What is Sterilization?

It is a method for the destruction of all forms of vegetative growth and endospores from a medium. In other words, sterilization can be defined as a method of killing every microbial organism such as bacteria, fungi, etc. present in an object or a medium. There are different strategies present to sterilize an object, like physical methods and chemical methods of microbial control.

Physical Methods of Microbial Control

The physical methods of microbial controls include applications of heat, cold, pressure, radiations, etc. 

Heat Sterilization

It is one of the most important and useful methods to kill bacteria and other microbes present in a culture medium. There are different techniques present to sterilize an object by applying heat on it, which are as follows,

Dry Heat 

It is a technique to kill microbial contents in an object either by direct flaming, incineration, or hot air sterilization.

Direct flaming

In the dry heat sterilization method, objects such as glassware and inoculating loops are sterilized by exposing them to the direct flame to kill all forms of microorganisms present in them.

Incineration

In incineration, objects such as paper materials, contaminated dressing equipment, animal carcasses are burned to ashes to kill all forms of microbes including endospores. The sterility of this process is very high.

Hot air 

In the hot-air sterilization procedure, different types of laboratory equipment such as empty glassware, needles, or glass syringes are oxidized by hot air to sterilize them. This process is done in a hot air oven machine and has a high sterility level.

Moist Heat Methods

There are two types of moist heat sterilization methods, autoclaving and flowing steam or boiling. These processes are also called steam sterilization.

Autoclaving

Autoclaving is the most secure method of destruction of all forms of vegetative growths and pathogens from the medium. Destruction of bacterial spores and postpartum sterilizations are also done by this sterilization system. The process is conducted in a machine called an autoclave. It is a forced sterilization system where high-level disinfection of the sample occurs. Samples are placed inside the autoclave in presence of steam. A sterilization cycle occurs at 121°C for 20 minutes. The steam and pressure created inside the autoclave kill all types of pathogens including bacteria, fungi, almost all viruses, and their spores. This is used to sterilize all glassware, plasticware, microbial and fungal cultures plates, broths, etc.

Boiling or Flowing Steam

Boiling in water is another effective sterilization method to kill microorganisms.  In this process, samples are boiled in water to sterilize them and kill the microbial pathogens within 10 minutes. Heat causes the death of the microbes by their protein denaturation. There are some other important terminologies related to moist or steam sterilization which are as follows.

• Thermal death point (TDP): It is the minimum temperature required at which all microbial organisms present in a liquid suspension died in 10 minutes.
• Thermal death time (TDT): TDT is defined as the minimum time required at which every bacteria will be killed in a liquid growth medium at a particular temperature.
• Decimal reduction time (DRT): It is defined as the time required to kill 90 % bacterial population from a liquid medium at a particular temperature.

Pasteurization

It is a very effective sterilization procedure used in the sterilization of milk, wine, beer, cream, etc. Here the sample is sterilized by heating them for 72°C for 15 minutes. Although there are different pasteurization methods available for different food items, pasteurization only kills the vegetative growths but not endospores. In this process the sterility assurance level is low.

Radiation

Radiation is another system to sterilize. There are two types of radiation used in the sterilization of microorganisms: ionizing radiation and nonionizing radiation.

Ionizing radiation

In this radiation sterilization process, ionizing radiations such as X-rays, gamma rays, high-energy electron beams having a wavelength of less than 1 nm are used. Due to the smaller wavelength, they possess higher energy and having the ability to generate free radicals from water. These radicals of oxygen such as hydroxyl ion, superoxide radicle damages the cellular components and their organic compounds, especially DNA. Ionizing radiation is commonly used for sterilization in food industries for the preservation of food.

Nonionizing radiation

It contains a wavelength greater than 1 nm, i.e., bigger wavelength than that of ionizing radiation. UV rays are the best example of nonionizing radiation. It kills microbes by damaging DNA. UV light causes DNA damage by thymine dimer formation and several other mutations. It is mainly used in laboratories, hospitals to sterilize a room or equipment such as laminar airflow.

High pressure

High pressure is another method of sterilization for liquids. When high pressure is applied to a culture media or liquid food medium it alters the structures of proteins and carbohydrates and prevents bacterial growth in it. This sterilization process is commonly used at high temperatures for the preservation of food.

Chemical Methods

There are several chemicals and their derivatives that act as a sterilant and which are used as a sterilizer, disinfectant, antiseptics, and sanitizer. Here is the list of the following chemicals which are used to kill microorganisms.

Phenol, Phenolics, and Bisphenols

Phenols and their derivatives such as phenolic compounds and bisphenol kill bacteria by disrupting their plasma membrane proteins and denaturation of their enzymes. Phenols are sensitive to the skin and are not recommended to be directly applied to the skin. On the other hand, phenolic compounds and bisphenols are commonly used as antiseptics, i.e. they kill the bacterial population on the surface of living organisms.

Alcohols

Alcohols are the types of sterilant that are commonly used as a bactericidal and fungicidal agent. It kills microorganisms by protein denaturation and dissolving the plasma membrane lipids. 60--70% ethanol and isopropyl alcoholic solutions are commonly used as a sanitizer.

Ethylene oxide

It is one of the most potent chemical sterilant which is frequently used in hospitals. In pure form ethylene oxide is a toxic and explosive chemical, so it is always mixed with a non-flammable gas before the use. Ethylene oxide kills all microorganisms and endospores, but it requires a comparatively longer exposure than other chemicals. Ethylene oxide kills pathogens by the alkylation of proteins and nucleic acids. Ethylene oxide forms active radicles that remove hydrogen atoms from the proteins and nucleic acids followed by cross-linking occurs in proteins and nucleic acids. One big advantage of ethylene oxide is that the sterilization technique does not require an elevated temperature.

Halogens

Iodine and compounds of chlorine are a strong oxidizing agent and used as a sterilizer. Iodine interferes in the protein functions of bacteria and acts as an oxidizing agent. Compounds of chlorine such as hypochlorous acid and chlorine gas itself act as a very strong oxidizing agent and prevent the intracellular functions of the bacterial cell. When chlorine gas is mixed with water it forms hypochlorous acid (HOCL) which is further auto-dissociated to form hydrogen ion and hypochlorite ion. Municipal water frequently treated with chlorine gas to prevent microbial growth and used in disease control.

Aldehydes

Aldehydes such as formaldehyde, glutaraldehyde are potent sterilizers and the latter one is frequently used in the sterilization process of medical equipment. They kill bacteria by protein denaturation.

Heavy Metals and their Compounds

Several heavy metals and their compounds have sterilization ability, i.e., they can kill microbes and act as bactericidal and antiseptic agents. Copper, silver, and mercury have bactericidal abilities. Silver compounds such as 1% silver nitrate act as an antiseptic and are used in eye disorders of new-born. Silver sulfadiazine is used as an antiseptic cream for burn injuries. On the other hand, copper sulphate is used as an algicidal agent. Heavy metals like mercury and silver also interfere with cellular functions, enzyme activities and act as biocidal agents.

Peroxygen and Other Forms of Oxygen

Peroxygen or hydrogen peroxide is an antiseptic agent used to sterilize anaerobes. Their mode of action is the oxidation of the organic compounds of bacteria. Hydrogen peroxide is also used as a disinfectant to clean floors. The most common form of oxygen used as a sterilizer is Ozon. It is used as a supplement to chlorination.

Context and Applications

This topic is significant in the professional exams for both under-graduate and post-graduate courses, especially for

• B.Sc. in Microbiology
• M.Sc. in Microbiology
• M.Sc. in Biochemistry
• M.Sc. in Biotechnology

Related Concepts

• Disinfectants
• Antiseptics
• Antibiotics
• Antimicrobial drugs
• Pathogenicity