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The two main types of autoclaves differ in the way that air is removed from the chamber. In general, the air in the chamber of an autoclave is removed and replaced with increasing amounts of steam trapped within the enclosed chamber, resulting in increased interior pressure and temperatures above the boiling point of water. Outside laboratory and clinical settings, large industrial autoclaves called retort s allow for moist-heat sterilization on a large scale. The autoclave is still considered the most effective method of sterilization. Charles Chamberland (1851–1908) designed the modern autoclave in 1879 while working in the laboratory of Louis Pasteur. They are used to raise temperatures above the boiling point of water to sterilize items such as surgical equipment from vegetative cells, viruses, and especially endospores, which are known to survive boiling temperatures, without damaging the items. (credit a: modification of work by Anh-Hue Tu credit b: modification of work by Brian Forster) AutoclavesĪutoclaves rely on moist-heat sterilization. These are examples of dry-heat sterilization by the direct application of high heat capable of incineration. (b) Alternatively, a bactericinerator may be used to reduce aerosolization of microbes and remove the presence of an open flame in the laboratory. (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. However, moist-heat sterilization is typically the more effective protocol because it penetrates cells better than dry heat does.įigure 1. Dry heat can also be applied for relatively long periods of time (at least 2 hours) at temperatures up to 170 ☌ by using a dry-heat sterilizer, such as an oven. Incineration at very high temperatures destroys all microorganisms.
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Aseptic technique in the laboratory typically involves some dry-heat sterilization protocols using direct application of high heat, such as sterilizing inoculating loops. 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. For these reasons, boiling is not considered a useful sterilization technique in the laboratory or clinical setting. 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. However, boiling is less effective at killing endospores some endospores are able to survive up to 20 hours of boiling. 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. These parameters are often used to describe sterilization procedures that use high heat, such as autoclaving. 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. Different microorganisms will respond differently to high temperatures, with some (e.g., endospore-formers such as C.
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The thermal death point (TDP) of a microorganism is the lowest temperature at which all microbes are killed in a 10-minute exposure. Heat can kill microbes by altering their membranes and denaturing proteins.
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It is used in simple techniques like cooking and canning. Heating is one of the most common-and oldest-forms of microbial control. Various physical methods used for microbial control are described in this section. 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. Common control methods include the application of high temperatures, radiation, filtration, and desiccation (drying), among others. Understand and compare various physical methods of controlling microbial growth, including heating, refrigeration, freezing, high-pressure treatment, desiccation, lyophilization, irradiation, and filtrationįor thousands of years, humans have used various physical methods of microbial control for food preservation.