MacConkey Broth w/ Neutral Red

MacConkey Broth w/ Neutral Red is used for selective enrichment and enumeration of coliforms.

Composition

Final pH (at 25°C): 7.5±0.2

Formula adjusted, standardized to suit performance parameters

Directions

Suspend 40.07 grams in 1000 ml distilled water. Heat if necessary to dissolve the medium completely. Distribute into test tubes with inverted Durham tubes and sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes. Cool the tubes before inoculation.

Principle And Interpretation

MacConkey Broth is the earliest selective and differential medium for cultivation of enteric microorganisms from a variety of clinical specimens (4,5). Subsequently MacConkey Broth have been recommended for use in microbiological examination of foodstuffs (6) and for direct inoculation of water samples for coliform counts (7). This medium is also accepted by the Standard Methods for the Examination of Milk and Dairy Products (8) and pharmaceutical preparations (9).

MacConkey Broth w/ Neutral Red is recommended by BIS under the specification IS:5887 (Part I) 1976 for Detection of Bacteria responsible for food poisoning Part I: for Isolation, Identification and Enumeration of Escherichia coli. This culture media is considered as a standard medium for the primary isolation as well as presumptive identification of coliaerogenes group in food and water which contains neutral red as an indicator (1-3).

Bile salt inhibits most species of gram positive bacteria. Gram-negative bacteria usually grow well on the medium and are differentiated by their ability to ferment lactose. Lactose fermenting strains grow as red. The red colour is due to production of acid from lactose, absorption of neutral red and a subsequent colour change of the dye when the pH of medium falls below 6.8. Lactose non-fermenting strains, such as Shigella and Salmonella are colourless and typically do not alter appearance of the medium.

Quality Control

Appearance: Pale yellow to pink coloured homogeneous free flowing powder

Colour and Clarity of prepared medium: Red clear solution without any precipitate

Reaction: Reaction of 4.0% w/v aqueous solution at 25°C. pH: 7.5±0.2

pH: 7.30-7.70

Cultural Response

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. Bureau of Indian Standards IS: 5401 - 1969.
2. Bureau of Indian Standards IS: 5887 (Part I) - 1976, reaffirm 1986.
3. Bureau of Indian Standards IS: 5887 (Part II) - 1976, reaffirm 1986.
4. MacConkey, 1900, The Lancet, ii:20.
5. MacConkey, 1905, J. Hyg., 5:333.
6. Speck M. (Ed.), 1985, Compendium of Methods for the Microbiological Examination of Foods, 2nd ed., APHA, Washington, D.C.
7. Greenberg A. E., Clesceri L. S. and Eaton A. D., (Eds.), 1992, Standard Methods for the Examination of Water and Wastewater, 18th ed., APHA, Washington, D.C.
8. Marshall R. (Ed.), 1992, Standard Methods For the Examination of Dairy Products, 16th ed., APHA, Washington, D.C.
9. The United States Pharmacopoeia XXI and the National Formulary, 16th ed., 1985, United States Pharmacopeial Convention, Inc., Washington, D.C.