|Abstract:||ppGpp is a small molecule that works as a global master regulator of the E. coli physiology, most notably during the stringent response. Systems biology studies have emphasized its role in understanding the host response to the burden of introduced synthetic genetic circuits. Traditionally, researchers rely on in vitro methods to measure the intracellular ppGpp levels. In vivo ppGpp reporters, however, would allow the possibility of closely monitoring the ppGpp levels during different processes, especially the host response given the functioning of synthetic circuits with relatively fast dynamics. In this study, a series of in vivo ppGpp reporters were developed in E. coli by placing a fluorescent protein under the control of promoters from genes known to be controlled by ppGpp levels. Reporter performance was tested by monitoring the fluorescence level with microscope during different stages of growth, induced stringent response, and different growth conditions. The four positive ppGpp reporters showed strong positive correlation with ppGpp levels, including two with particularly high sensitivity and signal intensity. The testing process also shed new light on the dynamics of ppGpp production during batch culture growth and under nutrient limitation. With further development, these in vivo ppGpp reporters promise to be very useful when studying the dynamics of host resource partitioning.