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Microbial Sterilization

Microbial Sterilization is the process of removal or deactivation of microorganisms by using a variety of methodologies. The type of methods used in sterilization is broadly classified based on their mechanism of action into physical sterilization, chemical sterilization as well as radiation sterilization.

No matter which technique of sterilization is used, the outcome should prove to be deleterious for the target microbes in order to provide a pure condition. However, when exposed to a particular sterilization method, microorganisms do not get killed immediately. The sterilization treatment causes a constant decline in the population of microbes at certain intervals if time. In this post, we will study how effective each of these methodologies is as well as their applications.

1. Physical methods

It utilizes the principle of subjecting microbes under extreme conditions of temperature as well as radiation in order to cause their lysis or deactivation.

Heat treatment

Microbes tend to show an increased growth at optimum temperature. As the optimum temperature is lowered or increased, microorganisms tend to show a decrease in their growth. Changes in temperature could cause the death of microorganisms. 
It should be noted that this method is quite effective for vegetative cells but not against endospores.d due to the addition or removal of heat.

Heat Addition: The elevation of temperature causes the killing of microorganism. The minimum temperature at which the microorganisms are killed is referred to as thermal death time (TDT). It is quite important to determine the thermal death time as it helps one to determine the effectiveness of the applied temperature. The two types of methods include:

1. Dry heat sterilization: As the name suggests, this method kills the microbes under the absence of water. It causes the lysis of microbes by causing the oxidation of cell components and denaturation of proteins.

2. Moist heat sterilization:  The killing of microbes is caused in the presence of water content. It kills microorganisms by disrupting the cell membrane, denaturing important enzymes as well as degrading nucleic acids.

Heat Removal: Freezing can slow down the growth of microorganisms but are not effective in killing them. At extremely low temperatures, slow growth of microorganisms is caused due to the absence of water.


Radiation waves of a particular frequency could cause lysis of various microorganisms. The killing of microorganisms by using harmful rays at a certain frequency is called sterilization by radiation.

The efficiency of this method higher than the other two methods mentioned.

UV Radiation: Microbial lysis is caused due to the short wavelength and high energy nature of UV light.

X-Ray: They are responsible for the generation of free radicals in the microbial cells. Free radicals are highly reactive and could result in toxic reactions of the cells intracellularly.

Filtration: Filtration can prove to be an effective measure in removing microbes for most particularly air medium. High-Efficiency Particulate Air filters are commonly used in the laboratory for a sterile supply of air.

2. Chemical methods

In these type of methodologies, chemical agents are used in order to create an imbalance in the microbial cells in order to kill them. The mechanism of action includes oxidizing the cell components, disrupting the cell membrane, denaturing proteins and enzymes and binding with cell constituents in order to inhibit their activities.
It proves to be efficient for both vegetative cells as well as bacterial spores.

Alcohol: 70% ethanol is used in laboratories in order to sterilize. Another alcohol used for the purpose of sterilization includes isopropanol. Alcohols cause effective killing machines in a short period of time.

Halides: Most commonly used halides for sterilization includes chlorine and iodine. Chlorine is an effective sterilization agent for vegetative cells but proves to be ineffective for spore cells. However, iodine could be effective in eradicating vegetative as well as spore cells.

Aldehydes: The two major aldehydes used for the purpose of sterilization includes formaldehyde and glutaraldehyde.

It is quite important to study the effectiveness of each method. You can select a method according to the strain and type of microorganisms.

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