i) Ionizing radiation

This radiation is a type of high-energy radiation that has enough energy to penetrate cells and alter molecular structures, causing damage to cell components and DNA molecules. Ionizing radiation, such as X-rays and gamma rays, have high energy and penetrating power, and can ionize water and other molecules to form radicals that disrupt DNA molecules and proteins. This can lead to mutations and cell death in microorganisms. This radiation is commonly used to sterilize medical equipment, pharmaceuticals, and other materials that cannot be sterilized by heat or chemicals. Such ionising radiation generates toxic oxygen metabolites that act as powerful oxidizing agents and destroy cellular components of bacteria, leading to their death. Some examples of ionizing radiation include X-rays, gamma rays, and cathode rays. Sterilization by ionizing radiation is known as cold sterilization.

ii) Ultraviolet and UV Radiation:

  • It is Non-ionizing radiation is used for disinfection and uses less energy than ionizing radiation.
  • Ultraviolet (UV) light causes thymine dimers to form between adjacent thymines within a single strand of DNA.
  • UV light does not penetrate cells or packaging.
  • UV lamps are commonly incorporated into water purification systems for use in homes.
  • Small portable UV lights are commonly used by campers to purify water from natural environments before drinking.
  • Germicidal lamps are used in surgical suites, biological safety cabinets, and transfer hoods emitting UV light at a wavelength of 260 nm.
  • UV light does not penetrate surfaces and will not pass through plastics or glass.
  • Sunlight can be effective against certain bacteria due to the formation of thymine dimers by UV light and the production of reactive oxygen products induced in low amounts by exposure to visible light.

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