How many tubes are necessary for performing the decarboxylase test?

The decarboxylase test is designed to determine the ability of bacteria to decarboxylate specific amino acids, primarily lysine, ornithine, and arginine. This test typically requires a control tube along with those prepared for each of the amino acids being tested. Therefore, for testing the decarboxylation of three amino acids, plus a control, the total number of tubes needed would indeed be four. One tube is prepared for lysine, another for ornithine, a third for arginine, and a fourth serves as a control to ensure the experiment's accuracy and help interpret the results correctly.

The choice indicating two tubes is not sufficient, as testing typically examines three substrates, requiring at least one for each. The option suggesting three tubes is also incomplete since it does not include a control which is critical for validating the test results. The choice with five tubes would unnecessarily complicate the process by suggesting the use of additional substrates, which is not the standard approach for this test in microbiology. Thus, four tubes—one for each substrate and one control—accurately reflect the requirements for conducting a thorough decarboxylase test.