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42: Beta-Galactosidase Test (ONPG)

Objectives

  • Identify the ability of the bacterium to make the enzyme beta - galactosidase

\(\beta\)-GALACTOSIDASE

Lactose utilization requires a couple of enzymes, one of which is beta - galactosidase. In this test, you use a molecular decoy called ONPG (Ortho-nitrophenyl-b-D-galactopyranoside) that will turn to a yellow color in the presence of this enzyme. ONPG is an analog of lactose that the enzyme can break down to produce a yellow colored end-product, O-nitrophenol. Since this enzyme is made ONLY in the presence of the lactose substrate, you need to be sure to grow this organism on media high in lactose (e.g., from lactose broth or on TSIA agar).

                      

Chemical structure of (left) Ortho-Nitrophenyl-β-galactoside and p-Nitrophenol . Image used with permission from Wikipedia (credit:  Edgar181 and NEUROtiker respectively)

Note

This test is generally for gram – bacteria (also to identify some Staphylococcus species)

MATERIALS NEEDED 

  • ONPG discs
  • 0.85-0.9% NaCl 1 ml
  • pipettes and pi-pumps
  • sterile small tubes and caps

THE PROCEDURE

  1. FIRST, your organism needs to be growing in/ on any medium with lactose (to induce the production of the galactosidase enzyme)
  2. Pipet 0.5 ml of the saline into a sterile tube.
  3. Inoculate with the bacterium and add the ONPG disc in a sterile manner (forceps dipped in alcohol and flamed) to the tube.
  4. Incubate at 37º C for 4 hours. There is so little fluid in the tube, it will dehydrate if you do not read it in a few hours (alternatively, you can cover it tightly with parafilm).

INTERPRETATION

The fluid and disc will turn any shade of yellow if + for galactosidase enzyme. 

QUESTIONS

  1. The enzyme, galactosidase, tests for the breakdown of the sugar_______.
  2. Why do you want to grow the bacterium on a high-lactose medium to run this test?

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