The Significance of the Urea Hydrolysis Test

There are many kinds of bacteria. Some are harmful, some are beneficial and some are neither. The ability to distinguish between these bacteria is crucial to disease prevention and treatment. Often, bacteria can be identified by their ability to react with chemical compounds. Urea is one such compound and the urea hydrolysis test is used to distinguish groups of bacterias from each other.

The Reaction

• When a bacteria contains the enzyme urease it can break down urea. The products of this hydrolysis reaction are ammonia and carbon dioxide. By itself, this would not be noticeable. When the reaction happens in the presence of pH indicators, however, it produces a noticeable color. Ammonia causes a basic (alkaline) pH to develop and the pH indicator will therefore change color. The most commonly used indicator for this test is phenol red, which will turn pink upon exposure to ammonia.

Performing the Test

• Liquid tests are carried out in a test tube.

  The test can be performed in liquid or solid media. For a liquid-based test, the bacteria being investigated is dissolved in a liquid containing urea and phenol red. For the solid test, bacteria is grown on a solid, agar-based media containing urea and phenol red. In both cases, the appearance of pink coloration is a positive result. It usually occurs within a day or two. This means the bacteria contains urease.

Uses

• Many bacteria found in the gut contain urease. However, few can break down urea rapidly. It is therefore possible to narrow down the identity of a bacteria based on urea hydrolysis. The most commonly tested groups that are positive are Proteus, Morganella and Providencia.

Limitations

• It takes many tests to identify a bacteria.

  While the urea hydrolysis test provides valuable information, it is not definitive. There are several bacteria that can break down urea and additional tests are needed to distinguish between them. Such tests give information about the organisms morphology and structure, oxygen requirements and its ability to react with hydrogen peroxide, sucrose and citrate. All this information taken in aggregate is usually enough to classify and name an unknown bacteria.

Special Circumstances

• The test described above applies to identifying a bacteria that can be grown outside the human body. In some situations, however, obtaining a bacterial sample is not possible or requires an invasive procedure. This occurs when a physician suspects a patient has Heliobacter pylori, the organism responsible for many ulcer disorders. In this situation, Heliobacter can still be identified by its ability to break down urea. The patient is given a capsule of labeled urea to swallow. Since carbon dioxide is a product of urea breakdown, the patient will breathe out labeled carbon dioxide if Heliobacter pylori is present.