The present invention relates to a method for assessing cell viability of bacterial and fungal cells and to a method for the detection of bacterial and fungal cells with specific enzyme activities. The methods of the present invention rely on the real-time measurement of the level of luminescence signal from a luciferase enzyme directly from a growing culture of bacterial or fungal cells. Furthermore, the present invention relates to a method for assessing susceptibility of bacterial cells to antibiotics by measuring ATP levels using a luciferase assay system.