This chemical, known as CTC (5-cyano-2,3-di-p-tolyltetrazolium chloride), is a tetrazolium salt primarily used in microbiology to detect and quantify respiratory active microorganisms. It functions as a redox indicator, where metabolically active cells reduce it via the electron transport chain to form an insoluble, intensely fluorescent formazan product (CTC-formazan), which emits red fluorescence. This allows for the visualization and enumeration of viable bacteria using fluorescence microscopy, flow cytometry, or epifluorescence techniques. It is widely applied in environmental microbiology, biofilm studies, wastewater treatment monitoring, and assessments of microbial activity in soil and aquatic samples. Its specificity for respiratory activity makes it valuable for distinguishing active from dormant cells in complex microbial communities. While primarily microbial-focused, similar tetrazolium salts are used in broader biological assays for cell viability.