Timely and effective detection of bacterial pathogens is of great importance to reduce morbidity rates from bacterial infections. Recently, enzyme-activated fluorogenic probes, which invoke enzymatic catalysis to trigger fluorescence emission, have been superior sensors for bacterial infections needed for accurate diagnoses. Here, a fluorescent sensor for nitroreductase (NTR) detection is described. It is based on a cyanine fluorophore and utilizes photoinduced electron transfer to generate a rapid 10-fold fluorescence response after being catalytically reduced by NTR. It has enabled selective and sensitive visualization of NTR activity in vitro and in living bacterial pathogens. Thus, the probe has great potential to provide a rapid, noninvasive tool to diagnose infections and guide antimicrobial selection.