Interpretation - The rate of hydrolysis of ONPG to orthonitrophenol may be rapid for some organisms, producing a visible yellow color reaction within 5 to 10 minutes. The yellow color is usually distinct, and indicates that the organism has produced orthonitrophenol from the ONPG substrate through the action Of B-galactosidase. Of the Enterobacteriaceae, only members of the Proteus, Morganella, and Providencia genera possess the deaminase enzyme necessary for this conversion. Principle - The phenylalanine test depends upon the detection of phenylpyruvic acid in the test medium after growth of the test organism. Meat extracts or protein hydrolysates cannot be used because of their varying natural content of phenylalanine.
Yeast extract serves as the carbon and nitrogen source.
The agar slant of the medium is inoculated with a single colony of the test organism isolated in pure culture of primary plating agar. As the reagent is added, the tube is rotated to dislodge the surface colonies. Interpretation - The immediate appearance of an intense green color indicates the presence of phenylpyruvic acid and a positive test. The medium contains ten times as much lactose and sucrose as glucose. Bacteria that ferment glucose produce a variety of acids, turning the color of the medium from red to yellow.