Question

phenylalanine medium sometimes is prepared in broth form with a pH indicator simiar to that used in decarboxylase medium so that increases in pH then can be detectedby development of purple color. when testing an organism for the ability to deaminate phenylalanine, would you expect to overlay this medium with mineral oil? why orwhy not?

Answer 1

No, overlaying of oil is not required in deaminase test.

While testing for Decarboxylase activity, the mineral oil is layered in the tube so as to prevent the oxygen to enter the test media. This test requires anaerobic conditions because the color change in the positive result depends on pH. If pH rises (above 6.8) due to removal of carboxyl group of amino acid, the indicator gives purple color which shows positive test.

In case of Phenylalanine deaminase test, the indicator is ferric chloride and it reacts with the acid produced after deamination in reaction. The reaction of ferric chloride with acid results in greenish precipitation which indicates positive test. The test is not much affected by pH and is also independent of presence of oxygen. Because of this, no overlaying of oil is required in this test.