Microbiology Lab Practical Practice Exam 2026 – The All-in-One Guide to Master Your Lab Skills!

Phase II in the nitrate reduction test looks for organisms that push nitrate reduction beyond nitrite to other forms (like nitrogen gas). After the first step, if there’s no red color, you add zinc. If zinc then turns the solution red, that means nitrate was still present and the organism didn’t reduce past nitrite (negative Phase II). If there’s no color change after zinc, it means the organism reduced nitrate beyond nitrite to other products, leaving no nitrate for zinc to reduce—so Phase II is positive. Therefore, no color change is the sign of a positive Phase II nitrate reduction test.