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Sterilization is defined as the process of making an object or material sterile (i.e. totally free from microbial cells, inclusive of viruses). Chemical agents that accomplish sterilization can be referred to as sterilizers or sterilants. Sterilization can be achieved in the microbiology laboratory by a variety of processes including but not limited to autoclaving, incineration (burning), tyndallization (process of killing microbes by heating and boiling) and radiation (by use of UV light) which are all physical sterilization procedures. Chemical sterilization which involves the use of chemicals strong enough to kill all forms of microbial life can also bu used to achieve some level of sterilization. Sterilization processes achieves complete cleaning in that it removes and kills all forms of microbes including viruses, viable organisms, viable spores and viroids. In the microbiology laboratory, sterilization is used to decontaminate old culture plates, reagents and other objects contaminated by microbes before their disposal.

An object or material that is free of all viable microorganism, viruses and spores is said to be sterile. Sterilization can be achieved by either physical methods (as exemplified above) or chemical methods (e.g. use of ethylene oxide). Ethylene oxide (EtO) gas is widely used as chemical sterilants to sterilize heat-labile or sensitive medical materials such as sutures, syringes, and catheters. EtO gas is used as a sterilizing gas to sterilize (i.e. to make germ free) materials and packaged goods because the gas effectively penetrate packaged and plastic wrapped goods or materials. The mechanism of action of EtO gas is based on its ability to react with microbial cell proteins. They are capable of destroying all forms of microbial life.   

The autoclave is a commonly used equipment that is used to achieve sterilization of materials in the microbiology laboratory (Figure 1). There are various sizes of the autoclave ranging from smaller ones that can be used on laboratory benches to immovable autoclaves that are used in industries and hospitals for large-scale sterilization or autoclaving purposes.

Figure 1. Autoclave. The autoclave is used for sterilization in the microbiology laboratory. Photo courtesy:


Filtration is a sterilization technique that removes suspended microbial particles and/or cells from liquids or solutions by passing them through special types of permeable membranes generally known as membrane filters. Membrane filters are filters with varying pore sizes (usually in the range of 0.1-10 µm). They are used to remove the vegetative forms of microbes (excluding viruses) from solutions. Membrane filters are specifically used in pharmaceutical companies, food industries and in order laboratory practices to reduce the microbial load of some heat-sensitive solutions such as laboratory reagents, culture media, antibiotic solutions, and ophthalmic solutions. Membrane filters unlike other methods employed to kill or inhibit microbial growth does not destroy or stop the growth of microorganisms. They only sieve out and trap microorganisms in their porous casing. Laminar flow biological safety cabinet or hood (Figure 2) employs an air filtration system (in this case, depth filters) that traps microbes and prevent the further spread of pathogenic microorganisms and the possible direct infection of the scientist working in the hood.

A biological laminar flow cabinet is a carefully enclosed bench designed to prevent contamination of biological samples, or any sensitive materials including the researcher working with dangerous samples. Air is drawn through a high efficiency particulate air (HEPA) filter and blown in a very smooth, laminar flow towards the user. The cabinet is usually made of stainless steel with no gaps or joints where spores might collect. Biological laminar flow cabinet can also be called the hood, or laminar flow closet or tissue culture hood. HEPA is a type of air filter; and it is critical in the prevention of the spread of airborne bacterial and viral organisms and, therefore, microbial infection when using the hood. HEPA is incorporated in many medical devices and other equipment capable of spreading infectious agents via contaminated air.

Figure 2. Biological laminar flow cabinet. Photo courtesy:

This system of sterilization or control of microorganisms (as obtainable in the hood) helps to sterilize air by a mechanism of filtration, and this is achieved by special types of filters known as depth filters fitted into the hood to trap pathogenic organisms. Face masks used in dusty areas and surgical masks are some examples of materials that can help in the sterilization of air and thus minimize the rate of infection or transmission of disease causing agents. Depth filters are used in biological safety cabinet and in other biosafety applications to reduce contamination of the environment and worker. The hood is mostly used for the manipulation of cell cultures and in the evaluation of pathogenic microorganisms or materials. Membrane filters and depth filters are the two methods of reducing microbial population by the process of filtration (Figure 3). While membrane filters are strictly used for the sterilization of heat-sensitive liquids, depth filters are used to remove microorganisms and other contaminants from air and liquids.

Figure 3. Membrane filters. Photo courtesy:


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