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. 2025 Mar;56(1):105-116.
doi: 10.1007/s42770-024-01564-y. Epub 2024 Nov 19.

Xylitol bioproduction by Candida tropicalis: effects of glucose/xylose ratio and pH on fermentation and gene expression

Sarah S Queiroz  1 , Isabela S Campos  1 , Tatiane F Silva  1 , Maria das Graças A Felipe  2
Affiliations

Xylitol bioproduction by Candida tropicalis: effects of glucose/xylose ratio and pH on fermentation and gene expression

Sarah S Queiroz et al. Braz J Microbiol. 2025 Mar.

Abstract

Xylitol is a highly demanded polyol in the food, pharmaceutical, and chemical industries. However, its current production methods are considered energy-intensive, require the use of hazardous chemical catalysts, and depend on complex and costly equipment. The biotechnological route of xylitol production is proposed as a sustainable alternative, but it still requires process improvements, such as enhanced fermentation capabilities, to be economically competitive. This study examined Candida tropicalis yeast to improve xylose-to-xylitol conversion via glucose: xylose ratio and pH modulation. Key parameters evaluated included xylose consumption rate (rS), xylose-to-xylitol yield (YP/S), and xylitol volumetric productivity (QP). Conditions with 50 g/L xylose at pH 3.5 exhibited superior xylitol production: 29.81 g/L, QP of 0.52 g/L/h, and YP/S of 0.54 g/g at 48 h. The statistical model demonstrated that the maximum YP/S and QP values have not yet been achieved. This could present an opportunity to be explored through yeast genetic engineering approaches. Additionally, the quantitative expression of the xylose transporter genes (XUT1 and STL2) and the xylose reductase gene (XYL1), previously identified in C. tropicalis, was evaluated under all tested conditions. Upregulation of the XUT1 was correlated with higher xylose concentrations, while STL2 was favored at lower xylose concentrations. The expression of XYL1 showed upregulation over time with higher xylose ratios. The high transcription levels and expression profile suggest that Xut1p-mediated xylose transport occurs through a proton symport mechanism. The results indicate that the pH factor indirectly influences XUT1 gene transcription, possibly as a compensatory response to the reduced transporter efficiency under high pH conditions. The present work underscores the influence of glucose ratios and pH in xylitol production, as well as the gene expression of xylose transporters and the key enzyme xylose reductase. Leveraging these insights can significantly enhance xylitol production from hemicellulosic hydrolysates through biotechnological pathways.

Keywords: Non-conventional yeast; Polyol bioproduction; RT-qPCR; Sugars assimilation.

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Conflict of interest statement

Declarations. Competing interests: The authors declare that they are not aware of conflicts of interest that may influence the present work.

References

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