Effects of substrates and phosphate on INT (2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl tetrazolium chloride) and CTC (5-cyano-2,3-ditolyl tetrazolium chloride) reduction in Escherichia coli

The effects of substrates of primary aerobic dehydrogenases, and inorganic phosphate on aerobic INT and CTC reduction in Escherichia coli were examined. In general, INT produced less formazan than CTC, but INT (+) cell counts remained near values of CTC (+) cells. INT and CTC (+) cell numbers were higher than plate counts on R2A medium using succinate, formate, lactate, casamino acids, glucose, glycerol (INT only) and no substrate. Formate resulted in the greatest amount of INT and CTC formazan. Reduction of both INT and CTC was inhibited above 10 mmol 1^-1 phosphate, and this appeared to be related to decreased rates of O₂ consumption. Formation of fluorescent CTC (+), but not INT (+) cells was also inhibited in a concentration dependent manner by phosphate above 10 mmol 1^-1. From light microscopic observations it appeared CTC formed increasing amounts of poorly or non-fluorescent formazan with increasing phosphate. Therefore, use of phosphate buffer in excess of 10 mmol 1^-1 may not be appropriate in CTC and INT reduction assays.