M-EMB Broth

M-EMB Broth is a selective differential medium for the detection of members of the coliform group by the membrane filter technique.

Composition**

Final pH (at 25°C): 7.2±0.2

**Formula adjusted, standardized to suit performance parameters

Directions

Suspend 84.33 grams in 1000 ml distilled water. Heat, if necessary to dissolve the medium completely. Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes. Autoclave sterilization is not required if the medium is used the same day of preparation.

Principle And Interpretation

The membrane filter technique is extremely useful in monitoring water samples. As related to the membrane filter technique, the coliform group is defined as those facultative anaerobic, gram-negative, non-spore-forming, rod-shaped bacteria that develop red colonies with a metallic sheen within 24 hours at 35-37°C on an Endo-type medium containing lactose (1). M-EMB Broth is a selective differential medium used for the detection of members of the coliform group by membrane filtration. M-EMB Broth is formulated as per Clark et al (2).

Proteose peptone and yeast extract serve as sources of carbonaceous and nitrogenous growth nutrients. Lactose is the fermentable carbohydrate and energy source. Bile salts mixture helps to inhibit the accompanying non-coliforms. Sodium chloride maintains the osmotic equilibrium of the medium while dipotassium phosphate buffers the medium. Eosin-Y and methylene blue serve as the indicator system in the medium. The two dyes combine to form a precipitate at acidic pH, due to lactose fermentation. Eosin Y also serves as an inhibitor (3).

Membrane filters through which the test water sample has been passed are initially enriched for 2 hours by incubating the filter on M-Enrichment Broth (M1109). These enriched cultures (filters) are then transferred onto sterile absorbent cotton pads saturated with 2 ml of M-EMB Broth (M1105). Incubation is done at 35-37°C for 18-24 hours. Lactose fermenting coliforms produce pink coloured colonies while non-lactose fermenting coliforms will form colourless colonies.

Quality Control

Appearance Pinkish purple to purple homogeneous free flowing powder

Colour and Clarity of prepared medium Green with orange cast opalescent solution with a flocculent precipitate

Reaction Reaction of 8.43% w/v aqueous solution at 25°C. pH: 7.2±0.2

pH 7.00-7.40

Cultural Response

M1105: Cultural characteristics observed after an incubation at 35-37°C for 18-24 hours.

Storage and Shelf Life

Store below 30°C in tightly closed container and the prepared medium at 2-8°C. Use before expiry date on the label.

Reference

1. Eaton A. D., Clesceri L. S. and Greenberg A. E., (Eds.), 1998, Standard Methods for the Examination of water and Waste water, 20th Ed., American Public Health Association, Washington, D.C.
2. Clark H. F., Geldreich E. E., Jeter M. L. and Kabler P. W., Public Health Rep., 66, 951
3. MacFaddin J. F., 1985, Media for Isolation-Cultivation-Identification-Maintenance of Medical Bacteria, Vol. 1, Williams and Wilkins, Baltimore.