1 . Scope
This standard specifies a method for using Compact Dry EC discs to rapidly enumerate 1) Escherichia coli (excluding strain E. coli O157) and total coliforms in food.
Annex A provides information on the method validation results and on the results of the collaborative studies performed by AOAC [9],[13],[14], NMKL - NordVal [11] and Microval [12]
2. Terms and definitions
Escherichia coli ( E. Coli )
E. coli is a bacterium which at 37 ºC forms typical blue or violet colonies on the medium under the conditions specified in this standard
Coliform (coliforms) Bacteria which at 37 ºC form typical blue, violet, pink or red colonies on the medium under the conditions specified in this standard
Colony
A visible mass of microorganisms growing on or within a solid nutrient medium, developed from a single living cell
3. Principle
Use a ready-to-use medium disc containing culture medium, a gelling agent and two colour-forming enzyme substrates. The disc containing the medium is moistened with the prepared test sample, after which the gelling agent solidifies the medium. Incubate the medium containing the test sample at 37 ºC ± 1 ºC for 24 h ± 2 h. E. coli forms blue or violet colonies, and coliform bacteria other than E. coli form red or pink colonies.
4. What is the Compact Dry EC disc?
The Compact Dry EC disc consists of a mixture of nutrients (peptone, potassium nitrate, sodium chloride and sodium pyruvate), phosphate buffer, selective agents against Gram-positive bacteria, a gelling agent (polysaccharide derived from agar) and two colour-forming enzyme substrates, 5-bromo-6-chloro-3-indoxyl-beta-D galactopyranoside (Magenta-GAL) and the cyclohexylammonium salt of 5-bromo-4-chloro-3-indoxyl-beta-D-glucuronic acid (X-Gluc). The disc measures 20 cm2, and the reverse side of the disc has a square grid of 1 cm × 1 cm and 0,5 cm × 0,5 cm to make colony counting easier. Store the disc at a temperature from 1 ºC to 30 ºC.
5. Method validation results and collaborative study
Method validation studies of the method specified in this standard were first carried out by AOAC in 2006 [13], supplemented in 2016 [14] and 2020 [9].
Repeatability assessment for E. coli: The method has a repeatability with R2 = 0.97. For Coliforms, this result is 0.99.
A method validation study of the method for enumerating E. coli and coliforms specified in this standard compared with the reference method carried out by NMKL - NordVal International in 2008 and supplemented in 2020 [11] has also been carried out,
A method validation study of the method for enumerating coliforms specified in this standard compared with the reference method given in TCVN 6848:2007 (ISO 4832:2007)[3] carried out by Microval in 2019 was also undertaken.
The test results confirming the validity, accuracy and repeatability of the method: Readers and customers are welcome to refer to TCVN 13875:2023.
