The invention relates to a method for real-time detection of viable microorganisms comprising: a. addition of a cell-permeable, phototautomeric compound to a micro-organism or other living cell; and b. measuring the fluorescent emission of said phototautomeric compound. Preferably the phototautomeric compound is salicylic acid, 2-hydroxy-1-naphtoic acid or 1-hydroxy-2-naphtoic acid. Further, the assay can he used to assess the antibiotic effect of a test compound. This test can be used as a high-throughput screening for compounds with antibiotic activity. Also part of the invention is the use of a cell permeable phototautomeric compound in a method for determining the viability of micro-organisms and for assessing the antibiotic effect of a test compound