v   Principle:-

This test is used to detect an organism which produces B-galactosidase enzyme, that
hydrolyses ONGP to produce the yellow colored end product orthonitrophenol
(ONP).

v Requirements:-

1.      ONPG disk

2.      Pure culture of suspected organism

3.      Normal saline

v  Procedure:-

1.      Suspend loopful of the organisms in normal saline.

2.      Place an ONPG disk in the tube

3.      Incubate for 4 hours at 37°C.

4.      Observe the tubes for color change

 v Observations:-

1             Yellow color of disk: Organism like E. coli

2              No change of color of disk: Negative test

v Quality control:-

1.       Use following microorganisms to confirm the reliability of reagents:

2.       Positive test: E. coli

3.       Negative test: S. typhimurium