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Principle
- The urea broth base is developed by the Rustigian and Stuart (1941) for the differentiation of Proteus species from Salmonella and Shigella species from clinical samples.
- They are differentiated on the basis of urea utilization, gram-negative enteric bacilli are unable to utilize urea in scanty of nutrients and highly buffered medium. Urea broth is composed of yeast extract, monobasic potassium phosphate, dibasic potassium phosphate, and phenol red.
- Yeast extract provides carbon and nitrogen, vitamins, and other growth factors required for the growth of organisms. Phosphates act as buffering agents. Many urinary tract infections causing organisms to produce urease enzyme, which catalyzes the splitting of urea in the presence of water to release two molecules of ammonia and carbon dioxide.
- The ammonia combines with the carbon dioxide and water to form ammonium carbonate, which turns the medium alkaline, and changes the color of the medium from light orange at pH 6.8 to magenta pink at pH 8.1 due to the presence of phenol red. Rapid urease-positive organisms turn the entire medium pink within 24 hours.
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