Dehydrogenase activity in E. coli cultures determined by CTC treatment. Samples were taken every 3 h over a total growth period of 36 h. An untreated E. coli culture was used as control. Fluorescence emissions were determined by flow cytometry (A) and by confocal laser scanning microscopy (B-D). Images B - D show CTC treated E. coli cells after growth of 3 h (B), 6 h (C), and 9 h (D). Total cell counts were determined by counting SYTO60 stained cells (red color). CTC-formazan fluorescence is shown in blue (outside cells) or white (inside cells). Micrographs are overlays of sequential scans. Scale bar equals 10 μm.