Keywords

14.1 Introduction

The six-legged critters called insects belonging to the invertebrate group of animals are the most abundant multicellular organisms on the planet. They play incredible role to maintain the milieu, as Prof. EO Wilson, famous biologist, rightly said “if insects were to vanish, the environment would collapse into chaos.” Class Insecta of phylum Arthropoda of the Kingdom Animalia is a widely diverse brunch that includes a wide range species. The number of existing insect species is estimated from six to ten million (Chapman 2006) and potentially corresponds to over 90% of the diverse animal life forms on Earth (Erwin 1982). Insects, present in almost all inhabited condition, typically possess the basic body plan composed of three segments: head, thorax, and abdomen and having three pairs of jointed appendages (legs). Major groups of insect species belong to orders like Coleoptera (sheet winged, e.g., Beetles), Lepidoptera (paired wings, e.g., Butterflies), Diptera (e.g., Flies), and Hymenoptera (e.g., Bees, Ants, etc.) (Wheeler et al. 2001).

Due to diverse habitation, the food habits of insects vary widely. A number of diverse microorganisms play a major role in digestion, metabolism, and nutrition in insect gut (Russell and Moran 2005; Douglas 2011; Krishnan et al. 2014; Sudakaran et al. 2015). Typically, insect’s digestive tract consists of foregut, midgut (or ventriculus), and hindgut (Chapman et al. 2013) although diversified modifications of the tract are eminently related to the adaptation to different feeding modes (Chapman et al. 2013; Engel and Moran 2013a, b). Many insects utilize the lignocellulosic material as their primary food source by degrading the complex polysaccharide using inhabiting gut bacteria and subsequently converting it into glucose monomer molecule (Sun and Scharf 2010). Gut bacteria of such insects are capable of producing various cellulolytic and ligininolytic enzymes that can break down the most abundant biological macromolecule. Lignocellulosic compounds are the basic structural components of plants. Cellulose is known as the world’s most abundant organic polymer, and plants produce approximately 4 × 109 tons of cellulose per year (Irfan et al. 2012; Chatterjee et al. 2015). As they are aplenty and producing a huge biomass, having molecular complexity, are difficult to degrade (Sari et al. 2016). Due to this reason, the lignocellulosic biomass also creates environmental nuisance and pollution. However, proper utilization of the material may help in generating different products like ethanol, biogas (methane). Generally, expensive, instrument intensive various chemical and physical treatment processes are followed to convert this natural component into energy resources (Vandenbossche et al. 2014). Enzyme-based biodegradation can be a choice to develop appropriate method for proper utilization of the biomass into a productive formulation. In this context, lignocellulolytic enzymes produced by the gut bacteria of insects have been drawing interests to scientific community recently, due to its enormous scope of exploration of potential species.

The present review work encompasses the potential role of insect gut bacteria in degradation of lignocellulosic biomass. Detailed literature survey was carried out to incorporate aspects of microbial colonization in insect gut, cellulolytic enzyme production, genomic evolution, and role of insect diet in cellulase production. Further, the importance of prospective bacteria for harnessing the cellulase enzyme and appropriateness of application in lignocellulosic wastes degradation is also discussed in this review.

14.2 Insect Gut Environment

Physiochemical conditions of insect gut are important factor that affects the microbial colonization. The actively regulated lumen of different gut compartments varies in pH and oxygen availability (Dow 1992; Hyun et al. 2014). The pH of gut lumen may vary extensively in insects. Gut microbiota variation within the insect depends upon various factors like the insect order, morphological state (metamorphosis stages), gut condition that varies over life cycle of the particular insect. The aerobic or anaerobic conditions of insect gut depends upon the shape of the gut, size of insect (more anaerobic conditions in bigger insects), availability and partial pressure of oxygen in gut etc. (Elbert and Brune 1997; Hyun et al. 2014). Microbial colonization and metabolism also dynamically shape within the insect gut as per the state of different compartments.

Interestingly, the pH of lepidopteran guts is extremely alkaline (with pH around 11–12) in nature that helps them to digest tannin-rich leaves enhancing their nutrient availability within gut; however, the microbial population is very less as a consequence (Berenbaum 1980; Appel and Martin 1990; Dow 1992; Harrison 2001; Engel and Moran 2013a, b). This alkalinity is maintained through recycling of H+ into the cytoplasm by midgut electrogenic K+ pump which is energized by a H+-pumping V-ATPase and net transport of alkali metal is attained by linking it to a nH+/alkali metal exchanger; the electrical field generated by the V-type ATPase that confers high luminal pH in lepidopteran insects is explained as a model of passive (Nernstian) distribution of proton (Dow 1992). Midgut of the lepidopteran insect has the potentiality to generate pH gradient using metabolic energy and pH profiles observed along the gut is basically due to morphological difference in gut sub regions and differential acid–base transportability of midgut (Dow 1992). Similarly, Boudko et al. (2001) stated that alkaline environment in the midgut (anterior region) is dependent on V-ATPase pumps that maintain strong gradients in hydrogen ion concentrations in mosquito larvae. Less extreme pH gradients reported by Appel and Martin (1990) found in the lumens of a number of nonholometabolous insects. However, guts of few soil-feeding termite species show extreme variation with pH ranging from 5 to >12, having selective alkaline-tolerant symbiotic bacteria from Firmicutes, Clostridium, and Planctomycetes (Brune and Ohkuma 2010; Bignell 2010; Kohler et al. 2012; Engel and Moran 2013a, b). Termite guts have several hindgut compartments or paunches harboring distinct sets of microbial communities acting as bioreactors with high rates of turnover of hydrogen pools (Pester and Brune 2007; Engel and Moran 2013a, b). Microbial fermentation producing acetate, lactate, and formate are abundant in the hindgut and midgut regions in the larvae scarab beetle (Pachnoda ephippiata) (Lemke et al. 2003; Cazemier et al. 2003).

14.3 Microbial Colonization Within Insect Gut

In holometabolous insects, four phases (egg, larvae, pupa, and adult) are eminent in the life cycle, which takes place through metamorphosis of molting and demolting (Moll et al. 2001; Minard et al. 2013). During the phases of metamorphosis, the microbial habitat of insect gut is affected considerably. While, during molting, insect midgut, constantly renewing peritrophic matrix along with microbial population; however, due to peeling of exoskeleton at foregut and hindgut, they are subjected to significant changes in terms of microbial population (Fukatsu and Hosokawa 2002; Minard et al. 2013; Engel and Moran 2013a, b). Moll et al. (2001) reported that during metamorphosis in some insects (as, e.g., mosquito) total to near total eradication of the gut bacteria takes place (Moll et al. 2001; Engel and Moran 2013a, b). Habitat of insects and the source of food consumed by them also affect the gut microflora colonization (Oliver 2003; Douglas 2015). Again, many insects, in their adult stages, represent specialized crypts or paunches that support microbial habitat (Fukatsu and Hosokawa 2002; Engel and Moran 2013a, b).

Gut epithelium of insect consists of folded membrane with either epithelial enterocyte or endocrine cells (Marianes et al. 2013). Enterocytes having microvilli secretes varied enzyme and help in assimilation of nutrients, while endocrine cells, devoid of microvilli, produce peptide and hormones (Beehler-Evans and Micchelli 2015). Other cell types such as goblet cells (in Lepidoptera), cuprophilic cells (in Dipterans) also play an important role in ion transport, with H+ pumps, that results in either alkaline or acidic conditions inside the insect gut (Huang et al. 2015). The gut epithelium functions as a selective barrier which helps in the uptake of nutrients and exchange of ions and water (Simpson et al. 2015). The transport is facilitated by two routes: transcellular route (across the epithelial cells) and paracellular route (between the epithelial cells). Beside this, water content of the body fluids is also regulated by the gut epithelium through channels known as aquaporins (Spring et al. 2009; Huang et al. 2015) and during water and heat stress condition this water moved down the osmotic gradient (from high concentration to low compartment) across biological membranes, hence, help the insects to survive in severe condition (Fig. 14.1).

Fig. 14.1
figure 1

Schematic representation of exchange of microbes between insect and environment

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14.4 Insect Gut Microbial Composition

The gut of insects has a varied group of microorganisms, which are usually mutualistic in nature and help in digestion of intractable plant polymers, supplying nutrients, stimulus of midgut self-renewal providing resistance to parasite invasion, and host fitness with different environmental conditions (Hosokawa et al. 2006; Oliver 2003; Douglas 2015). In this manner, a mutualistic association is formed between insect and intracellular microbes to play diverse metabolic roles to their host, even, working with new metabolic pathways to utilize nutrients which may otherwise be missing from their conventional food sources (Baumann 2005; Carrasco et al. 2014). Reports suggest that the extracellular symbiotic microbes associated with the alimentary tract of stinkbugs insect (Pentatomidae sp.) sustain in the uneven environment due to their potential adaptation as evident in γ-proteobacteria (Nikoh et al. 2011; Hosokawa et al. 2006; Kikuchi et al. 2009). Fukatsu and Hosokawa (2002) reported that vertical transmission of microorganisms to newly hatched nymph takes place through a symbiont capsule being ingested deposited in the midgut by the nymphs of insect. Similar observations have been reported by Kikuchi et al. (2009). During the development of midgut, the anterior portion becomes free from symbiont microorganisms, while the posterior part transformed into a baggy organ having diverse groups of symbiont cells (Hosokawa et al. 2006; Nikoh et al. 2011). However, regarding structural, functional, and evolutionary studies and their correlation on symbiont microorganisms and insect gut, the available scientific information needs to be augmented (Douglas 2015).

The insect gut bacteria also acts as an iron reservoir for the host that helps as iron sink and source for physiological activities. Pesek et al. (2011) reported that Microbacterium arborescens in the larval gut of Spodoptera exigua (Beet armyworm) possess iron reservoirs that help bacterial enzyme (Peroxidase) to inhibit the occurrence of cell-damaging oxygen radicals. Several other structurally similar enzymes (also known as DNA protecting proteins) also help the host insect during starvation (Pesek et al. 2011). Gut bacteria also contribute in maintaining biogeochemical cycle by recycling nitrogen, as members of enterobacteriace species Klebsiella, Roseateles aquatilis found in larvae of southern pine beetle accumulate nitrogen in the environment (Krishnan et al. 2014). Insect feeding on wood (lignocellulose) sources, as a biochemical catalyst, has enormous impact in carbon cycling in nature (Sun and Scharf 2010; Taggar 2015). These lignocellulosic enzymes have a wide range of potential applications including industries for various application purposes.

14.4.1 According to Diet

Anatomy and physiology of insect’s intestines differ greatly and act as the host a variety of microorganisms as per their food habit. The insect gut microbial diversity represents a large source of unexplored microbes that participate in various activities from utilization of different organic polymers, nitrogen fixation, methanogenesis, pesticide degradation, pheromone production to pathogen prevention (Nardi et al. 2002; Reeson et al. 2003; Mrázek et al. 2008). As per the report, the estimated number of bacteria ranges from 109 in honey bees, to 105 in a fruit fly, to negligible numbers in the plant sap-feeders (downloaded from http://schaechter.asmblog.org/schaechter/2013/06/). Different food habits trigger more diversity in gut bacteria. Hernández et al. (2013) reported that polyphagous Iberian geotrupid dung beetles Thorectes lusitanicus that feeds on wide range of food material from dung, acorns, fungi, fruits, to carrion, harbors assorted group of aerobic, facultative anaerobic, and aerotolerant gut bacteria. However, environment plays a critical role in acquisition of gut microbial population, which are more or less constant with respect to the habitats it shares (Wang et al. 2011). The relationships between the insect and its gut microbiome are dynamic one, and resident bacteria play a major role in colonization in the gut even by non-indigenous species. The insect gut is thus a “hot spot” for gene transfer (transfer of plasmids and transconjugation) between bacterial strains that inevitably contribute toward insect’s food habit and nutrition (Dillon and Dillon 2004). Anderson et al. (2013) reported that six of seven bacterial phylogenetic groups present in the hindgut of honey bee (Apis mellifera) play a critical role to manage important functions related to the health of host. Engel and Moran (2013a, b) suggested that high levels of genetic diversity and functional differences within gut possibly due to niche partitioning within the species during evolution. Gut bacteria also play a role in pathogen protection in insects. Reports on Wolbachia, a maternally inherited intracellular bacterium which is found in 40% of insects and other arthropod species, suggest that, it augments pathogen protection, survival against viral-infection in many insect species (Moreira et al. 2009; Bian et al. 2010; Friberg et al. 2011; Zug and Hammerstein 2012; Kuraishi et al. 2013a, b; Ye et al. 2013).

Insect gut microbial composition depends upon several biological and ecological factors (Nikoh et al. 2011; Colman et al. 2012). However, host diet plays a major role in gut bacterial diversity in insect species. Xylophagus insects feeding on decaying woods possess the abundance gut flora (102.8 ± 71.7 species-level OTUs/sample, 11.8 ± 5.9 phylogenetic diversity (PD)/sample) while the insects like bees feeding on relatively simpler food materials have low abundance in bacterial flora (11.0 species-level OTUs/sample ± 5.4, 2.6 ± 0.8 PD/sample) (Colman et al. 2012). Although insect guts can also harbor protists, fungi, archaea; however, bacterial population plays dominant role in insect physiology, food habit, nutrition, etc.; however, their maintenance depends upon the social transmission (Hongoh 2010). Wood or detritus eaters have fungi in their guts, while methanogenic archaea are present in the guts of beetles and termites that feed on dung, detritus, or wood (Brune 2010; Engel and Moran 2013a, b).

On the other hand, food components play a dominating role in microbial population having little impact on host species characterization. Pernice et al. (2014) reported that phylogenetically distantly related insect species with different gut microbial composition when feed and cultured on similar food substrates exhibited similar microbial communities in their gut. Further, Mikaelyan et al. (2015) found that diverse gut bacteria help termite to effectively digest the different wood sources through a synergistic and symbiotic effort. However, apart from bacteria, termite gut also contains other intestinal flora, like cellulolytic flagellates, prokaryotic communities, and archaeal populations (Lozupone et al. 2012; Mikaelyan et al. 2015; Brune 2010). Distinct phylogenetic pattern occurs in the termite gut microflora from different subfamilies that show the diet is the main factor which formulates the bacterial community structure having distinctive microenvironmental conditions (Mikaelyan et al. 2017).

14.4.2 Role in Partner Selection

As discussed above, gut microbiome manipulates a number of aspects including fitness of organism that otherwise influences its mating preferences. Dodd (1989) reported the insect Drosophila pseudoobscura preferred positive assortative mating where it favored mating partners reared for more than 25 generations in the same media; whereas, the flies reared on other media (either starch-based or maltose-based media) preferred mating partner came out from their own rearing media, becoming a population of either “starch flies” or “maltose flies.” Similarly, Sharon et al. (2010) examined the mating preference of Drosophila melanogaster using molasses and starch as rearing media. It was found that the mating preference of these flies appeared only after one generation and was maintained for at least 37 generations; however, mating preference was eliminated after treating with antibiotic, which signifying the vital role of microbiota of fly gut for the phenomenon (Sharon et al. 2010). These observations triggered the scientific community to study further on the role of gut microbial population based on dietary substances. As reported, gut microbiomes have an effect on longevity and reproduction capacity to an organism. A study on antibiotic-treated termites Zootermopsis angusticollis and Reticulitermes flavipes showed the reduced diversity and decrease of useful microbes in their gut flora and subsequent malnourishment that led to the production of significantly less number of eggs (Rosengaus et al. 2011). Brucker and Bordenstein (2013) showed, in their study on the parasitic wasp Nasonia sp., that the bacteria in this insect gut of wasp species (Nasonia giraulti and Nasonia vitripennis) execute themselves as a living barrier that prevents their evolutionary trail from mating with each other and precisely preserve a different sets of gut microbiomes. However, after their forceful crossbreed, the hybrids develop an indistinct microbial population in the gut that causes their premature deaths (Brucker and Bordenstein 2013). Further, in this study, it was found that bacterial constituents and abundance are unequal in hybrids in comparison with the parental species. While Providencia sp. is the major gut bacteria in parental species, Proteus mirabilis became dominant in the hybrid one, which signifies that interbreeding between two species caused damaging modification to the gut flora; therefore, the microbiome of Nasonia helps to remain the two species separate (Brucker and Bordenstein 2013).

14.4.3 Genome Evolution

Symbiotic association with the gut microbes is believed to be one of the key factors that assist the largest phylum of the animal kingdom to be so successful over the centuries (Warnecke et al. 2007; Lize et al. 2014; Brown and Wernegreen 2016). Through the mechanism of differentiation during the process of vertical transmission, among environmentally acquired varied groups of bacteria, symbiotic microorganisms can selectively be preserved by the insects (Kiers et al. 2008). Gut environment has profound impact toward evolution of gut associates either obligate or facultative (Kikuchi et al. 2009; Nikoh et al. 2011). Transfer of gut-associated bacteria through vertical (queen to daughter) or horizontal transmission (between workers) assists the host by serving in the developmental phases (Kwong and Moran 2015). Kikuchi et al. (2009) reported the transmission of symbiotic mutualistic bacteria like Buchnera aphidicola sp. (in aphids), Wigglesworthia glossinidia sp. (in tsetse flies) take place packed in mycetocytes. This vertical transmission plays an important role in nutrition (Kikuchi et al. 2009). Further, accelerated molecular evolution, AT-biased nucleotide composition, and reduced genome size changes have been noticed conferring to major evolutionary pattern in these symbionts (Wernegreen 2002; Brown and Wernegreen 2016). The study of microorganism diversity in insect gut can be done individually or metagenomic approaches. Culture-dependent classical methods have many inadequacies in terms of species diversity study, as many of the insect gut bacteria may not be cultured in laboratory conditions. However, metagenomic approaches can provide an insight to all the microbial community presents in the gut, along with scope to ascertain inter- and intra-specific role (Brune 2010; Ellegaard and Engel 2016).

14.5 Lignocellulose as a Component: Physiological Property

“Lignocellulose” is the combination of three biopolymers: cellulose, hemicellulose, and lignin which make the natural, rigid structural component of plants. Cellulose is a polysaccharide linear chain of thousands of β(1 → 4) linked D-glucose units, with chemical formula (C6H10O5)n (Fig. 14.2). Molecular weight of cellulose ranges from 200,000 to 2,000,000, corresponding to 1250–12,500 glucose molecules per residues (Bashir et al. 2013; Chatterjee et al. 2015) molecule. The cellulose polymer is subdivided into four different categories (Cellulose I, II, III, and IV) which vary in physical and chemical properties (Zhang and Zhang 2013). Lignocellulosic biomass makes a large portion of the plant biomass, approximately 50% in the world (Sanchez and Cardona 2008). It is the most abundant component of terrestrial ecosystem and thus represents a massive source of food and energy for diverse group of microorganisms (Shaikh et al. 2013). Cellulose, a tasteless, odorless, hydrophilic, homopolysaccharide, is more crystalline in nature than starch. Cellulose requires temperature beyond 320 °C to attain the amorphous state in contrast to temperature above 60–70 °C, which converts the crystalline starch into amorphous state. Application of strong acid can break amorphous state of cellulose and produces nano-crystalline cellulose (Peng et al. 2011).

Fig. 14.2
figure 2

Schematic representation of cellulose molecule in plant cell

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In lignocellulosic biomass, lignin comprises around 10–30% and is the second most abundant natural organic polymer that enables plants to generate rigid structures and gives protection against hydrolysis of cellulose and hemicellulose (de Gonzalo et al. 2016). Lignin is a highly cross-linked polymer, formation of which is activated by plant laccases and/or peroxidases. A range of ether and carbon–carbon bonds (β–β, β–O–4, and β–5 bonds) polymerizes the 4-hydroxyphenylpropanoid monomers (monolignols having phenolic moieties, like p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) groups), and the composition of which varies depending on the plant tissue and species (Vanholme al. 2010; de Gonzalo et al. 2016). The lignin degradation usually takes place in two stages. The first stage involves extracellular, non-specific depolymerization, forming aryl and biaryl compounds (like β-aryl ethers) following mineralization of these compounds through specific catabolic enzymes and pathways (Sainsbury et al. 2013). Most of the lignin degradation studies show that enzymes such as manganese peroxidase (MnP), lignin peroxidase (LiP), and laccase from fungi (white rot fungi) are the best enzyme that degrades the lignin component (Sainsbury et al. 2013; de Gonzalo et al. 2016).

Bacterial ligninolytic enzymes are peroxidase in nature, which belong to the family of heme-containing peroxidases, better known as dye-decolorizing peroxidases (DyPs, EC 1.11.1.19), contains a non-covalently bound heme b cofactor (Van Bloois et al. 2010; Colpa et al. 2014; Yoshida and Sugano 2015; Singh and Eltis 2015). In the year 1999, the first member of this family, DyP from Bjerkandera adusta (order: Polyporales), was isolated and characterized (Kim and Shoda 1999). Till then, different bacterial DyPs have been studied and have been reported, which suggest that putative DyP-encoding genes are amply present in bacterial genomes (Lambertz et al. 2016). Other Dyp enzymes that have been reported in bacteria are PpDyP from Pseudomonas putida MET94 (Santos et al. 2014), BsDyP from Bacillus subtilis KCTC2023 (Min et al. 2015), SviDyP from Saccharomonospora viridis DSM 43017 (Yu et al. 2014), and TfuDyP from Thermobifida fusca (Van Bloois et al. 2010). Interestingly, in the year 1988, in laboratory studies, bacterial “lignin peroxidase” has been reported from Streptomyces viridosporus (Wang et al. 1990; Thomas and Crawford 1998; de Gonzalo et al. 2016). Recently, Davis et al. (2013) reported gene encoding a putative Tat-secreted DyP in Streptomyces isolate. However, in bacteria, due to the complexity of the proteins having several disulfide bonds, integrate calcium ions and a heme cofactor, DyP is usually glycosylated, which is in contrast to the regular peroxidases present in fungi (Lambertz et al. 2016). There are some special conditions which the bacterial machinery requires for folding and processing in manufacturing protein. Although genetic engineering technique has been used to express various DyPs in E. coli, which is comparable to that of fungal peroxidases (de Gonzalo et al. 2016; Lambertz et al. 2016). Bacterial peroxidases and laccases are recently being used for large-scale recombinant enzyme development. Studies have been reported that bacterial laccases can be produced in E. coli (Ihssen et al. 2015; de Gonzalo et al. 2016). Therefore, bacterial enzymes are comparatively easier to produce and have potential application in lignin biodegradation.

14.6 Enzymatic Breakdown of Lignocellulose

Natural degradation lignocellulosic biomass occurs through coordinated action of a set of enzymes. The conversion of lignocellulose into simpler molecule, including glucose, is essential to utilize the biomass in a productive manner. Plant biomass-derived products (like aromatic products, carbohydrates, ethanol) can be used as food and flavor compounds, polymer precursors, pharmaceutical building blocks, fuel, etc. (Asgher et al. 2014; Ragauskas et al. 2014; Kawaguchi et al. 2016; de Gonzalo et al. 2016). However, biochemical reactions for various processes use to operate at favorable pH, temperature, pressure, and other biotic and abiotic conditions. Lignocellulosic biomass hydrolysis requires a set of coordinated action of multiple enzymes. Cellulase enzyme is a complex package of three different classes of enzymes: (1) Endo-1,4-β-endoglucanase binds to non-crystalline part of cellulose, cleaves glucosidic linkages, (2) Exo-1,4-β-exoglucanase binds to crystalline part of the cellulose, and cleaves the molecule while, (3) β-glucosidase enzyme cleaves the cellobiose (a disaccharide molecule) releasing glucose monomers. These three classes of cellulase enzyme are necessary to breakdown the crystalline cellulose into simpler forms such as glucose (Willis et al. 2010; Chatterjee et al. 2015). Nature of cellulose, source of cellulolytic enzymes, optimal condition for catalytic activity, and production of enzymes also play critical role in bioconversion of cellulose (Chatterjee et al. 2015).

14.7 Cellulosomes Complex

Due to its high recalcitrant crystal structure, cellulose degradation is limited to few microorganisms and is a complicated chore. In anaerobic bacteria, cellulases are bound to scaffoldin, forming multicomponent, multienzyme cellulosome complexes that efficiently can degrade cellulosic substrates (Béguin and Lemaire 1996; Bayer et al. 2004; Bae et al. 2013). Cohesin–dockerin interaction helps the non-catalytic subunit called scaffoldin, to bind the various enzyme subunits into the complex. The interaction is highly specific between the scaffoldin-based cohesin modules and the enzyme-borne dockerin domains, which forms the assembly of the cellulosome (Bayer et al. 2004; Bae et al. 2013; Haitjema et al. 2017). This multienzyme complex facilitated by cohesin–dockerin interaction, which is the basis for newly emerging field of synthetic biology (Haitjema et al. 2017). Investigation of the growth substrate-dependent variations in cellulosomal systems has been studied with the advances in proteomics study approach. Further, deigned minicellulosomes have contributed to investigate the immediacy and targeting effects of synergistic action of cellulosomal complex. The arrangement of genes in multiple-scaffoldin or enzyme-linked group on the genome contributes toward the diversity in cellulosome structural design (Bayer et al. 2004; Haitjema et al. 2017). Chimeric cohesin-bearing scaffoldins have been used for amalgamation of recombinant dockerin-containing enzymes, for assembling the designer cellulosomes (Stern et al. 2016). Interestingly, chimeric scaffoldin, having six cohesins, has been reported to form the largest designer cellulosome. However, this has resulted in the instability of the scaffoldin polypeptide, limited numbers of available cohesin–dockerin specificities, and low expression levels (Stern et al. 2016). Again, study related to the regulation of cellulosome-related genes through genetic engineering tools and approach and promising genomics of cellulosome-producing bacteria has facilitated in examining the assembly and consequences of the multienzyme complex (Bayer et al. 2004; Bae et al. 2013). Stern et al. (2016) reported that a designer cellulosome complex having a hexavalent scaffoldin attached to adaptor scaffoldin, having a type-II cohesin forms an effective enzyme complex which is having potential capacity up to 70% as compared to that of native cellulosomes for solubilization of natural lignocellulosic substrates.

14.8 Biotechnological Application of Cellulase Enzyme

Importance and application of microbial enzymes increased rapidly in mid-1980s, where different industrial applications have also been identified. Research on insect gut micobiome has indicated the potential use of bacterial enzymes, especially cellulase for biotechnological application (Kuhad et al. 2011; Su et al. 2017). The list of insects producing cellulase has been reviewed elsewhere (Chatterjee et al. 2015). It is obvious that inside the gut of insect these cellulase enzymes help in nutrition by deconstruction of food materials. As for example, heterotermitidae and rhinotermitidae groups of termite possess highly potential cellulolytic enzymes that help in digestion of complex polysaccharide food materials including wood and wood-based products (Martin et al. 1983; Chatterjee et al. 2015; de Gonzalo et al. 2016). It has been reported that the cellulosome complex presents in the bacteria residing at hindgut area of termites that have the capacity to degrade lignocellulosic material with their cell wall by surrounding the food substrates (Bayer et al. 2004; Tokuda et al. 2005; Scharf et al. 2011; Bae et al. 2013; Chatterjee et al. 2015; Stern et al. 2016; Haitjema et al. 2017). The catalyst of this complex system together is more effective than a single enzyme unit for lignocellulosic degradation (Stern et al. 2016). However, the termite gut has a complex niche of community of bacterial, archaeal, and eukaryotic gut symbionts that synergistically break down the plant fibers into the products like acetate, hydrogen, and methane (Brune 2014; Brune and Dietrich 2015; Mikaelyan et al. 2017). It has been estimated that termite gut can digest 74–99% of cellulose and 65–87% of hemicelluloses within hours (Li et al. 2017). More than 4700 bacterial phylotypes have been detected in the lower termite Reticulitermes, where Bacteriodetes, Proteobacteria, Spirochetes, Firmicutes, and Eubacteria are prominent members of this microbiota that help in biomass degradation (Cragg et al. 2015). Some Archaeal species present in the termite gut can degrade lignocellulose at higher temperature. Reports suggest that endoglucanase GH12 gene presents in archaeon Pyrococcus, and genes encoding laccase enzymes from Halobacteriales, and Thermoproteales are potential element that caters lignocellulosic degradation in archaea (Graham et al. 2011; de Gannes et al. 2013; Tian et al. 2014; Cragg et al. 2015).

14.8.1 In Waste Management

Most of the agricultural and household wastes contain lignocellulose as major components. The waste amelioration process can easily be achieved by treating wastes using bacterial cellulases and lignin-degrading enzymes (Kuhad et al. 2011; Gupta et al. 2011; Brune and Dietrich 2015; Chatterjee et al. 2015). Composition and dynamics of microflora play a major role during this process of enzymatic degradation; however, a detailed study of microbial succession and selection to accelerate the process is important for effective and appropriate management of biowastes (Kuhad et al. 2011; Gupta et al. 2011).

14.8.2 Food and Brewage Industry

Increase demand of fruit and vegetable juice has drawn the attention of food and brewage industries toward macerating enzymes like cellulase, pectinase, and related enzymes to ease in processing of fruit and vegetable juice. The conventional system involves multistep processing like maceration, extraction, clarification, and stabilization. Using cellulase for macerating fruit pulp can yield better in starch and protein extraction (Ventorino et al. 2015). In addition, the better maceration helps in color and carotenoid extraction of fruits and vegetables which in turn help in improved texture, quality, flavor, aroma, and viscosity of fruit purees (Kuhad et al. 2011; Chatterjee et al. 2015). Further using enzymes’ mixture or in combination like pectinases, cellulases, and hemicellulases improves extraction, malaxation, and quality of olive-based oil and paste (Wongputtisin et al. 2014). It has also been observed that infusion of enzymes such as pectinases and β-glucosidases reduces bitterness of citrus fruits by some extent (Sharada et al. 2014). Similarly, these microbial enzymes have a key role in alcoholic beverages production also. Brewing of beer initiates with barley malt or malted sorghum which contains raw starch and protein material which require enzymes’ to convert it into simpler form like sugars, amino acids, and peptides. Enzymes help in improving skin maceration, color extraction, clarification, filtration, and stability (Singh et al. 2007). Further, to produce liquor controlled fermenting conditions, along with microbial enzymes play a major role in deciding the quality and yields of the fermented products.

14.8.3 Ethanol Production from Lignocellulosic Biomasses

Nowadays, through the use of starch or sucrose as provided by agricultural crops such as corn, wheat, or sugarcane, fermentation of ethanol is being carried out on a larger scale. The biological conversion of the lignocellulosic wastes produces either ethanol, methanol, or hydrogen, which depends upon the process (biochemical or thermochemical) and ideal microorganism (Dutta et al. 2014). In nature, degradation of organic matter, i.e., lignocelluloses leads to methane generation, whereas ethanol and hydrogen are the intermediates by-products in anaerobic degradation (Ahring and Westermann 2007). Ethanol is an environmentally safe liquid transportation fuel as it does not contribute toward greenhouse gas emissions because ethanol produced from the renewable plant materials and CO2 generated from the ethanol burning is recycled by the plant body in their photosynthesis process (Limayem and Ricke 2012; Saini et al. 2015). The fermentation of lignocellulosic substrates is an eminent and collective process. This tough process of conversion of lignocellulose to ethanol has several steps, such as (i) detaching (or delignification) lignin from other molecules to release free cellulose and hemicellulose from the lignocellulosic material; (ii) depolymerization of carbohydrate to release sugars from cellulose and hemicelluloses; (iii) fermentation of hexose and pentose sugars to produce ethanol (Lee 1997; De Souza 2013) (Tables 14.1 and 14.2).

Table 14.1 Microbes from different insects gut having potential cellulolytic activity
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Table 14.2 Cellulose digestion in different insects with respect to enzyme produced (adopted from Martin et al. 1983; Fischer et al., 2013; Engel and Moran (2013a, b); Chatterjee et al. 2015)
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14.8.4 Pulp and Paper Industry

Annually, massive amount lignocellulosic biomass is being processed in pulp and paper industry. Use of cellulase enzyme for efficient conversion of lignocellulosic waste into quality paper is an eco-friendly and appropriate approach. Enzymes-based process includes pre-bleaching of pulp and deinking process that helps in pulp freeness and cleanliness, as a result, improves fiber brightness and strength properties (Kuhad et al. 2011; Chatterjee et al. 2015).

14.8.5 Textile Industry

Cellulase has been employed widely in textile industries for biostoning of jeans, biopolishing of cotton, and other cellulosic fabrics. Earlier biostoning was performed mechanically by pumice stone which use to cause damage to the fiber; however, after the introduction of cellulase enzyme, this process becomes easy with less damage to fiber (Kuhad et al. 2011; Chatterjee et al. 2015). Further, acidic cellulase takes care of biopolishing process as a result soft, smooth, and bright color fabric obtain.

14.9 Conclusion

Insect gut microbiota varies widely. As for examples, complex gut microbial communities can be found in termite gut, while, little or no gut microbiota are present in sap-feeding insects (Colman et al. 2012; Chapman et al. 2013). As a whole, less complexity of microbial community structure in insect gut can be found, which may be due to varied reasons like, the simple gut structure (that can afford fewer ecological niches), smaller retention time (due to their tiny structure), lacking a “classical” adaptive immune system etc. (Colman et al. 2012; Chapman et al. 2013; Engel and Moran 2013a, b). However, bacterial consortia in insect gut specifically help the host in digestion of food and various other activities (Engel and Moran 2013a, b). Lignocellulose eaters (scarab beetles and termite), however, retain food longer having fermentative guts with diverse gut microbial communities (Colman et al.2012). Recent developments of omics technologies have led the researchers to know more about the insect gut bacteria and their potential enzymatic activities upon lignocelluloses degradation. Further, several applications like production of biofuels from such wastes have pushed the researchers to understand different aspects related to biotechnological applications, like lignocellulose-active genes, substrate binding paradigms, oxidation of polysaccharides, architecture of enzyme domain, enzymatic synergies, and lignin bond breakdown (Brunecky et al. 2013; Agger et al. 2014; Cragg et al. 2015). However, there are several aspects like comprehensive individual enzyme-based sequence–structure–function relationships or synergistic action of cocktails of enzymes that work together within the insect gut are yet to be ascertained (Cragg et al. 2015). It is essential to explore these unknown aspects to optimize the enzyme activities in different industrial applications. In coming years, the relevant findings will enormously help to understand about the diversity of insect gut microbial community, in one hand, and various applications to develop sustainable eco-friendly technologies for generation of wealth (e.g., biofuel) from lignocellulosic wastes.