Question

You test two different bacterial strains with the nitrate test. One tube (organism 1) turns red after the addition of reagents A and B, while organism 2 stays colorless. You wait two minutes and add zinc to organism 2, after which it turns red. Are organisms 1 and 2 both positive for the nitrate test? Explain.

Answer 1

Answer-

According to the given question-

Here we have two different bacterial strains and we have to find which is positive for nitrate test. One tube contains microorganism 1 which  turns red the medium when we add  A and B, while microorganism 2 does not form red colour and remain colorless. after that we wait two minutes and add some amount of zinc dust in tube having  microorganism 2,and after adding we observe the medium turns red.

• We know that the Nitrate reduction test is done to differentiate facultative anaerobic microorganism from the group of microorganism because they carry nitrate respiration.
• Those microorganism which have carry nitrate respiration they have enzyme nitrate reductase and nitrous reductase which completely reduces nitrate into N2O or N2 .
• E. coli have nitrate reductase and can partially reduces nitrate into nitrite .
• We can determine the nitrate reduction test on the basis of end product which are formed in the medium either nitrate was completely reduced or unreduced or partially reduced.
• here the Two reagent which are used are sulpfanilic acid and second is dimethyl -alpha-napthhylamine.
• First we incubate the microorganism for nitrate reduction test to check presence of gas in Durham tube.
• when there is no gas then we add both the reagent sulpfanilic acid and dimethyl -alpha-napthhylamine to the medium and if there is red colour formation takes place due to complex formation between nitrite and reagent then it means that nitrate is converted into nitrite in the medium and if there is no red colour it means there will be two possibility either microorganism completely reduced nitrate into gaseous product N2O or N2  or nitrate was not reduced or unreduced.
• To check this we add few particle of zinc dust in the medium, if after adding there is red colour formation takes place which means that nitrate is present in the medium and the microorganism was unable to reduce the nitrate. And if there is no red colour formation takes place after adding zinc it means that microorganism completely reduces the nitrate into gaseous product N2O or N2 .  
• So microorganism 1 form red colour in tube 1 which means that it have the capability to reduce nitrate into nitrite with the help of enzyme nitrate reductase and can partially reduces the nitrate while when zinc was added in tube 2 for microorganism 2 then there is red colour formation indicates the presence of nitrate in medium which indicates that microorganism 2 cannot convert nitrate into nitrite or gaseous product due to lack of enzyme. so microorganism 1 gives positive result and microorganism 2 gives negative result for nitrate reduction test.