For evaluating the physiological status of cells, a stringent response network was used. Fluorescence from intact E. coli, which has a plasmid encoding the green fluorescence protein (GFP) under the regulation of rpoS promoter, was monitored. Comparison of the response of different E. coli strains demonstrated an essential role of ppGpp in the expression of GFP, as it activated the rpoS promoter. The physiological status of intact cells, that depends on ppGpp accumulation in response to the nutritional status such as amino acid starvation, could therefore be monitored by measuring fluorescent intensity using this reporter gene.