5-Cyano-2,3-ditolyl tetrazolium chloride (CTC) has been used to evaluate the respiratory activity of many bacterial populations as well as populations undergoing drug efficacy evaluations. Healthy cells respiring via the electron transport chain will absorb and reduce CTC into an insoluble, red fluorescent formazan product. Cells not respiring or respiring at slower rates will reduce less CTC, and consequently produce less fluorescent product, giving a semi-quantitative estimate of healthy vs. unhealthy bacteria.
