Xylanolytic Bacillus Species for Xylooligosaccharides Production: a Critical Review Rozina Rashid1,2 and Muhammad Sohail1*
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Rashid and Sohail Bioresour. Bioprocess. (2021) 8:16 https://doi.org/10.1186/s40643-021-00369-3 REVIEW Open Access Xylanolytic Bacillus species for xylooligosaccharides production: a critical review Rozina Rashid1,2 and Muhammad Sohail1* Abstract The capacity of diferent Bacillus species to produce large amounts of extracellular enzymes and ability to ferment various substrates at a wide range of pH and temperature has placed them among the most promising hosts for the industrial production of many improved and novel products. The global interest in prebiotics, for example, xylooli- gosaccharides (XOs) is ever increasing, rousing the quest for various forms with expanded productivity. This article provides an overview of xylanase producing bacilli, with more emphasis on their capacity to be used in the produc- tion of the XOs, followed by the purifcation strategies, characteristics and application of XOs from bacilli. The large- scale production of XOs is carried out from a number of xylan-rich lignocellulosic materials by chemical or enzymatic hydrolysis followed by purifcation through chromatography, vacuum evaporation, solvent extraction or membrane separation methods. Utilization of XOs in the production of functional products as food ingredients brings well-being to individuals by improving defense system and eliminating pathogens. In addition to the efects related to health, a variety of other biological impacts have also been discussed. Keywords: Bacillus, Prebiotics, Xylanase, Xylooligosaccharides Introduction (Marín-Manzano et al. 2013), pectic oligosaccharides Prebiotics are non-degradable ingredients or food sup- (POs) (Míguez et al. 2016) cellooligosaccharides (COs) plements that can signifcantly impact the physiology of (Karnaouri et al. 2019) and xylooligosaccharides (XOs) entire body and improve the health of host by specif- (Mäkeläinen et al. 2009). As described by Yang et al. cally promoting the growth of intestinal bacteria (Gib- (2015), XOs promote the growth of probiotics, such as, son et al. 2010). In the light of existing health awareness, Bifdobacterium spp. and prevent the proliferation of the interest in prebiotic food ingredients has been ris- cancer cells in human colon (Le et al. 2020), therefore, ing. According to an estimate by Grand View Research these can be utilized in food and feed preparations. XOs (2016), the worldwide prebiotic market will reach up to are composed of 2–10 residues of β1 → 4-linked xylose worth 7.11 billion $ by 2024. Tere are many compounds and are converted to short chains fatty acids when fer- comprising a group of essential functional oligosaccha- mented by probiotic bacteria. rides with signifcant interest that have been studied for Te spore-forming Bacillus species are broadly used their prebiotic capabilities such as fructooligosaccharides in the production of functional foods for their probiotic (FOs) (Jain et al. 2014), galactooligosaccharides (GOs) properties and food preservation potential. In various animal feed supplementation, Bacillus strains provide abundant benefts including improvement in intestinal *Correspondence: [email protected] microbiota, digestibility and immunomodulation (Ber- 1 Department of Microbiology, University of Karachi, Karachi 75270, Pakistan nardeau et al. 2017). Te excellent fermentation potential Full list of author information is available at the end of the article of these strains along with enhanced product yield and © The Author(s) 2021. 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(2021) 8:16 Page 2 of 14 total absence of harmful by-products, render them an and Bifdobacterium spp. and are found in human and excellent choice for industrial processes (Singh and Bajaj cow milk (Marín-Manzano et al. 2013). POs are derived 2017). Additionally, their enhanced survivability under from pectin and polydextrose constituted from glucose wrathful environment makes them more suitable candi- (Míguez et al. 2016). Pectin is plentiful in diferent agro- dates for the production of prebiotics (Elshaghabee et al. industrial bio-resources for example apple pomace, citrus 2017). Tis review article summarizes various reports peel, sugar beet and cranberry mash. Tese materials can regarding XOs production by Bacillus spp. particularly hence be considered as a source of possible POs (Holck by xylanolytic bacilli. Te strategies used for separation et al. 2014). COs derived from hydrolysis of cellulose, and purifcation of XOs have also been discussed. Lastly, consist of a group of signifcant functional oligosaccha- the health benefts conferred by XOs, in general, have rides with substantial interest as a potential material in been described. food and chemical industries (Karnaouri et al. 2019). XOs constitute yet another promising prebiotic, found in Prebiotics and their signifcance some fruits, vegetables, wheat bran and bamboo shoot, Prebiotics are typically non-digestible compounds, some plant-based milks, and honey (Vázquez et al. 2000; therefore they become available to the gut microbes and Samanta et al. 2015). As reported by Jaskari et al. (1998) serve as feed for the benefcial microfora. Tis stimu- XOs can enhance in vitro growth of Bifdobacterium lates the production of nutrients by the gut bacteria for spp. more efciently as compared to other oligosaccha- the colon cells leading to a healthier digestive system rides including FOs. In addition to the heat-resistant and (Davani-Davari et al. 2019). Te mutual benefcial rela- acid-stable properties (Samanta et al. 2015), XOs have tionship between prokaryotes and the colon plays a key also been reported to be efective even when consumed role in health and well-being which is why the potential in lower doses which support the preference of XOs over of prebiotics is being explored (Aachary and Prapulla other types of prebiotics (Sako and Tanaka 2011). 2011; Olaimat et al. 2020). Studies suggest that anaero- bic degradation of prebiotics leads to their fermentation Xylooligosaccharides structure and their health by probiotics that stimulate colonization of host intestine benefts by the probiotic bacteria (particularly, lactic acid bac- XOs are the xylose sugar polymers produced from xylan teria). Te fermentation of prebiotics also results in the component of plant fbers (Reque et al. 2019). Xylans are release of short-chain fatty acids (SCFA) as by-products generally categorized into glucuronoxylan in hardwoods like acetate, butyrate and propionate. Tese SCFA may and arabinoxylan and glucuronoarabinoxylan in grasses. exert an anticarcinogenic efect because of the enhanced In arabinoxylans, the main (xylan) chain is substituted acid content in the colon, increased mineral absorption with α-arabinosyl residues. In case of glucuronoxylan, and by elaborating anti-allergic efect. Tey also promote 4-O-methyl glucuronic acid is linked by α-(l → 2) link- the growth of Lactobacillus and Bifdobacterium spp. ages; while glucuronoarabinoxylans consist of backbones and restrict the growth of potential pathogenic species of 1,4-linked-β-d-xylose residues with heterogenous (Jain et al. 2014). Prebiotic ingestion is possibly obtained substitutions (Chakdar et al. 2016). Te major products through the diet such as certain fruits and vegetables; produced from the hydrolysis of xylan are xylose, xylo- however, the levels of the natural prebiotics are exces- biose, xylotriose and xylotetraose with some residual sively low, signifying the need for increasing the levels of oligosaccharides. Te XOs with low degree of polym- prebiotic intake (Míguez et al. 2016). erization (2–10 monomers) are considered as potential non-digestible sugars, while those with ˂ 4 monomeric Source and types of prebiotics units encompass prebiotic applications because they Prebiotics as natural components may be found in vari- encourage the benefcial bacteria in the human gut such ous food sources like milk, honey, sugarcane juice, fruits as Bifdobacteria and inhibit the growth of pathogens (de and vegetables such as onion, beans, legumes, Jerusalem Freitas et al. 2019). Initially, the role of XOs as food ingre- artichoke, chicory, peas, leek, garlic, banana, rye and bar- dient and their positive role for gastrointestinal health ley (Davani-Davari et al. 2019). were explored in Japan (Kobayashi et al. 1991). Tere are many types of prebiotics, amongst which Lately, XOs (particularly xylobiose) has attracted inter- FOs are widely described as naturally occurring oligo- est as an efective prebiotic that has benefcial efects for saccharides derived from natural inulin (Jain et al. 2014) animal and human digestion. XOs not only promote right and are found in asparagus, wheat, sugar beet, tomato, type of commensals, but also