MPN method and CFU method in microbiological examination - Database & Sql Blog Articles

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The MPN (most probable number) method, the most approximate number method, also known as the maximum possible number method, is a commonly used method in food testing. Its biochemical reaction is based on lactose fermentation, Gram-negative bacteria such as Bacillus, such as coliforms. Generally, lactose is fermented at 37 ° C for 24 hours to produce acid and produce gas. Using this principle, the test solution is added to the fermentation medium and cultured at 37 ° C for 24 hours. If no gas is produced, the control bacteria are reported to be negative; if gas is produced, Then, it was inoculated on the plate and cultured at 37 °C for 24 hours. The colony morphology, Gram staining and re-fermentation were observed.

Advantages: easy to operate, high sensitivity, short time, can directly observe the test tube to conclude that if the fermentation medium does not produce gas, the control bacteria can be reported negative for 24h.

Disadvantages: The sample is required to have uniformity, suitable for testing liquid samples, and can only be used to test the strains that produce gas in the fermentation medium. Note: The appropriate dilution and inoculum should be selected.

The CFU (colony forming units) method is a commonly used method in food and drug testing. The principle is the number of viable bacteria contained in lg or lml samples, and the growth is calculated by culturing the treated samples under certain conditions. The number of colonies (CFU), that is, the number of colonies formed on a solid medium in a certain volume of bacterial culture solution. The liquid sample can be directly diluted into a test solution, and the solid sample should be soaked in sterile physiological saline. Test solution. Pipette 1ml of test solution with different dilutions, put it into a sterilized plate, and pour the melted nutrient agar. After the agar is solidified, turn the plate and incubate at 37 °C for 4 days to calculate the number of colonies in the plate. CFU The method is widely used, but the inspection process must be strictly sterile to prevent contamination of the bacteria, and the inspection takes a long time, and it is also necessary to select the appropriate dilution and inoculum.