Nitrate Broth with Durham Tube


What is this medium used for?


Some bacteria have an enzyme called nitrate reductase that can convert nitrate to nitrite. Some also have an enzyme called nitrite reductase that can convert nitrite into nitrogen gas. This medium is used to determine whether the microbe has the enzyme nitrate reductase. If it is present, the medium will also allow determination of the presence of the second enzyme, nitrite reductase.



How is the presence of nitrate reductase determined? 


If a bacterium producing nitrate reductase is grown in a medium containing nitrate, the enzyme converts the nitrate to nitrite. Nitrite reacts with certain chemicals to yield a red-colored product.



How is the presence of nitrite reductase determined? 


If the bacterium also produces nitrite reductase, nitrogen gas will be liberated. Bubbles collecting in an inverted Durham tube indicate that nitrogen gas has been produced.



What is the content of this medium?


The medium used is nitrate broth with Durham tube, a nutrient broth with potassium nitrate added. The inverted Durham tube is placed in the medium as a trap for any nitrogen gas that might be generated.



How are these tests performed?  


For information on how to detect nitrate and nitrite reductase activity, refer to the nitrate reduction test.