Rapid detection of coliform bacteria (MPN method and plate method) (2)

[Sample Preparation]

Take 25mL (or 25g) sample with aseptic procedure and add it to a wide-mouth bottle (or Erlenmeyer flask) containing 225ml, sterile phosphate buffer (or saline) (preset appropriate number of glass beads in the bottle), Shake well or use a homogenizer to homogenize 1 min to 1:10 dilution at 8000) ~10000r/min

Use a 1 mL sterile pipette to draw 1.

Take another 1.

Figure 1-2 Inspection procedure of coliforms

According to the requirements of food hygiene standards or the estimation of the degree of sample contamination, select 3 suitable serial dilutions, and inoculate 3 tubes of culture medium (MPN method) or 2 plates (plate method) for each dilution

[MPN count of coliforms]

Inoculate the MUG al broth tube with the sample to be tested and the sample dilution, 1.

3 serial dilutions, each dilution is inoculated with 3 tubes of culture medium

Place the inoculated culture tube in a (37±1)℃ incubator for 18~24h

Place the culture tube in a dark place and irradiate it with an ultraviolet light with a wavelength of 366nm.

Result report: According to the number of coliform positive tubes, check the MPN table (see Table 1-2) and report the MPN value of coliforms per 100mL(g) of food

Table 1-2 The most probable number of coliforms in the MPN chair seat table

［Count of colonies of coliforms］

Use a sterile pipette to draw 1.

Pour 15mL of Aliz-gal agar at 45 ~ 50℃ into each sample plate, and turn the plate gently to make it evenly mixed

After the agar has solidified, turn the plate over and incubate in a (37 + 1) ℃ incubator for 18-24 hours

[Result report]

When the number of purple (or red) colonies on the plate is not more than 150, and at least one of the plate has not less than 15 purple (or red) colonies, the number of coliforms is calculated as follows:

In the formula, N—the number of coliforms in the sample, unit/mL or g;

Σ C—the sum of the number of purple (or red) colonies on all counting plates;

n ₁ -the number of plates with the lowest dilution factor counted;

n ₂ ——the number of plates with the higher dilution factor counted;

d——The lowest dilution of the sample for counting (such as 10 ^-1 , 10 ^-2 , 10 ^-3, etc.

If the number of purple (or red) colonies on all (3) diluted samples is inoculated on the plate is less than 15, the calculation is still based on the above formula, but the result should be added with a "*" sign next to it to indicate the estimated value

If the number of purple (or red) colonies on the plates of the undiluted sample and all the diluted samples is less than 15, the report result is: less than 15 coliforms per milliliter (g) of the sample
.

If the number of purple (or red) colonies is not found on the plate of the undiluted sample and all the diluted samples, the report result is: less than 1 coliform per milliliter (g) of the sample
.

If the number of purple (or red) colonies on the plate is more than 150, calculate according to the above formula, and add "*" beside the result to indicate the estimated value or select a higher dilution factor for determination as appropriate
.

Related links: Rapid detection of coliform bacteria (MPN method and plate method) (1)