biomolecules Article Xylan Decomposition in Plant Cell Walls as an Inducer of Surfactin Synthesis by Bacillus subtilis Ida Szmigiel 1 , Dorota Kwiatkowska 2 , Marcin Lukaszewicz 1 and Anna Krasowska 1, *   Citation: Szmigiel, I.; Kwiatkowska, D.; Lukaszewicz, M.; Krasowska, A. Xylan Decomposition in Plant Cell Walls as an Inducer of Surfactin Synthesis by Bacillus subtilis. Biomolecules 2021, 11, 239. https:// doi.org/10.3390/biom11020239 Academic Editor: Valdir Florencio Da Veiga Junior Received: 11 December 2020 Accepted: 4 February 2021 Published: 8 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Faculty of Biotechnology, University of Wroclaw, F. Joliot-Curie 14a, 50-383 Wroclaw, Poland; ida.szmigiel@uwr.edu.pl (I.S.); marcin.lukaszewicz@uwr.edu.pl (M.L.) 2 Faculty of Natural Sciences, Institute of Biology, Biotechnology and Environment Protection, University of Silesia in Katowice, Jagiello ´ nska 28, 40-032 Katowice, Poland; dorota.kwiatkowska@us.edu.pl * Correspondence: anna.krasowska@uwr.edu.pl; Tel.: +48-71-3756426 Abstract: Hemicellulose is the second most abundant plant heterogenous biopolymer. Among products obtained from a wide range of agro-residues, biosurfactants, e.g., surfactin (SU), are gaining increasing interest. Our previous studies have shown that a Bacillus subtilis strain can successfully produce a significant amount of SU using a rapeseed cake. This work aimed to investigate plant hemicellulose components as substrates promoting SU’s efficient production by B. subtilis 87Y. Analyses of SU production, enzymatic activity and cell wall composition of hulled oat caryopses suggest that the main ingredients of plant hemicellulose, in particular xylan and its derivatives, may be responsible for an increased biosurfactant yield. Keywords: Bacillus subtilis; surfactin; hemicellulose; xylan 1. Introduction Microbial hemi- and lignocellulosic hydrolysis of biomass has become increasingly important not only for reducing waste burden but also for producing a variety of high- value products [1–4]. Hemicellulose is the second most abundant plant heterogenous biopolymer. It is often composed of xylan [5,6], which has a backbone of β-(1→4)-linked xylose residues with side chains containing acetyl group, arabinose, or other sugars [7]. The precise structure of xylans varies between plant species and between different tissues [8]. Endo-β-1,4-xylanases and β-1,4-xylosidases degrade the xylan backbone while the xylan side-chains are hydrolyzed by α-L-arabinofuranosidase, α-glucuronidase, and acetylxylan esterase [9]. Among products obtained from a wide range of agro-residues, biosurfactants are gaining interest [10]. Biosurfactant surfactin (SU) is a well-known microbial-derived cyclic lipopeptide showing many useful properties. Among others, SU is known for its an- timicrobial and antitumor activity [11,12]. Our previous studies showed that Bacillus subtilis successfully produced SU in solid-state fermentation (SSF) on rapeseed cake [13]. Moreover, during preliminary investigations we observed increased SU production on several types of cereals, especially milled oats. Production of SU by B. subtilis has been connected to the degradation of plant cell wall elements, especially xylan [14]. Hence, we decided to investigate the relationship between degradation of oat cell wall components and SU production. In this study, we used B. subtilis 87Y previously isolated from soilworm Eisenia fetida [15] We decided to use the SSF process, which provides many benefits. Com- pared to liquid state fermentation, SSF requires lower energy, produces less wastewater, is environment-friendly, and most importantly, it resolves the huge problem of solid waste or by-product disposal [16,17]. It also better reflects the natural environmental conditions of the B. subtilis interaction with plant material, i.e., the degradation of plant debris in the soil. In this study we performed SSF on mixtures of rapeseed meal (RSM) and milled oats (MO) along with RSM with xylan supplementation. To find out the connection between oat cell Biomolecules 2021, 11, 239. https://doi.org/10.3390/biom11020239 https://www.mdpi.com/journal/biomolecules