The alkaline fermentation of urine (urea) with the resultant production of ammonia was hypothesized to occur due to the action of microorganisms by Reoch in 1875 (6). Many forms of urea-degrading bacteria were subsequently isolated and their urease activity studied (3). Urease activity is considered to be a major criterion for the identification of Proteus species and allows for Proteus to be distinguished from non-lactose-fermenting members of the Enterobacteriaceae (1). A chemically defined medium that was shown to be differential for Proteus species was formulated by Stuart (7). In this medium, Proteus are capable of using urea as a sole nitrogen source and produce sufficient quantities of ammonia to overcome the high buffering capacity of the medium resulting in a color change from yellow to bright pink (fuschia).
Christensen later formulated a medium that would allow growth of other members of the Enterobacteriaceae that cannot utilize the by-product of urea hydrolysis (ammonia) as a sole nitrogen source for growth (2). This medium also has a decreased buffering capacity allowing for the detection of smaller quantities of alkali produced from the degradation of urea. Thus organisms with a slight or delayed urease activity can be detected with Christensen’s urea agar.
The urease test identifies those organisms that are capable of hydrolyzing urea to produce ammonia and carbon dioxide. It is primarily used to distinguish urease-positive Proteeae from other Enterobacteriaceae.
Theory (5, 8)
Urease is a constitutively expressed enzyme that hydrolyzes urea to carbon dioxide and ammonia.
(NH2)2CO + H2O → CO2 + 2NH3
Urease test media contain 2% urea and phenol red as a pH indicator. An increase in pH due to the production of ammonia results in a color change from yellow (pH 6.8) to bright pink (pH 8.2). Urea broth (Stuart’s urea broth) is a highly buffered medium requiring large quantities of ammonia to raise the pH above 8.0 resulting in a color change. This medium provides all essential nutrients for Proteus, for which it is differential. Urea agar (Christensen’s urea agar) has a reduced buffer content and contains peptones and glucose. This medium supports the growth of many enterobacteria allowing for the observation of urease activity.
Two media types are commonly used to detect urease activity. Christensen’s urea agar is used to detect urease activity in a variety of microorganisms. Stuart’s urea broth is used primarily for the differentiation of Proteus species. Both media types are available commercially as prepared tubes or as a powder.
Christensen’s Urea Agar (2, 4, 5)
|Sodium chloride||5 g|
|Potassium phosphate, monobasic||2 g|
|Phenol red||0.012 g|
||15 to 20 g
To prepare the urea base, dissolve the first six ingredients in 100 ml of distilled water and filter sterilize (0.45-mm pore size). Suspend the agar in 900 ml of distilled water, boil to dissolve completely, and autoclave at 121oC and 15 psi for 15 minutes. Cool the agar to 50 to 55oC. Aseptically add 100 ml of filter-sterilized urea base to the cooled agar solution and mix thoroughly. Distribute 4 to 5 ml per sterile tube (13 x 100 mm) and slant the tubes during cooling until solidified. It is desirable to have a long slant and short butt. Prepared media will have a yellow-orange color. Store the prepared media in the refrigerator at 4 to 8oC until needed. Once prepared, do not reheat the medium as the urea will decompose.
Stuart’s Urea Broth (4, 5, 7)
|Yeast extract||0.1 g|
|Potassium phosphate, monobasic||9.1 g|
|Potassium phosphate, dibasic||9.5 g|
|| 0.01 g
Dissolve all ingredients in 1 liter of distilled water and filter sterilize (0.45-mm pore size). Distribute 3 ml of prepared broth per sterile tube (13 x 100 mm). Prepared media will have a yellow-orange color. Store the prepared broth in the refrigerator at 4 to 8oC until needed. Do not heat the medium as the urea will decompose.
Christensen’s Urea Agar (4, 5)
Use a heavy inoculum from an 18- to 24-hour pure culture to streak the entire slant surface. Do not stab the butt as it will serve as a color control (Fig. 1c). Incubate tubes with loosened caps at 35oC. Observe the slant for a color change at 6 hours, 24 hours, and every day for up to 6 days. Urease production is indicated by a bright pink (fuchsia) color on the slant that may extend into the butt. Note that any degree of pink is considered a positive reaction. Prolonged incubation may result in a false-positive test due to hydrolysis of proteins in the medium. To eliminate protein hydrolysis as the cause of a positive test, a control medium lacking urea should be used.
Rapidly urease-positive Proteeae (Proteus spp., Morganella morganii, and some Providencia stuartii strains) will produce a strong positive reaction within 1 to 6 hours of incubation. Delayed-positive organisms (e.g., Klebsiella or Enterobacter) will typically produce a weak positive reaction on the slant after 6 hours, but the reaction will intensify and spread to the butt on prolonged incubation (up to 6 days). The culture medium will remain a yellowish color if the organism is urease negative (Fig. 1).
a b c d
FIG. 1. Urea agar test results. Urea agar slants were inoculated as follows: (a) uninoculated, (b) Proteus mirabilis (rapidly urease positive), (c) Klebsiella pneumoniae (delayed urease positive), (d) Escherichia coli (urease negative). All samples were incubated at 37oC for 16 hours.
Stuart’s Urea Broth (4, 5)
Use a heavy inoculum from an 18- to 24-hour pure culture to inoculate the broth. Shake the tube gently to suspend the bacteria. Incubate tubes with loosened caps at 35oC. Observe the broth for a color change at 8, 12, 24, and 48 hours. Urease production is indicated by a bright pink (fuchsia) color throughout the broth.
Rapidly urease-positive Proteeae (Proteus spp., Morganella morganii, and some Providencia stuartii strains) for which this medium is differential, will produce a strong positive reaction as early as 8 hours, but always within 48 hours of incubation. Delayed-positive organisms (e.g., Enterobacter) will not produce a positive reaction due to the high buffering capacity of this medium.
FIG. 2. Urea broth test results. Urea broth test tubes were inoculated as follows: (a) Proteus vulgaris (urease positive) and (b) Escherichia coli (urease negative). All samples were incubated at 37oC for 16 hours.
The ASM advocates that students must successfully demonstrate the ability to explain and practice safe laboratory techniques. For more information, read the laboratory safety section of the ASM Curriculum Recommendations: Introductory Course in Microbiology and the Guidelines for Biosafety in Teaching Laboratories.
COMMENTS AND TIPS
Comments and tips are from discussion at the ASM Conference for Undergraduate Educators 2010.
Rapid urease test kits are available commercially.
Reliable results can also be obtained by inoculating 0.5 ml of Stuart’s urea broth with a loopful of culture.
Rapid results (<1 or 2 hours) can be obtained for Proteus spp. on urea agar slants by using a heavy inoculum in a single spot halfway up the slant.
Helicobacter pylori, which is of clinical importance, is also urease positive. Many special tests and media have been developed for the detection of H. pylori in the clinical lab.
1. Bailey, W. R., and E. G. Scott. 1974. Diagnostic microbiology, 4th ed. Mosby, St. Louis, MO.
2. Christensen, W. B. 1946. Urea decomposition as a means of differentiating Proteus and paracolon cultures from each other and from Salmonella and Shigella types. J. Bacteriol. 52:461–466.
3. Day, A. A., W. M. Gibbs, A. W. Walker, and R. E. Jung. 1930. Decomposition of urea by Bacillus proteus. J. Infect Dis. 47:490–502.
4. Difco. 1998. Difco manual, 11th ed. Difco Laboratories, Detroit, MI.
5. MacFaddin, J. F. 2000. Biochemical tests for the identification of medical bacteria, 3rd ed. Lippincott Williams & Wilkins, Philadelphia, PA.
6. Reoch, J. 1875. The decomposition of urea. J. Anat. Physiol. 9:368–385.
7. Stuart, C. A., E. Van Stratum, and R. Rustigan. 1945. Further studies on urease production by Proteus and related organisms. J. Bacteriol. 49:437–444.
8. Winn, W., S. Allen, W. Janda, E. Koneman, G. Procop, P. Schreckenberger, and G. Woods. 2006. Koneman’s color atlas and textbook of diagnostic microbiology, 6th ed. Lippincott Williams & Wilkins, Philadelphia, PA.
This resource was peer reviewed at the ASM Conference for Undergraduate Educators 2010.
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