Increased Ta and time to pH 5.5 demonstrated the negative impact of fluorescent labelling on the acidification activity of L. lactis subsp. lactis S1 compared with the respective control (Table 2). In contrast to cFDA, single staining with 5 or 50 mM cFDA-SE resulted in significant (P < 0.05) differences with a 6.3 and 9.4% higher Ta. The effect of TOTO-1 on the acidification activity of L. lactis subsp. Lactis S1 was again proven to depend on the dye concentration and was higher than for PI: compared with the respective control 18.4% higher Ta for 0.2 mM TOTO-1, 78.1% for 2 mM TOTO-1