## 3 Major Routes of Xylose Transport and Metabolism in Bacteria
### Metabolic Pathways to Xylitol and Xylonic Acid
#### 3.3.1 Xylitol
Under aerobic conditions, the NADH formed during xylose metabolism can be reoxidized into NAD\({}^{+}\) in the electron transport system, and consequently, xylitol is not produced, since the high NAD\({}^{+}\)/NADH ratio favors xylitol oxidation to xylulose. Under anaerobic conditions, microorganisms are unable to metabolize d-xylose because of the redox imbalance between NAD\({}^{+}\) and NADH. Under oxygen-limited conditions, the electron transport system is unable to oxidize intracellular NADH completely, increasing the NADH concentrations, which subsequently favors the xylulose conversion into xylitol by the XOHOHOH enzyme or the reduction of xylose into xylitol by the XR enzyme (Figure 1) [107]. However, other authors claim that an increase in the amount of dissolved oxygen (DO) also increases the quantity of xylitol produced, since the activity of the enzymes depends not only on the regeneration of cofactors but also on oxygen availability [38]. Table 1 summarizes the contents of this section regarding a selection of bacterial strains able to convert xylose into xylitol. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Strains** & **C-Source** & \begin{tabular}{c} **Genetic** \\ **Modification** \\ \end{tabular} & **Growth Conditions** & \begin{tabular}{c} **Xylitol** \\ **(g L\({}^{-1}\)) * \\ \end{tabular} & \
\begin{tabular}{c} **Y\({}_{\text{yiltrolulose}}\)** \\ **(g g\({}^{-1}\)) \\ \end{tabular} &
\begin{tabular}{c} **Productivity** \\ **(g L\({}^{-1}\) * \\ \end{tabular} & **Ref.** \\ \hline \multirow{4}{*}{_Corgubacterium_} & arabinose & \multirow{4}{*}{Yes} & Batch shake flask & 6.7 & n.a. & n.a. \\  & glucose & & Fed-batch shake flask & 31 & n.a. & 0.28 g g\({}_{\text{cdw}}\)\({}^{-1}\) h\({}^{-1}\) \\  & xylose & & Fed-batch shake flask & 27 & n.a. & 0.22 g g\({}^{-1}\)\({}_{\text{cdw}}\) h\({}^{-1}\) \\  & acid pre-treated liquor & & & & & \\  & of sorghum stover & & & Shake flask & 1.7 & 0.57 & 0.071 \\ \hline \multirow{4}{*}{_Corgubacterium_} & \multirow{4}{*}{xylose} & \multirow{4}{*}{No} & \multicolumn{2}{c}{6-phosphogluconate } & \multirow{4}{*}{10} & \multirow{4}{*}{n.a.} & \multirow{4}{*}{0.067} & [108] \\  & & & & & & & \\  & & & & & & \\  & & & & & & \\ \hline \multirow{4}{*}{_Corgubacterium_} & \multirow{4}{*}{xylose} & \multirow{4}{*}{No} & \multicolumn{2}{c}{6-phosphogluconate } & \multirow{4}{*}{69} & \multirow{4}{*}{n.a.} & \multirow{4}{*}{0.21} & [123] \\  & & & & & & & \\  & & & & & & \\ \hline \multirow{4}{*}{_Enterobacter_} & \multirow{4}{*}{xylose} & \multirow{4}{*}{No} & \multicolumn{2}{c}{Shake flask} & \multirow{4}{*}{33} & \multirow{4}{*}{n.a.} & \multirow{4}{*}{0.35} & [110] \\  & & & & & & & \\ \cline{1-1} \cline{6-6}  & & & & & & & \\ \hline \hline \multirow{4}{*}{_E. coli_ BL21(DE3)} & \multirow{4}{*}{xylose} & \multirow{4}{*}{Yes} & \multicolumn{2}{c}{Shake flask} & \multirow{4}{*}{202} & \multirow{4}{*}{1.0} & \multirow{4}{*}{6.37} & [113] \\ \cline{1-1} \cline{6-6}  & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of bacteria (wild type and genetically modified) with potential for D-xylitol production from lignocellulosic biomass (n.a.—information not available).