As can be seen in Fig. S1A, membrane polarization decreased
markedly upon the onset of the lactobionic acid production phase
regardless the higher C-source availability conditions encountered in a whey/glucose co-fermentation. In comparison to a
non-supplemented fermentation, P. taetrolens cells displayed lower
depolarization levels at later cultivation stages (60 h) for all the different
glucose concentrations, suggesting the presence of readily
assimilated C-sources like glucose or gluconic acid in the fermentation
broth. In terms of metabolic activity, glucose co-supply led
to reduced metabolic activity levels at early cultivation times
(12 h) (Fig. 2A). However, the prolonged exposure to extreme
acidic conditions encountered in a non-supplemented fermentation
led to find similar metabolic activity levels at the end of the
fermentation (60 h) regardless whether the system was
co-supplied with glucose or not (Fig. 2A). After 60 h, all the
fermentations also presented low membrane integrity levels,
suggesting the emergence of a stress-induced physiological
response featured by the loss of membrane integrity and metabolic
activity. Interestingly, physiological segregation with two
major cell subpopulations at early culture stages (12 h) arose when
glucose was employed as co-substrate (Fig. 2B). Such cell
segregation may be translated into an asynchronous burst of producing
and non-producing lactobionic acid cells with detrimental
impact on the fermentation performance (Alonso et al., 2012b,
2013c).