What test would you perform last to differentiate Enterobacteriaceae based on urease activity?

The urease test is indeed the last test to perform when differentiating Enterobacteriaceae based on urease activity. This is because the urease test specifically measures the ability of an organism to hydrolyze urea to ammonia and carbon dioxide, which raises the pH and changes the color of the medium.

In laboratory practice, tests are often conducted in a specific order to efficiently narrow down the identity of a bacterium. The urease test is typically performed after other tests that help classify the organism in broader categories. For example, the indole test determines the organism's ability to produce indole from tryptophan, the oxidase test assesses the presence of cytochrome c oxidase, and the nitrate reduction test checks for the ability to reduce nitrate. These tests help identify organisms based on other metabolic characteristics before confirming urease activity specifically.

By conducting the urease test last, a microbiologist can ensure that they have already ruled out other characteristics, allowing for a clearer interpretation of urease activity in the context of Enterobacteriaceae. Therefore, performing this test as the final step provides focused information regarding the specific urease activity, solidifying the identification process.