availability conditions. Therefore, lactose feeding supported a
higher metabolic activity status, resulting in a meaningful increase
in the number of metabolically active cells during a whey/lactose
co-fermentation (96%) in comparison to lower percentages
achieved during whey/glucose (78%) and whey/glycerol
co-fermentations (60%) (Fig. S2). The appearance of
non-lactobionic acid producing cells, featured by a loss in the
membrane integrity as well as lower metabolic activity levels, constitutes
a major drawback behind culture robustness. When multimodal
phenotypes with subpopulations of reduced fitness are
present, fermentation production outputs are strongly affected
(Delvigne and Goffin, 2014). In fact, both growth fitness and lactobionic
acid production capabilities are greatly jeopardized when
physiologically and structurally compromised P. taetrolens cell subpopulations
arise due to deleterious bioprocessing conditions
(Alonso et al., 2012b, 2013c). Physiological cell segregation has
likewise been found to occur due to C-source shifts, affecting both
metabolite production capabilities and physiological cellular
responses (Lopes da Silva et al., 2011). Overall, higher cellular
robustness and increased metabolic activities were supported
under whey/lactose fed-batch co-fermentation. In contrast, the
higher C-source availability found in both whey/glycerol and
whey/glucose fed-batch co-fermentations was translated into
lower metabolic activity levels as well as longer cell polarization
periods