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Christensen Urea Agar (Oxoid, England)

Christensen urea agar is a very useful medium to check the urease activity of various micro-organisms like bacteria and fungi.

Composition of  Christensen Urea Agar

Ingredients                                                       gm/liter

Peptone                                                            1.0

Dextrose                                                          1.0

Sodium Chloride                                           5.0

Dipotassium Phosphate                           1.2

Mono-potassium phosphate                0.8

Phenol Red                                                    0.012

Agar                                                                   15.0

Final pH (at 25ºC)                                       7.4±0.2

Requirements for urea agar preparation

• Christensen Urea Agar base
• Conical flask
• Measuring cylinder
• Distilled water
• Test tubes
• Cotton plug
• 40% urea

Preparation of urea agar

1. Suspend 24 grams of the medium  in 950 ml distilled water
2. Sterilize by autoclaving at 115ºC for 15 minutes.
3. After cooling at about 45ºC,  add 50 ml of 40% urea and mix well.
4. Then dispense 5 ml in a test tube and set at slant position.

Note:  For 100 ml urea agar, warm the refrigerated 40% urea solution to room temperature. Aseptically add 5 mL to 95 mL of sterile molten Urea Agar Base. Mix well and dispense in sterile tubes and set at slant position.

40% urea preparation

• Aseptically weigh 40 gram.
• Dissolve in 50 ml distilled water.
• Add the extra volume of distilled water to reach the final volume of 100 ml.
• Store it in a refrigerator.

Uses of urea agar

This medium uses for urease test which  is useful to identify the organisms on the basis of urease activity and the list of urease positive bacteria and fungi are as follows-

Urease positive bacteria

strong urease producers

Brucella species

Helicobacter pylori

Rapid urease producers

Proteus species

Morgenella species

Slow urease producers

Klebsiella species

Enterobacter species

Urease positive fungi

Cryptococcus neoformans

Trichophyton mentagrophytes

Further Readings

1. Cowan & Steel’s Manual for identification of Medical Bacteria. Editors: G.I. Barron & R.K. Felthani, 3rd ed 1993, Publisher Cambridge University Press.
2. Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.
3. Clinical Microbiology Procedure Handbook Vol. I & II, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
4. Colour Atlas and Textbook of Diagnostic Microbiology. Editors: Koneman E.W., Allen D.D., Dowell V.R. Jr, and Sommers H.M.
5. Jawetz, Melnick and Adelberg’s Medical Microbiology. Editors: Geo. F. Brook, Janet S. Butel & Stephen A. Morse, 21st ed 1998, Publisher Appleton & Lance, Co Stamford Connecticut.
6. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
7.  Textbook of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.