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J Biol Eng. 2018 Dec 18;12:34. doi: 10.1186/s13036-018-0128-1. eCollection 2018.

Online evaluation of the metabolic activity of Ustilago maydis on (poly)galacturonic acid.

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1AVT - Biochemical Engineering, RWTH Aachen University, Jochen Büchs, Forckenbeckstr. 51, 52074 Aachen, Germany.
Bioeconomy Science Center (BioSC), 52426 Jülich, Germany.
2Institute for Microbiology, Cluster of Excellence on Plant Sciences, Heinrich-Heine University Düsseldorf, Universitätsstr. 1, 40225 Düsseldorf, Germany.



Pectin is a rather complex and highly branched polysaccharide strengthening the plant cell wall. Thus, many different pectinases are required for an efficient microbial conversion of biomass waste streams with a high pectin content like citrus peel, apple pomace or sugar beet pulp. The screening and optimization of strains growing on pectic substrates requires both, quantification of the residual substrate and an accurate determination of the enzymatic activity. Galacturonic acid, the main sugar unit of pectin, is an uncommon substrate for microbial fermentations. Thus, growth and enzyme production of the applied strain has to be characterized in detail to understand the microbial system. An essential step to reach this goal is the development of online monitoring tools.


In this study, a method for the online determination of residual substrate was developed for the growth of the plant pathogenic fungus Ustilago maydis on pectic substrates such as galacturonic acid. To this end, an U. maydis strain was used that expressed a heterologous exo-polygalacturonase for growth on polygalacturonic acid. The growth behavior on galacturonic acid was analyzed by online measurement of the respiration activity. A method for the online prediction of the residual galacturonic acid concentration during the cultivation, based on the overall oxygen consumption, was developed and verified by offline sampling. This sensitive method was extended towards polygalacturonic acid, which is challenging to quantify via offline measurements. Finally, the enzymatic activity in the culture supernatant was calculated and the enzyme stability during the course of the cultivation was confirmed.


The introduced method can reliably predict the residual (poly)galacturonic acid concentration based on the overall oxygen consumption. Based on this method, the enzymatic activity of the culture broth of an U. maydis strain expressing a heterologous exo-polygalacturonase could be calculated. It was demonstrated that the method is especially advantageous for determination of low enzymatic activities. In future, it will be applied to U. maydis strains in which the number of produced hydrolytic enzymes is increased for more efficient degradation.


Enzymatic activity; Exo-polygalacturonase; Galacturonic acid; Online measurement; Oxygen transfer rate; Pectin degradation; Ustilago maydis

Conflict of interest statement

Not applicable.Not applicable.The authors declare that they have no competing interests.Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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