Urea Agar Base (Christensen)

Composition**

Final pH (at 25°C): 6.8±0.2

**Formula adjusted, standardized to suit performance parameters

Directions

Suspend 24.51 grams in 950 ml distilled water. Heat to boiling to dissolve the medium completely. Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes. Cool to 50°C and aseptically add 50 ml of sterile 40% Urea Solution (FD048) and mix well. Dispense into sterile tubes and allow to set in the slanting position. Do not overheat or reheat the medium as urea decomposes very easily.

Principle And Interpretation

Urea Agar Base Media is a slight modification of Christensen formulation (1, 2) and is recommended by BIS (3, 4) for identification of urease activity. Rustigian and Stuart (5) had originally formulated a medium to detect urease activity. These media differentiate between rapid urease positive Proteus species and other urease positive organisms like Citrobacter, Enterobacter and Klebsiella and the bacteria other than Enterobacteriaceae. Christensen observed that addition of peptic digest of animal tissue, dextrose and reduced content of buffer helps to support an early luxuriant growth.

Heavy inoculum of growth is inoculated on the surface of the slants. When urea is utilized, ammonia is formed during incubation which makes the medium alkaline, showing a pink-red colour by the change in the phenol red indicator. Prolonged incubation may cause alkaline reaction in the medium. Check using medium without urea as the negative control.

Quality Control

Appearance: Light pink coloured homogeneous free flowing powder

Gelling: Firm, comparable with 1.5% Agar gel

Colour and Clarity of prepared medium: Yellowish orange coloured clear gel forms in tubes as slants.

Reaction: Reaction of 2.45% w/v aqueous solution at 25°C. pH: 6.8±0.2

pH: 6.60-7.00

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. Christensen, W.B., 1946, J. Bact., 52:461.
2. MacFaddin J., 1980, Biochemical Tests for Identification of Medical Bacteria, 2nd ed., Williams and Wilkins, Baltimore.
3. Bureau of Indian Standards, IS: 5887 (Part I) - 1976, reaffirmed 1986.
4. Bureau of Indian Standards, IS: 5887 (Part III) - 1999.
5. Rustigian and Stuart, 1941, Proc. Soc. Exp. Biol. Med., 47:108.