What test can be performed to confirm the presence of a true negative reaction in the nitrate reduction test?

To confirm the presence of a true negative reaction in the nitrate reduction test, adding zinc dust is the correct approach. This step is significant because it clarifies whether nitrate is present or if the bacteria have completely reduced it to nitrogen gas or other nitrogenous compounds.

When zinc dust is added to a nitrate broth that initially yielded a negative result (meaning that no gas or color change was observed), it acts as a reducing agent. If nitrate is still present in the medium, zinc will reduce it to nitrite, which would subsequently react with the reagents present in the test and produce a color change. A color change after the addition of zinc indicates that the original bacteria did not reduce nitrate and confirms a negative result. If no color change occurs, this suggests that nitrate was fully reduced beyond nitrite, resulting in a true negative reaction.

Other options do not serve the purpose of confirming a true negative in this context. For example, measuring pH does not provide information about nitrate reduction status. Inoculating on blood agar pertains to different growth and hemolysis characteristics of bacteria rather than nitrate reduction. Performing an oxidase test is also unrelated, as it tests for the presence of cytochrome oxidase and does not confirm nitrate reduction directly. Thus