The nitrate reduction test identifies whether the given strain of bacteria is able to reduce nitrate to nitrite using anaerobic respiration.
In this test, the solution is incubated after adding the bacteria strain. After the incubation period is over, additional solutions Sulfuric acid (a) and Naphthylamine (b) are added to it.
If the reduction has occurred and the nitrite is present, then the solution gives red color.
If there is no color change, then the solution is tested for the presence of other reduction products like N2 gas , NH3 gas etc. This is done by adding zinc to the solution.
Zinc is able to reduce nitrate to nitrite only. Therefore a color change after the addition of zinc indicates that the bacterial strain was not able to reduce nitrate and nitrates are still present which were reduced by zinc.
If there is no color change, that means the nitrates are absent and the bacteria has reduced nitrate to some other species like N2 or NH3 gas.
Therefore , In the nitrate reduction test, only after the addition of zinc to your 24-hour incubated solutions a and b is there a color change. This indicates that the bacteria was not able to reduce nitrate to nitrite.
