Question

3 Suppose yout has a gas bubble in the Durham tube. Knowing that fermenters frequenthy produce gas, you ignore the bubl and proced to the next step. Adding reagents produces no chang consistent with what you have learned about this test? Why? remove your test cultures from the incubator and notice that one of them-a known fermenter- e, and neither does addng zinc. Is this occurrence 4 Would you change your answer to question 3 if the control broth did not change color after the adition of reagents? What if the control broth had changed color only after the addition of reagents and zinc?

Nitrate reduction test

Answer 1

Sol).(A).No,this occurance is not consistent with what we have learned about this test.In the test,the organism is incubated in nitrate broth.After 4 hrs, sulfanilic acid reagent & $\alpha$ -naphthylamine is added.

If the organism reduces nitrate to nitrite, the nitrites in the medium will form nitrous acid. When sulfanilic acid is added, it will react with the nitrous acid to produce diazotized sulfanilic acid. This will react with the α-naphthylamine to form a red-colored compound. Therefore, if the medium turns red after the addition of the nitrate reagents, it will be considered a positive result for nitrate reduction.

If the medium does not turn red after the addition of the reagents, it means that the organism is unable to reduce the nitrate. Another step is needed for the confirmation of the negative test. Add a small amount of powdered zinc. If the tube turns red after the addition of the zinc, it means that unreduced nitrate is present*.Therefore, a red color on the second step is a negative result.

(B).If the control broth did not change colour after the addition of reagents, then the answer will be that the test is negative and there is no nitrate reduction taking place in the broth.Since nitrate is not reduced to nitrite which will not form nitrous acid This nitrous acid will not react with sulphanilic acid to produce diazotized sulphanilic acid and not form a red coloured compound with $\alpha$ -naphthylamine.

Colour change only after addition of reagent and zinc acts as a confirmation test for the negative result and no nitrate reduction since the addition of the zinc reduces the nitrate to nitrite, and the nitrite in the medium forms nitrous acid, which reacts with sulfanilic acid. The diazotized sulfanilic acid that is produced reacts with the α-naphthylamine to create the red complex.