Phenolphthalein Phosphate Agar

Principle And Interpretation

Bacteria in the genus Staphylococcus are pathogens of man and other mammals. Traditionally they were divided into two groups on the basis of their ability to clot blood plasma (the coagulase reaction). The coagulase-positive staphylococci constitute the most pathogenic species Staphylococcus aureus. The presence of staphylococci in a lesion might first be suspected after examination of a direct gram stain. However, small numbers of bacteria in blood preclude microscopic examination and require culturing first (1). Phosphatase has been implicated as a virulence factor for S. aureus. The organisms produce both an acid and alkaline phosphates, the latter being repressed in the presence of inorganic phosphate in the medium.

Phenolphthalein Phosphate Agar is used for the identification of phosphatase-positive colonies of S. aureus, which is a coagulase-positive pathogenic strain (2).

Peptic digest of animal tissue and beef extract supply the nitrogenous compounds, growth factors and trace ingredients essential for the growth of Staphylococcus aureus. Sodium phenolphthalein phosphate serves as a substrate for the phosphatase enzyme. Sodium chloride maintains osmotic equilibrium. Phosphatase production is determined by the liberation of phenolphthalein, which is indicated by the change in colour of the medium (3). When alkali is added to this medium, the liberated phenolphthalein gives a bright pink-red colouration (4). Alternatively phosphatase production can be determined by following technique.

Technique: Grow staphylococci overnight at 37°C on the medium. Invert the plate and pour few drops of ammonia solution into the lid, read a positive culture whose colonies turn bright pink within a few minutes. The colour soon fades.