Conjugates of 2,3-dihydroxynaphthalene and its derivatives with enzyme cleavable groups are chromogenic substrates that form colored compounds when complexed with metal ions, e.g. iron ions, on cleavage by enzymes, and are useful in microbial detection and identification. The cleavage products form purple or red-brown colored complexes, that can easily be observed by the naked eye. Microbes can be grown in the presence of the substrates and the metal salts that provide the metal ion for complexing with the 2,3-dihydroxynaphthalene product. Substituents in the naphthalene ring may affect the solubility of the substrates and also the diffusibility and color of the metal complexes. Some of the substrates yield soluble complexes on cleavage and are of particular value in liquid growth media. Other substrates produce less soluble complexes that are more suitable for use in solid agar media. Methods of synthesizing the substrates are described.