Abstract: 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-myoinositol-1-phosphate compound of formula (I) ##STR1## wherein R is selected from the group consisting of hydrogen and C.sub.1-4 alkyl, while R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are radicals selected from the group consisting of hydrogen and chromogenic substituents, or of a salt of said formula I compound. The invention provides for a safe, sensitive and commercially viable detection of potentially pathogenic bacterial activity of such microbes as Bacillus cereus, B. Thuringiensis, Staphylococcus aureus and various Listeria strains in potentially infected materials including physiological samples or consumable goods such as foods and beverages.