If a culture turns red after zinc is added in the nitrate reduction test, what does this mean?

In the context of the nitrate reduction test, a culture turning red after zinc is added indicates that nitrate was initially present in the medium, and zinc serves to confirm this.

During the test, if an organism can reduce nitrate (NO3-) to nitrite (NO2-), the addition of reagents will turn the medium red due to the formation of a red azo dye. If no color change occurs after adding reagents, zinc dust is introduced to assess if nitrate is still present. Zinc reduces any remaining nitrate to nitrite, leading to a red color if nitrate was indeed there to begin with. If the medium turns red after the addition of zinc, it suggests that the organism did not reduce the nitrate, confirming that nitrate remained in the culture throughout the testing process. Therefore, in this instance, the appearance of red after zinc addition confirms the presence of nitrate in the original culture.