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Learning objectives

  • Understand and compare various physical methods of controlling microbial growth, including heating, refrigeration, freezing, high-pressure treatment, desiccation, lyophilization, irradiation, and filtration

For thousands of years, humans have used various physical methods of microbial control for food preservation . Common control methods include the application of high temperatures, radiation, filtration, and desiccation (drying), among others. Many of these methods nonspecifically kill cells by disrupting membranes, changing membrane permeability, or damaging proteins and nucleic acids by denaturation, degradation, or chemical modification. Various physical methods used for microbial control are described in this section.

Heat

Heating is one of the most common—and oldest—forms of microbial control. It is used in simple techniques like cooking and canning . Heat can kill microbes by altering their membranes and denaturing proteins. The thermal death point (TDP) of a microorganism is the lowest temperature at which all microbes are killed in a 10-minute exposure. Different microorganisms will respond differently to high temperatures, with some (e.g., endospore-formers such as C. botulinum ) being more heat tolerant. A similar parameter, the thermal death time (TDT) , is the length of time needed to kill all microorganisms in a sample at a given temperature. These parameters are often used to describe sterilization procedures that use high heat, such as autoclaving . Boiling is one of the oldest methods of moist-heat control of microbes, and it is typically quite effective at killing vegetative cells and some viruses. However, boiling is less effective at killing endospores; some endospores are able to survive up to 20 hours of boiling. Additionally, boiling may be less effective at higher altitudes, where the boiling point of water is lower and the boiling time needed to kill microbes is therefore longer. For these reasons, boiling is not considered a useful sterilization technique in the laboratory or clinical setting.

Many different heating protocols can be used for sterilization in the laboratory or clinic, and these protocols can be broken down into two main categories: dry-heat sterilization and moist-heat sterilization . Aseptic technique in the laboratory typically involves some dry-heat sterilization protocols using direct application of high heat, such as sterilizing inoculating loops ( [link] ). Incineration at very high temperatures destroys all microorganisms. Dry heat can also be applied for relatively long periods of time (at least 2 hours) at temperatures up to 170 °C by using a dry-heat sterilizer, such as an oven. However, moist-heat sterilization is typically the more effective protocol because it penetrates cells better than dry heat does.

a) A picture of a hand holding a loop over an open flame. A machine with a wire metal cylinder that fits a loop in the center.
(a) Sterilizing a loop, often referred to as “flaming a loop,” is a common component of aseptic technique in the microbiology laboratory and is used to incinerate any microorganisms on the loop. (b) Alternatively, a bactericinerator may be used to reduce aerosolization of microbes and remove the presence of an open flame in the laboratory. These are examples of dry-heat sterilization by the direct application of high heat capable of incineration. (credit a: modification of work by Anh-Hue Tu; credit b: modification of work by Brian Forster)

Questions & Answers

scope of microbiology
Mahira Reply
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Abdullahi
which microbals are likely to be find on the skin but they don't cause infection.
osward Reply
what is the difference between prokaryotic and eukaryotic?
Brian Reply
how many types of cell
Brian Reply
Two PLANT AND ANIMAL CELL
Shafi
actually we have two major types eukaryotic and prokaryotic eukaryotic may me unicellular as yeast or multicellular as plant ,animal, algae,fungi prokaryotic as bacteria
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natural
It's made of many bonds and proteins that are used to block
Lee
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ANTHONY
the history of cell theory.
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Hussaina
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Khaled
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Khaled
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e coli staphylococcus streptococcus pseudomomas
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What are the convectional methods for microorganism detection
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Margrete
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Kamaluddeen
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Williams
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stephen
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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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