Control of Microorganisms – 8 Physical methods

When you are dealing with microbes in your lab, one thing that is always problematic is the growth of unwanted microorganisms. Contamination can cause trouble not only in hospitals or diagnostic labs but also in the food and pharmaceutical industries as well. It is necessary to take decisions for the control of microorganisms

Hence the only way to control the unwanted growth at the place is to kill the microbes over there! The best way to make your space free from contamination is to sterilize it! Sterilization can be defined as the killing of microbes from a surface or area to make it contamination-free!

Why do we need to do Sterilization?

One thing to keep in mind is that all microbes are not human-friendly. These unseen organisms can prove themselves as the worst enemy if they intrude into the body, and find out optimum conditions for bacterial growth.

Moreover, in the case of non-sterilized surfaces, there are chances of acquiring an infection! Not only that, if the surgical equipment isn’t contamination-free then there is a high risk of post surgical infections.

In the case of the pharmaceutical industry, the microbial count in intravenous injections must be 0, otherwise, the person may have to face life-threatening consequences.

In the food industry, contamination in milk, meat, or canned food may lead to severe food poisoning or sometimes death. Hence it is necessary to ensure that the products prepared for the consumer’s well-being shouldn’t be the cause of his health’s deterioration.

Efficient Physical Ways for Sterilization

Complete removal of microbes is possible with chemicals, but some chemicals may also impart negative impacts on the human body and environment. Therefore physical methods for sterilization are preferred all over as they don’t require any technical knowledge or precautions.

There is a list of physical means for sterilization. It entirely depends upon the person which method he is going to choose, and the conditions as well!

1-Control of Micro-organisms by Filtration

Filtration is one of the easiest and most efficient ways to control the microbial load in the product or area! It’s quite interesting that the most common way of contamination, by which these microbes spread is air!

Hence to reduce the microbial load or to eradicate them from the workspace is filtration of the air. For this purpose, HEPA filters are used in different labs. HEPA filters not only trap microbes but also act as a firm barrier to capture dirt particles.

The pore size of the HEPA filter is 0.3 microns. So the particle which will be smaller than 0.3 microns will pass through it otherwise it will trap inside it.

The filtration method is also used in the food industry to filter the water which is utilized in making items. So filtration is not limited to the pharmaceutical industry or diagnostic labs.

2-Sterilization through Moist Heat

Moist heat is an effective way to kill microbes. The steam is used as the source to cause the bactericidal effect. The technique is efficient to make the glassware and media plates safe and reusable.

The temperature for moist heat sterilization is 121 degrees for 20 minutes. The pressure must be 15 PSi. The principle of sterilization by moist heat is the coagulation of the proteins inside the cell. Steam heat has more penetrating power, hence it can easily enter the cell.

The best and most appropriate example for sterilization through moist heat is the autoclave which is common in labs.

If the temperature for moist heat sterilization is below 100 degrees then we are going to use it in the dairy industry for the pasteurization of milk. As most food pathogens can tolerate temperatures above 50 degrees. So the item is treated at 62.5 degrees for around half an hour for the complete removal of bacteria

If the temperature is exactly 100 degrees, then it is likely used to kill the spores by bringing them back to their vegetative forms. The method is called tantalization and it takes around 3 days to complete.

3-Control of Microbes through Dry Heat

Dry heat is an effective way to treat those items which cannot be treated with moist heat like powders, pipettes, and greasy stuff.

There is no use of steam for this procedure! The products are directly exposed to the source of heat to kill the microbes. Dry heat exposure leads to the denaturation of essential proteins inside the cell.

Furthermore, it also leads to a redox reaction inside the cell which cannot be repaired by the cell’s repairing mechanism. That irreversible change leads to the death of the cells. The best equipment for the dry heat method for sterilization is a hot air oven.

4-Control of Microbes through Low temperatures (Below 100ºC)

   This method has two subcategories, one of them is the holder method and the other is the flash method. The former one involves heating at 63ºC for 30 minutes and the latter one involves 73ºC for 20 seconds.

The most common method that is used to kill microbes in the fluids and reduce their number up to a certain level so that they don’t cause disease is known as Pasteurization. This method is most commonly used for killing microbes present in milk.

This method is also used for killing microbes for vaccines that are used against non-sporing bacteria are also killed by heating at 60ºC for 1 hour.

Serum and Body fluids can also be cleaned using this method.

5-Control of Microbes through High temperatures (Above 100ºC)

   For above 100ºC sterilization, steam is used under high pressure. In an autoclave, pressure is increased to 15psi which causes the boiling point of water to raise up to 121ºC.

Steam, under high pressure easily kills all kinds of bacteria. It gives off latent heat that causes bacteria to die immediately.

Cultural media and contaminated instruments are used for this purpose.

6-Control of Microbes through Osmotic Pressure

        Salt has been used for centuries to kill microbes. Ancient Egyptians used to wash their teeth with salt. Many freshwater fish diseases are easily cured by salt. The mechanism of salt action is very simple. It causes an imbalance in the osmosis process as the concentration of salt outside the cell membrane is different from that within the cell membrane. This is called osmotic shock. It causes the cell membrane of the microorganism to rupture. This helps a lot in the treatment of many fungal diseases. It also cures a lot of mouth diseases.

7-Control of Microbes through High Pressure (Pascalization)

      Using high pressure for the preservation of food and brewery is a very modern technique. It helps very much in preserving food as it will cause many microorganisms and bacteria to go inert completely and die soon after. Sterilizing food with chemicals or drying the food products denatures the protein to an extent that the food losses its natural nutritional value and hence, food will not have a good effect in improving health.  

 High-pressure sterilization is also known as bridgmanization, high-pressure processing, or high hydrostatic pressure (HHP) processing. This technique has less effect on covalent bonds within the food and it does not affect the nutritional value of food and the taste of food is also not affected.

A French scientist, named Blaise Pascal was working on fluids and studying the effect of pressure on the fluids. This lead him to discover the effect of pressure on microorganisms and that is how this technique of preserving food using high pressure came into being. Then, after a few years a 

a great physicist named Percy 

Williams Bridgman developed this technique and 

Then, this technique started to be used for purpose of sterilization.

  For the inactivation of bacteria and yeasts and fungus, very high pressure is applied. In some instances, a pressure as high as 50,000 pounds per square inch is applied for almost fifteen minutes and this causes the microbes to die.

8-Control of Microbes through Desiccation

    Desiccation simply means removing moisture and water from the products to preserve them for long periods. When water is removed from the products, microorganisms such as bacteria and yeasts and fungi are unable to grow and they are also unable to propagate. But they can survive for some time depending upon the type of microorganisms.

   This job is done by many pharmaceutical companies as they have to preserve medicines for longer periods. The biggest example is the silica ball bag that is present in the medicine bottles, which is used to absorb water from the medicine and helps prevent the medicine from being rotten or spoiled.

 The most common method used for drying medicines and food is Freeze drying. It is also known as lyophilisation, lyophilization, or cryodesiccation is a dehydration Process. The method of freeze drying is most commonly used to prevent food from being spoiled or any medicine that needs to be prevented from being spoiled. It is also used to prepare medicines and food, which are to be transported over long distances.

There is one problem with desiccation, that it does not provide complete sterilization. Desiccation works till there is no water in the product. As soon as, water is added to the product, the bacteria start growing again.

Mubashir Iqbal
Mubashir Iqbal

Mubashir Iqbal is a highly dedicated and motivated Microbiologist with an MPhil in Microbiology from the University of Veterinary and Animal Sciences. Currently, he is researching the efficacy of commercially available SARS Cov-2 vaccines to neutralize the omicron variant in Pakistan. He holds a Bachelor's degree in Microbiology and has experience in chemical and microbiological analysis of water samples, managing SOPs and documents according to standard ISO 17025. Additionally, he has worked as an internee in BSL 3, Institute of Microbiology, UVAS, where he gained experience in RNA extraction, sample processing, and microscopy.

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