The coliform bacteria are mostly found in the feces of warm-blooded animals, places where humans are often active, and places where feces are contaminated. The pollution of human and animal manure to the external environment is the main reason for the presence of coliforms in nature. Coliform is one of the important indicators for evaluating the quality of food hygiene. Among the methods for detecting coliforms in food, the domestically used method for detecting coliforms of imported and exported foods mainly has national standards, the national standard three-step nine-tube method, namely lactose fermentation test, separation culture, and confirmatory test.
Since the coliform group refers to a group of bacteria associated with fecal contamination with certain characteristics, namely, aerobic and facultative anaerobic, Gram-negative Bacillus sp. which can decompose lactose-producing gas at 37 °C. Therefore, the detection of coliforms is generally carried out according to its definition.
1. Lactose fermentation test: After the sample is diluted, three dilutions are selected, and each dilution is inoculated with three tubes of lactose bile salt fermentation tube. Incubate at 36±1°C for 48±2h to observe whether gas is produced.
2. Isolation and culture: The gas-producing fermentation tube culture was transferred to Eosin blue agar plate, cultured at 36±1°C for 18-24 hours, and the colony morphology was observed.
3. Confirmation test: Suspicious colonies on the plate were picked and observed by Gram staining. At the same time, the lactose fermentation tube was inoculated at 36±1°C for 24±2h, and the gas production was observed.
The lactose fermentation test work, can often be seen in the inverted tube fermentation at very small bubbles (sometimes smaller than the small grain), small bubbles may sometimes be encountered in the early acidogenic fermentation or slowly along the wall has a floating . Experiments have shown that the gas production of coliforms can cause the fermentation tube to be filled with gas, and the smaller ones can produce smaller bubbles than the millet grains.
During the test, an autoclave is used for sterilization. When the nine-tube method is used, coliforms will generate bubbles at high temperatures. If an ALP high-pressure sterilization pot is used, it can be accessed through the engineer menu. Adjusting the exhaust time during the heating process can effectively remove the air bubbles remaining in the medium.
Initial interface
Sterilized
End of sterilization
Bubbles in the medium
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