Principle of Sterilization

Sterilization – Introduction 

Sterilization is the removal of all forms of microorganisms from the surface of an object. It includes both spore and vegetative forms. Here, let’s glance at the definition and classification of sterilization notes.

Table of Contents

• What is Sterilization? – Sterilization Meaning
• Classification of Sterilization
  • Physical Methods of Sterilization
    • Heat Sterilization
    • Filtration
    • Irradiation
    • Sound Waves Vibration
    • Fractional Sterilization
  • Chemical Methods of Sterilization
    • Gaseous Sterilization
    • Liquid Sterilization
• Frequently Asked Questions

What is Sterilization? – Sterilization Meaning

In microbiology, sterilization can be defined as the complete removal of all forms of microorganisms, both vegetative and spore forms, from a surface or an object. Sterilization is carried out by various physical and chemical methods such that it eliminates around 10⁶ log colony-forming units.

Sterilization is done to avoid the growth of microorganisms which may grow on the surface of an object if left without killing the germs. It is, however, different from disinfection or sanitisation where only reduction of the microorganisms takes place, rather than total elimination. After sterilization, an object becomes sterile or aseptic.

Classification of Sterilization

Sterilization is achieved by different physical and chemical methods in microbiology. Sterilization is classified into 2 types – physical sterilization and chemical sterilization. Let us discuss them in detail.

Physical Methods of Sterilization

• Heat Sterilization

Heat sterilization is the most effective method of sterilization, where the elimination of microbes is achieved by the destruction of cell constituents and enzymes. It is done by two methods:

1. 1. 1. Moist Heat Sterilization: It is one of the best methods of sterilization. Moist heat sterilization is done with the help of an instrument called an autoclave. An autoclave works on the principle of producing steam under pressure. Thus moist heat sterilization is also known as steam sterilization. The water is boiled in an autoclave at 121-134℃ at a pressure of 15psi. This leads to coagulation of proteins in the microorganism, and they are effectively killed.
      2. Dry Heat Sterilization: This method is used on objects that are sensitive to moisture. Moisture-free heat or dry heat is applied on the surface or objects such that there is denaturation and lysis of proteins which leads to oxidative damage, and ultimately the microbial cell dies out or may even burn. Some methods of dry heat sterilization include incinerators, hot air ovens and flaming techniques.

• Filtration

This is a mechanical method of sterilization in microbiology. This method uses membranous filters with small pores to filter out the liquid so that all the bigger particles and microbes cannot pass through. The three steps of filtration are sieving, adsorption and trapping.

• Irradiation

Irradiation is the process of exposing surfaces or objects to different kinds of radiation for sterilization. It is of two types:

1. 1. 1. Non-ionising Radiation: Ultraviolet radiation is exposed to the object, which is absorbed by nucleic acids of the microorganisms. This leads to the formation of pyrimidine dimers in the DNA strand, which causes the replicative error, and eventually, the microbe dies.
      2. Ionising Radiation: Upon exposure to ionising radiations such as gamma rays and X-rays, reactive oxygen species such as hydrogen peroxide and superoxide ions are formed that oxidise the cellular components of the microbe, and they die.

• Sound Waves Vibration

Sonix sound waves ranging from 20-40 kHz in frequency are applied across the fluid to be sterilized. These ultrasonic waves produce an alternation of compressive and tensile forces forming cavities in the solution. These cavities suddenly collapse, which creates submicroscopic voids and removes microorganisms from the container.

• Fractional Sterilization

Fractional sterilization or tyndallisation is a method used for media containing gelatin or sugar. Typically, exposure to 100°C for 20 minutes on 3 successive days is required. The principle is that the first exposure kills all spores and vegetative bacteria. If they germinate, they will be killed in the subsequent exposures. However, this method may fail to kill spores of certain thermophiles and anaerobes.

Chemical Methods of Sterilization

Chemical methods of sterilization are used in microbiology for biological specimens and plastic equipment. In this method, several chemicals work as bactericidal agents. They can be of two types: gaseous or liquid.

• Gaseous Sterilization

Gaseous sterilization is the method where the object is exposed to gas in a closed, heated and pressurised chamber. The gaseous chemical agents used for sterilization include ethylene oxide, formaldehyde, nitrogen dioxide and ozone.

• Liquid Sterilization

Liquid sterilization is the process of immersing the object in a liquid such that it kills all the viable microorganisms and their spores. This method is less effective than gaseous sterilization and is used to remove low levels of contamination. Common liquid chemical agents that are used for sterilization include hydrogen peroxide, glutaraldehyde and hypochlorite solution.

Cold Sterilization Definition – It is a process in which sterilization is carried out at low temperatures with the help of chemicals, filters, radiation and all other means excluding high temperatures.  It is done for products that contain heat-sensitive ingredients and yet require sterilization.

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Also Read:

• Difference between Disinfection and Sterilization
• Do autoclaves kill everything?