Evaluation of the redox dye 5-cyano-2,3-tolyl-tetrazolium chloride for activity studies by simultaneous use of microautoradiography and fluorescence in situ hybridization.

Three microscopic in situ techniques were used simultaneously to investigate viability and activity on a single-cell level in activated sludge. The redox dye 5-cyano-2,3-tolyl-tetrazolium chloride (CTC) was compared with microautoradiography (MAR) and fluorescence in situ hybridization (FISH) to indicate activity of cells in Thiothrix filaments and in single floc-forming bacteria. The signals from MAR and FISH correlated well, whereas only 65% of the active Thiothrix cells and 41% of all single cells were detectable by CTC reduction, which mainly targeted the most active cells.