A method of detecting a phosphatidylinositol-specific phospholipase C enzyme by means of a substrate which is cleaved by said enzyme and yields a dye when the chromophoric portion of the substrate is dimerized and oxidized; the invention teaches using in such method, as a novel substrate, a 3-indoxyl-myo-inositol-1-phosphate compound of formula (I) wherein R is selected from the group consisting of hydrogen and C1-4 alkyl, while R1, R2, R3, and R4 are radicals selected from the group consisting of hydrogen, halogen, cyano, nitro, carboxy, amino, alkyl-substituted amino and sulphonyl; or of a salt of said formula I compound. The invention provides for a safe and sensitive method of detection of potentially pathogenic bacterial activity of such microbes as Bacillus cereus, B. Thuringiensis, Staphy lococcus aureus and various Listeria strains in potentially infected materials including physiological samples or consumable goods such as foods and beverages.