Overall, results showed that higher carbon availability led either
to the hindrance, overproduction, or simultaneous co-production of
lactobionic and gluconic acid depending on the co-substrate supplied.
Whereas glycerol was the most preferred C-source for P. taetrolens’
growth, the presence of glucose modulated the oxidative
machinery by channelling glucose towards its oxidation, resulting
in the formation of gluconic acid via the direct oxidative pathway
(Fig. 6). Such direct oxidative pathway involves the conversion of
glucose into gluconic acid or 2-ketogluconate which are transported
to the cytoplasm, then converted to 6-phosphogluconate
and further metabolized through the Entner-Doudoroff pathway
(del Castillo et al., 2007; Schleissner et al., 1997). Under batch
co-fermentation mode, glucose as a rapidly-assimilated compound
exerted a regulatory effect on lactose oxidation, inhibiting lactobionic
acid formation. Such hierarchy mechanism suggested that
glucose may compete with lactose for the membrane dehydrogenase,
eliciting a carbon catabolite repression. Carbon catabolite
repression in pseudomonads is featured first by glucose inhibition