As can be seen in Fig. 4A, P. taetrolens cells remained fully polarized
only until the onset of the lactobionic acid production phase
(10 h). However, the higher C-source availability, the lower membrane
depolarization levels were maintained upon the onset of the
lactobionic acid production phase. Such adaptive physiological
responses supported the evidence that increased membrane polarization
levels are related to higher nutrient availability (Müller
et al., 1999). Whereas cells were hyperdepolarized in a whey/lactose
co-fermentation after 48 h, glucose and glycerol feedings supported
lower membrane depolarization levels due to the higher
C-source availability in the fermentation broth (Fig. 4A).