Selection and identification of media
Most selective media have the ability to identify, and can distinguish the target microorganisms (that is, the tested microorganisms) grown on the media from other microorganisms. Select some suspicious colonies (the colonies have the typical identification characteristics of 3 standard microorganisms) to do the confirmation test. The standard method adopted by the International Organization for Standardization (ISO) usually stipulates that five colonies are selected from the plate, and the selective medium is used for the test .
It should be noted that the method of selecting only typical colonies for confirmation and identification tests often results in those atypical colonies being ignored. For example, when selecting Salmonella, if only colonies that do not ferment sucrose and lactose are selected from the culture medium, the Ignore some other bacteria in the genus Salmonella, so it is impossible to understand the frequency and replayability of these bacteria.
Obligate anaerobic bacteria must be cultivated in an anaerobic environment. This environment can be obtained either by using a natural medium containing reducing substances, or by adding various reducing substances (such as glucose and cysteine) to the medium. The storage time of the medium should not be too long. The dissolved oxygen in the medium can be removed by heating, the oxygen in the air can be isolated by sealing, or by the solidification of the deep agar medium, or the bacteria can be inoculated in an oxygen-free environment. A common method in actual operation is to use physical and chemical methods in combination to obtain anaerobic conditions.
Some very demanding anaerobic bacteria, especially the non-spore-forming bacteria of the genus Bacillus, require a complete anaerobic environment, otherwise these bacteria will die. For such microorganisms, special techniques are best used in sample processing, culture and medium preparation. However, because such anaerobic bacteria (usually intestinal bacteria) are extremely sensitive to oxygen and will die quickly in many foods, food microbiologists have not conducted extensive research on them.
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