Physical methods of controlling Microbial growth, Biology tutorial


Most of the microorganisms are advantageous to human beings; though, most have undesirable effects. They are the main reason for food spoilage and numerous diseases to animal, plants and human. To control and slow down the growth and activities of injurious microorganisms that is capable of causing diseases and contaminating water, food and substances utilized.

Definition of Sterilization:

Sterilization is the procedure by which all living cells, spores and acellular entities (example: viruses, viroids and prions) are either destroyed or eliminated from an object or habitat.  

It can as well be stated as the killing or elimination of all viable organisms in a growth medium.

A sterile object is completely free of viable microorganisms, spores and other infectious agents.

Pattern of Microbial Death:

Microbial population death is logarithmic or exponential, meaning the population will be decreased by the similar fraction at constant interval. When the logarithm of the population remaining is plotted against the time of exposure to the agent, a straight-line plot will answer. A bacterium is often stated as dead if it is not viable and doesn't grow and reproduce if inoculated to a culture medium which generally supports its growth.

Heat Sterilization:

1) The most general sterilization method employed for controlling and destroying the microbial growth is the utilization of heat.

2) Heat can kill microorganisms through denaturing the enzymes that prevent them from multiplying.

3) At temperature surpassing the maximal growth temperature, the death rate surpasses the growth rate.

4) Factors which find out the effectiveness of heat sterilization comprise the temperature and duration of the heat treatment and whether the heat is dry or moist.

Measuring Heat Sterilization:

All the microorganisms encompass a maximum growth temperature beyond which viability reduces. Viability is lost since at very high temperatures most of the macromolecules lose structure and function, a process termed as Denaturation. The efficiency of heat as a sterilant is evaluated by the time needed for a tenfold reduction in the viability of a microbial population at a given temperature. It is the decimal reduction time. The time and temperature, thus, should be adjusted to accomplish sterilization for each particular set of condition. Moreover, the kind of heat is as well significant. Moist heat has been observed to possess better penetrating power than dry heat and, at a given temperature, generates a faster reduction in the number of living organisms.

The decimal reduction time determination needs a large number of viable count measurements. A simpler way to characterize the heat sensitivity of an organism is to compute the thermal death point that is, the time it takes to kill all cells at a specific temperature. Thermal death point (TDP) is the lowest temperature needed to kill all the microorganisms in a liquid suspension in 10 minutes. Heat is most broadly applicable and efficient agent for killing the microorganisms and as well the most economical and effortlessly controlled. To find out the thermal death time, samples of a cell suspension are heated for various times, mixed by a culture medium and incubated. When all the cells have been killed, no growth is noticed in the incubated samples. The thermal death time based on the size of the population tested; a longer time is needed to kill all the cells in a big population than in a small one.

Moist Heat Sterilization:

Moist heat readily kills bacteria, viruses and fungi. It kills by degrading nucleic acids and by denaturing enzymes and other necessary proteins. Exposure of micro-organisms to boiling water will destroy vegetable cells and eukaryotic spores. Though, this temperature will not destroy bacterial endospores.

In order to kill or destroy bacterial endospores moist heat sterilization above 100oC using saturated steam under pressure is done. The steam sterilization is taken out by an autoclave.

The Autoclave:

The autoclave is a sealed heating machine or device which lets the entrance of steam under pressure. The killing of heat-resistant endospores needs heating at temperatures over 100oC (that is, the boiling point of water at normal atmospheric pressure). This is achieved by applying steam beneath pressure at a temperature of 121oC.

The materials to be sterilized are positioned in a chamber and the chamber is sealed. Steam is transferred from the jacket to the chamber forcing out all the air. The steam is held in the chamber for around 15 minutes at 121oC and then expelled from the chamber.

When an object being sterilized is bulky, heat transfer to the interior is retarded and the net heating time should be expanded to make sure that the whole object is at 121oC for 10 to 15 minutes. Extended times are as well needed if large volumes of liquid are being autoclaved since large volumes take longer time to reach sterilization temperatures. It should be noted that it is not the pressure within the autoclave that kills the micro-organisms though the high temperature which can be accomplished if steam is applied under pressure. 

Dry Heat Sterilization:

This is a process of heat sterilization in which the objects or materials are sterilized in the deficiency of water. A few items are sterilized through incineration, for illustration; inoculating loops employed in the laboratory throughout the culturing of bacteria can be sterilized in a small bench top incinerator.

The utilization of an oven at a temperature of 150 to 160oC for 2 to 3 hours can as well be utilized to sterilize the glass wares like pipettes, and test-tubes in the laboratories.

Dry heat kills the microorganisms through the oxidation of the cell components and Denaturation of proteins.

This process of sterilization consists of some definite benefits, it doesn't corrode glassware and metal instruments as moist heat does and it can be employed to sterilize powders, oils and other identical items. Though, it is much slow and less efficient than moist heat. For illustration: the spores of Clostridium botulinum the organism which causes botulism are killed in 5 minutes at 121oC by moist heat however just after two hours at 160oC by dry heat.

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