Minerals Modified Glutamate Broth (MMBG) ISO

In comparison to other media, this broth is better as it gives fewer false positive results. Sodium glutamate and sodium formate are the basis of the medium for the enumeration of coliform organisms in water. Lactose is the source of carbohydrates. The addition of vitamins, amino acids and magnesium sulfate heptahydrate increases fermentation, whereas the addition of ferric ammonium citrate permits an increase of gas production. Bromocresol purple is a pH indicator.

Preparation

Suspend 17.7 grams of the medium in one liter of distilled water. Add 2.5 grams of ammonium chloride. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Dispense into appropriate containers and sterilize in autoclave at 116 °C for 10 minutes.

According to ISO 16649

Enumeration of beta-glucuronidase-positive Escherichia coli according to ISO 16649:

1. Inoculate three tubes of a single strength and three tubes of a double strength medium.
2. The larger volumes of sample (10 mL) are added to equal volumes of double strength medium, whereas the 1 mL volumes (or dilutions of them) are added to 5 mL of single strength medium.
3. Incubate the tubes at 37 ± 1 °C and examine after 24 ± 2 hours for the acid production (lactose fermentation).
4. Each tube of selective enrichment medium showing acid production (yellow color) is subcultured to TBX Chromogenic Agar (LC-1151).
5. Incubate the TBX plates at 44 ± 1 °C for 22 ± 2 hours.
6. The presence of characteristic blue colonies on TBX indicates the presence of Escherichia coli in the original MMGB tube.

Those strains of Escherichia coli that do not grow at 44 °C and, in particular, those that are beta-glucuronide negative, such as Escherichia coli O157:H7, will not be detected.

• ISO 16649-3 Microbiology of food and animal feeding stuffs - Horizontal method for the enumeration of beta-glucuronidase-positive Escherichia coli - Part 3: Most probable number technique using 5-bromo-4-chloro-3-indolyl-beta-D-glucuronide.
• Departments of the Environment, Health & Social Security, and P.H.L.S. 1982. The bacteriological examination of drinking water supplies. Report on public Health and Medical Subjects No. 71., H.M.S.O., London, England.