with glucose:xylose = 2:1 (w/w), were investigated. The C/N ratio
of 15:3 was used by supplementing tryptone as the nitrogen
source. As shown in Table 3, CDW obtained in 100 g/L sugar were
the highest (32.2 g/L) and it was almost 6-fold of that obtained in
15 g/L sugar (5.33 g/L). PHB content reached 58.6%, and PHB production
reached 18.84 g/L, both being the highest among all the sugar
concentration tested. Further increase in sugar concentration
to 150 g/L resulted in the inhibition of cell growth and reduced
PHB content, both reaching the highest at 36 h. Glucose was not
fully utilized with 63.46 g/L left and xylose was entirely unconsumed.
Such inhibitory effects were consistent to the report by
Gouda et al. (2001) and they might be attributed to the osmotic
pressure from the high concentration of sugar (Zhang et al.,
2012). The lack of consumption of xylose under such condition
indicates that high sugar concentration inhibits the activities of
xylose-utilizing enzymes in B. megaterium R11. It is also noteworthy
that a sharp decrease of medium pH from 6.8 to 5.2 at 24 h was
observed in medium containing 150 g/L, whereas pH decrease in
medium containing sugar of 100 g/L and below was insignificant
(data now shown). The reduced pH at 150 g/L sugar might be
one of the factors that decreased PHB production. This is according
to the report by Faccin et al. (2009) that medium pH was one of the