Acidification activity of L. lactis subsp. lactis S1 was more affected by higher concentrations of cFDA/cFDA-SE and PI (e.g., 6.3% higher Ta for 5 mM cFDA-SE and 1.6 mM PI, 16.1% for 50 mM cFDA-SE and 40 mM PI; Table 2). Double staining with cFDA and TOTO-1 resulted in significantly (P < 0.05) higher Ta (27.8 and 68.1% for the two dye concentrations), but replacing cFDA with cFDA-SE further diminished the acidification activity of L. lactis subsp. Lactis S1 (38.1 and 92.3% higher Ta for the two dye concentrations).