pnp agar pnp agar pnp agar is an agar medium used in microbiology to identify staphylococcus species that have phosphatase activity. the medium changes color when p-nitrophenylphosphate disodium (pnp) is dephosphorylated. pnp agar is composed of mueller–hinton agar buffered to ph 5.6 to 5.8, with the addition of 0.495 mg/ml pnp.