## 3 Major Routes of Xylose Transport and Metabolism in Bacteria
### Metabolic Pathways to Xylitol and Xylonic Acid
#### 3.3.1 Xylitol  
The large-scale production of xylitol is generally carried out by a chemical route based on the catalytic hydrogenation of highly pure D-xylose in solution (obtained from hardwood hydrolysates) submitted to high temperature (140-200 \({}^{\circ}\)C) and pressure (50-60 bar) conditions, and in the presence of the toxic Raney nickel catalyst [101]. The chemical process requires several purification steps because only pure xylose can be used in this chemical reduction. The conversion efficiency of the chemical process can be estimated as 8-15% from the initial raw material (hardwood hydrolysates), 50-60% from the hemicellulose in the raw material, and almost 98% from pure xylose [102; 103]. Biotechnological methods are a promising choice that has been studied as alternatives to the conventional method, since they involve much milder conditions and can use a mixture of sugars as raw materials. This alleviates the purification step of the lignocellulosic hydrolysates used as carbon source, resulting in a less energy-demanding and cheaper process [104]. According to [PERSON] et al. [104], yeasts are preferred for xylitol production primarily due to their high pentose assimilation rates and xylitol productivity, as result of stable expression levels of XR and X\({}_{\mathrm{OH}}\)DH. Currently, there are a few studies on xylitol productions by yeast (namely different species of _Candida_) with impressive results, achieving conversion efficiencies of nearly 86%, with a volumetric productivity of 4.88 g L\({}^{-1}\) h\({}^{-1}\), which is relatively high when compared with other reported microorganisms [104; 105]. However, most of these yeasts are not considered as GRAS (Generally Recognized as Safe) by the FDA. Furthermore, filamentous fungi were also reported to be xylitol producers; however, their xylitol production is too low to be competitive. Thus, there is a need to look for other microorganisms capable of producing xylitol with high yields, namely bacterial species. Several studies have shown the ability of some bacterial strains, namely _Cellulomonas_, _Corynebacterium_ sp., _Enterobacter liquefaciens_, _Gluconobacter oxydans_, _Mycobacterium smegmatis_, and _Serratia_, to produce xylitol [106, 107, 108, 109, 110]. Although more common in yeasts or fungi, XR (Figure 1) is not totally specific of these microorganisms, as this enzyme was also present in some bacterial extracts. For example, _Corynebacterium_ sp. NRRL B 4247 exhibited both NADH- and NADPH-dependent XR activity in enzymatic assays using cell-free extracts. In these bacteria, the highest yield obtained was of 0.57 g g\({}^{-1}\) xylose, when an initial D-xylose concentration of 75 g L\({}^{-1}\) was used. This yield was however improved when potassium gluconate was added to the medium [108]. For _Corynebacterium_ sp. NRRL B 4247, xylitol production appears to be growth-associated [108]. XR (Figure 1) enzymatic activity is present in _Enterobacter liquefaciens_, with NADPH as co-factor, and a productivity of 0.35 g L\({}^{-1}\) h\({}^{-1}\) has been reported in this case [110]. The XR route has also evolved in enteric bacteria such as _E. coli_; however, the efficiency of the pathway is low. _P. sacchari_ is also able to accumulate xylitol. In this case, it was hypothesized that the metabolic pathway used by _P. sacchari_ is the isomerase pathway, since genes present in its genome include those that encode for XI, XK (Figure 1), and ABC xylose transporter, whereas the genes of the oxidoreductase route were not found, suggesting that _P. sacchari_ cannot assimilate xylose by this pathway [111]. In xylose-richer mixtures, _P. sacchari_ was able to accumulate both xylitol and xylonic acid, but the mechanism that leads to this production was not exploited [60]. As referred, the most common way used by microorganisms to produce xylitol is through a single-step mechanism, where xylose is reduced to xylitol by xylose reductase while one of the cofactors NADH/NADPH (depending on the species preference) undergoes oxidation.