1. Principle & application scope
Coliform is one of the crucial indicators to evaluate the sanitary quality of food and water. It has been widely used for the detection of food hygiene. Coliforms mostly exist in feces of warm-blooded animals and the places that contaminated by feces. The quantity of coliform indicates the level of food contamination in the process of food production. Coliform count plate contains selective medium, specific chromogenic agent of galactosidase and macromolecule bibulous gel. This product is suitable for counting the quantity of coliform in food, drinking water and raw material; also it’s suitable for hygiene detection of instrument surface.
Executive standard: National food safety standard
Food microbiological examination: Enumeration of coliforms (GB 4789.3)
2. Using method
2.1 Sample preparation: Prepare the 1:10 solution by placing 25mL (g) of sample into 225mL of sterilized phosphate buffer solution or sterilized saline, then prepare 1:100 solution by sucking 1mL of solution (1:10) into 9mL of sterilized phosphate buffer solution or sterilized saline.
2.2 Inoculation: Select 2~3 appropriate diluted solution to do the test for ordinary food, while for some liquid sample which has little coliform, directly suck original liquid to do the test. Place coliform count plate onto sterile experiment table, uncover the film on the surface. Suck 1mL of solution with sterile pipette, and then drip it slowly and evenly onto the count plate, cover the film gently and stand it for 10s to let the medium solidify. Inoculate two count plates for each dilution, and in the meantime do a blank control.
2.3 Culture: Superimpose these count plates and place them back into original ziplock bag. Seal it well and put it in the incubator at 36℃±1℃ for 15~24hs with the transparent side up. The quantity of superimposed plates should not be more than 12.
3. Results
3.1 Those show blue spots after culture are coliforms. Select the count plates of which the colony quantity is between 15CFU and 150CFU to do the count.
3.2 If the colony numbers on count plates from two serial dilutions are both between 15CFU and 150CFU, multiply the average number of colonies by corresponding dilution multiple as the result data.
3.3 If the colony numbers on count plates from all the dilutions are less than 15CFU, multiply the average number of colonies on the minimum dilution multiple count plates by minimum dilution multiple as result data.
3.4 If no colony was found on all the count plates, report the result in terms of less than one multiplied minimum dilution multiple.
3.5 If the colony numbers on count plates from all the different dilutions are more than 150CFU, count the average number of colonies on count plates of maximum dilution, then multiply it by the maximum dilution multiple as result data. Or count colonies in one or two squares having representative colony distribution to get the average number of colonies per square, and then multiply this number by 20 (the count plate area is 20cm2) as report data.
Use CFU/mL or CFU/g as report unit.
4. Additional explanatory note
4.1 Coliform count plate can shorten the detecting time, and the results are consistent with the traditional method, the coincidence rate reaches 90%.
4.2 The PH value of sample solution should be between 6.5 and 7.5, adjust the PH value with 1mol/L NaOH or 1mol/L HCl if necessary.
5. Preservation condition
This kind of count plate should be placed in the fridge at 4℃-10℃ and the expiration time is one year. Once the package is opened, those unused count plates should be placed back to the package and then seal it well. Store them in the fridge and exhaust them within one month. Condensation may occur in high humid environment, it is better to warm it to room temperature before unfolding the package.
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