Phenylalanine Agar medium is used to differentiate the Proteus and Providencia groups of Enterobacteriaceae by oxidative deamination of Phenylalanine to phenylpyruvic acid. Yeast Extract is a source of B-complex vitamins. Sodium Chloride maintains the osmotic balance and Disodium Phosphate serves as pH buffer. The addition of Ferric Chloride results in the appearance of green color (positive reaction with phenylpyruvic acid) or no color change (negative reaction). Agar is the gelifying agent.