[Steps]
1. Water sample taking
(1) Sterilize the tap water with a flame for 3 minutes, then open the tap to let the water flow for 5 minutes, and then take the water sample with a sterilized Erlenmeyer flask for analysis.
(2) Pool water, river water or lake water should take a deep water sample 10-15 cm from the surface of the water. First, immerse the sterilized bottle with glass stopper into the water, then turn it over, remove the glass stopper, and the water will flow into the bottle. After it is full, cover the cork and take it out of the water. It is best to check it immediately, otherwise it needs to be stored in the refrigerator.
2. Determination of the total number of bacteria
(1) Tap water
(A) Use a sterile straw to suck 1ml of water sample and pour it into a sterile petri dish. Make two plates in total.
(B) Pour about 15ml of meat extract peptone agar medium that has been melted and cooled to about 45°C, and immediately shake it on the table to make the water sample and the medium thoroughly mixed.
(C) Take another empty sterile petri dish and pour 15ml meat extract peptone agar medium as a blank control.
(D) After the medium has solidified, place it upside down in a 37°C incubator, incubate for 24 hours, and count the colonies.
The average number of colonies on two plates is the total number of bacteria in 1ml of water sample.
(2) Pool water, river water or lake water, etc.
(A) Dilute the water sample Take 3 sterile empty test tubes and add 9ml sterile water to each. Take 1ml of water sample into the first tube of 9ml of sterile water, shake it, and then take 1ml from the first tube to the next tube of sterile water, and then dilute to the third tube with dilutions of 10-1 and 10- 2 and 10-3. The dilution factor depends on the degree of turbidity of the water sample. The most suitable dilution is that the number of colonies on the plate after culture is between 30 and 300. If the number of bacteria in the three dilutions is too large to be counted or too small to be counted, then Need to continue to dilute or reduce the dilution factor. Generally, three serial dilutions of 10-1, 10-2, and 10-3 are used for moderately polluted water samples, and three serial dilutions of 10-2, 10-3, and 10-4 are used for severely polluted water samples.
(B) Take 1ml of dilution water from each of the last three dilution test tubes and add it to an empty sterile petri dish. Make two petri dishes for each dilution.
(C) Pour 15ml of meat extract peptone agar medium that has been melted and cooled to about 45°C, and immediately put it on the table and shake it well.
(D) After solidification, place it upside down in a 37°C incubator and incubate for 24 hours.
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Post time: Dec-15-2021