A positive urease test for ureaplasmas is indicated by what observation?

In microbiology, a urease test is utilized to determine the ability of an organism to produce the enzyme urease, which hydrolyzes urea to produce ammonia and carbon dioxide. This reaction increases the pH of the medium, leading to a color change.

When ureaplasmas are present and urease is produced, the test medium often changes color due to the alkalinization caused by ammonia. A positive urease test is indicated by the formation of a brown halo surrounding the colonies. This brown color results from the pH indicator in the medium, which changes color in response to the increase in pH. The presence of ureaplasmas and their urease activity will clearly demonstrate this characteristic, making it evident that the organism possesses the urease enzyme.

Other color indicators or zones may suggest different biochemical activities or the presence of other organisms, but the specific brown halo provides a clear positive result for ureaplasmas in the context of a urease test.