Agar based medium, as a fundamental tool in microbiology experiments, plays a crucial role in bacterial culture, identification, isolation, and preservation due to its physical and chemical properties. This article will provide a detailed exposition on the definition, preparation, use, and storage of agar based medium.
Agar based medium, is a terminology published in 2019 in the field of tuberculosis. It is a solid culture medium using agar as an excipient, providing a stable growth environment for microorganisms through the solidifying action of agar. Agar powder is generally white, pale yellow, or light gray in powder form. It is insoluble in cold water but dissolves when boiled or autoclaved and solidifies upon cooling.
The preparation process of agar based medium is relatively complex, but every step is crucial. Here are the basic steps for preparing an agar based medium:
Preparation of raw materials
According to experimental needs, measure an appropriate amount of agar powder, peptone, yeast extract, glucose, etc.
Dissolving and mixing
Add the raw materials into an appropriate amount of purified water, stir well to ensure no lumps. Note that the water temperature should not be too high to avoid hardening of the agar surface due to water absorption.
Adjusting pH
Use acidic or alkaline solutions to adjust the pH of the medium to an appropriate range, usually around 7.0.
Heating and sterilizing
Sterilize the well-mixed medium at high temperatures and pressure to kill any bacteria and other microorganisms. Typical sterilization conditions are 121°C for 15-20 minutes.
Dispensing and solidifying
Dispense the sterilized medium into test tubes or Petri dishes and let it cool and solidify.
Agar based medium is widely used in microbiology experiments. Here are its main applications:
Bacterial culture
Inoculate the bacteria to be cultured onto the agar medium using streaking, spot inoculation, or smearing techniques to allow bacteria to grow and form colonies. By observing the morphology, size, color, and other characteristics of the colonies, bacteria can be further identified and isolated.
Colony counting
Use agar medium for bacterial counting. By counting the number of colonies on the medium, the bacterial content in the sample can be evaluated.
Strain preservation
Agar medium can also be used for long-term preservation of strains. By adjusting the amount of agar, semi-solid media suitable for strain preservation can be prepared.
Proper storage of agar based medium is critical to maintaining its quality and prolonging its shelf life. Here are some storage considerations:
Storage temperature
Purchased disposable bacterial culture plates should generally be stored in a 4°C refrigerator, though some literature suggests that room temperature storage may be more beneficial for bacterial culture medium. However, for opened packages or homemade media, long-term room temperature storage should be avoided to prevent bacterial contamination.
Storage duration
The storage time depends on factors such as the sealing degree of the plate, the thickness of the medium, and the refrigerator environment. Purchased disposable media usually have sealed packaging bags, with a storage time ranging from one month to six months. Opened packages or homemade media have shorter storage times, usually less than half a month.
Storage method
The medium should be stored upside down to prevent condensation droplets from falling onto the medium. Additionally, the medium should be placed in a refrigerator area away from fans, bacterial cultures, and water vapor circulation to avoid moisture evaporation and mold growth.
Inspection
Before use, carefully observe the medium for cracks or contamination by foreign bacteria. If abnormalities are found, promptly process and replace with new medium.
As a fundamental tool in microbiology experiments, the preparation, use, and storage of agar based medium must strictly follow relevant operating procedures. By scientifically and rationally preparing and storing the media, the quality and stability of the agar based medium can be ensured, providing reliable support for microbiology experiments.
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