Lactose Glucuronide m-Agar (mlGA)

Peptone and proteose peptone provide nitrogen, amino acids and minerals. Yeast extract is a source of vitamins, particularly of the B group. Lactose is the source of carbon and energy, and when fermented, the pH indicator phenol red promotes the appearance of yellow colonies due to the acid production. Sodium pyruvate protects injured cells to enhance the growth of coliforms. Sodium lauryl sulfate is the selective agent which inhibits Gram-positive organisms. E. coli absorbs the chromogenic agent X-beta-D-glucuronide, and the intracellular glucuronidase enzyme activity breaks the bond between the chromophore and the glucuronide. The released chromophore is colored and builds up within the cells, causing the E. coli colonies to be green, in combination with the lactose fermentation. Bacteriological agar is the solidifying agent.

To sum up, the identification of Escherichia coli and coliforms is facilitated through two biochemical reactions within the medium: lactose fermentation is detected by the dye phenol red, which gives yellow colonies when acid is produced. The chromogenic substrate X-Glucuronide is cleaved by the enzyme glucuronidase and produces a blue chromophore which builds up in the bacterial cells. Coliforms are lactose-positive so colonies will be yellow; Escherichia coli is both lactose-positive and possesses glucuronidase so will appear as green colonies.

Typical formula in g/L, adjusted and/or supplemented as required to meet performance criteria.

Preparation

Suspend 88 grams of the medium in one liter of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Sterilize in autoclave at 121 °C for 15 minutes and dispense in Petri dishes.

Instructions for use

Membrane filtration method:

1. Filter the water sample through a membrane filter (47 nm diameter, 0.45 µm pore size).
2. The volume and dilution of water filtered should be chosen so that the number of colonies to be counted on the membrane is 20-80.
3. Place the membrane filter onto the Lactose Glucuronide m-Agar (MLGA), ensuring that no air bubbles are trapped under the membrane.
4. Incubate the plates at 30 °C for 4 hours, then at 37 °C for 14 hours.

• Mara, D. D. 1973. A single medium for the rapid detection of Escherichia coli at 44 °C. J. Hyg. 71:783-785.
• Pugsley, A. P., L. J. Evision, and A. James. 1973. A simple technique for the differentiation of Escherichia coli in water examination. Water Res. 7:1431-1437.
• The Environment Agency - Methods for Examination of Waters and Associated Material - The Microbiology of Drinking Water 2002.