The curdlan-type exopolysaccharide produced by Cellulomonas flavigena KU forms part of an extracellular glycocalyx involved in cellulose degradation

Genetic Engineering of Agrobacterium Increases Curdlan Production through Increased Expression of the crdASC Genes

Curdlan is a water-insoluble polymer that has structure and gelling properties that are useful in a wide variety of applications such as in medicine, cosmetics, packaging and the food and building industries. The capacity to produce curdlan has been detected in certain soil-dwelling bacteria of various phyla, although the role of curdlan in their survival remains unclear. One of the major limitations of the extensive use of curdlan in industry is the high cost of production during fermentation, partly because production involves specific nutritional requirements such as nitrogen limitation. Engineering of the industrially relevant curdlan-producing strain Agrobacterium sp. ATTC31749 is a promising approach that could decrease the cost of production. Here, during investigations on curdlan production, it was found that curdlan was deposited as a capsule. Curiously, only a part of the bacterial population produced a curdlan capsule. This heterogeneous distribution appeared to be due to the activity of Pcrd, the native promoter responsible for the expression of the crdASC biosynthetic gene cluster. To improve curdlan production, Pcrd was replaced by a promoter (PphaP) from another Alphaproteobacterium, Rhodobacter sphaeroides. Compared to Pcrd, PphaP was stronger and only mildly affected by nitrogen levels. Consequently, PphaP dramatically boosted crdASC gene expression and curdlan production. Importantly, the genetic modification overrode the strict nitrogen depletion regulation that presents a hindrance for maximal curdlan production and from nitrogen rich, complex media, demonstrating excellent commercial potential for achieving high yields using cheap substrates under relaxed fermentation conditions.

Biofilm-Mediated Fragmentation and Degradation of Microcrystalline Cellulose by Cellulomonas flavigena KU (ATCC 53703)

Cellulomonas flavigena KU (ATCC 53703) produces an extracellular matrix involved in the degradation of microcrystalline cellulose. This extracellular material is primarily composed of the gel-forming, β-1,3-glucan known as curdlan and associated, cellulose-degrading enzymes. In this study, the effects of various forms of nutrient limitation on cellulose attachment, cellular aggregation, curdlan production, and biofilm formation were investigated throughout a 7-day incubation period by using phase-contrast microscopy. Compared to cultures grown in non-limiting media, nitrogen-limitation promoted early attachment of C. flavigena KU cells to the cellulose surface, and cellulose attachment was congruent with cellular aggregation and curdlan production. Over the course of the experiment, microcolonies of attached cells grew into curdlan-producing biofilms on the cellulose. By contrast, bacterial cells grown on cellulose in non-limiting media remained unattached and unaggregated throughout most of the incubation period. By 7 days of incubation, bacterial aggregation was ninefold greater in N-limited cultures compared to nutritionally complete cultures. In a similar way, phosphorus- and vitamin-limitation (i.e., yeast extract-limitation) also resulted in early cellulose attachment and biofilm formation. Furthermore, nutrient limitation promoted more rapid and efficient fragmentation and degradation of cellulose, with cellulose fragments in low-N media averaging half the size of those in high-N media after 7 days. Two modes of cellulose degradation are proposed for C. flavigena KU, a "planktonic mode" and a "biofilm mode". Similar observations have been reported for other curdlan-producing cellulomonads, and these differing cellulose degradation strategies may ultimately prove to reflect sequential stages of a multifaceted biofilm cycle important in the bioconversion of this abundant and renewable natural resource.

β-1,3-Glucan synthesis, novel supramolecular self-assembly, characterization and application

β-1,3-Glucans are ubiquitously observed in various biological systems with diverse physio-ecological functions, yet their underlying assembly mechanism and multiscale complexation in vitro remains poorly understood. Here, we provide for the first-time evidence of unidentified β-1,3-glucan supramolecular complexation into intricate hierarchical architectures over several length scales. We mediated these unique assemblies using a recombinantly produced β-1,3-glucan phosphorylase (Ta1,3BGP) by fine-tuning solution conditions during particle nucleation and growth. We report a synthesis of interconnected parallel hexagonal lamellae composed of 8 nm thick sheets of highly expanded paracrystals. The architecture consists of β-1,3-glucan triple-helices with considerable inter-intra hydrogen bonding within, as well as in between adjacent triple-helices. The results extend our understanding of β-1,3-glucan molecular organization and shed light on different aspects of the crystallization processes of biomolecules into structures unseen by nature. The presented versatile synthesis yields new materials for diverse medical and industrial applications.

Chitin-Active Lytic Polysaccharide Monooxygenases Are Rare in Cellulomonas Species

Cellulomonas flavigena is a saprotrophic bacterium that encodes, within its genome, four predicted lytic polysaccharide monooxygenases (LPMOs) from Auxiliary Activity family 10 (AA10). We showed previously that three of these cleave the plant polysaccharide cellulose by oxidation at carbon-1 (J. Li, L. Solhi, E.D. Goddard-Borger, Y. Mattieu et al., Biotechnol Biofuels 14:29, 2021, https://doi.org/10.1186/s13068-020-01860-3). Here, we present the biochemical characterization of the fourth C. flavigena AA10 member (CflaLPMO10D) as a chitin-active LPMO. Both the full-length CflaLPMO10D-Carbohydrate-Binding Module family 2 (CBM2) and catalytic module-only proteins were produced in Escherichia coli using the native general secretory (Sec) signal peptide. To quantify chitinolytic activity, we developed a high-performance anion-exchange chromatography-pulsed amperometric detection (HPAEC-PAD) method as an alternative to the established hydrophilic interaction liquid ion chromatography coupled with UV detection (HILIC-UV) method for separation and detection of released oxidized chito-oligosaccharides. Using this method, we demonstrated that CflaLPMO10D is strictly active on the β-allomorph of chitin, with optimal activity at pH 5 to 6 and a preference for ascorbic acid as the reducing agent. We also demonstrated the importance of the CBM2 member for both mediating enzyme localization to substrates and prolonging LPMO activity. Together with previous work, the present study defines the distinct substrate specificities of the suite of C. flavigena AA10 members. Notably, a cross-genome survey of AA10 members indicated that chitinolytic LPMOs are, in fact, rare among Cellulomonas bacteria. IMPORTANCE Species from the genus Cellulomonas have a long history of study due to their roles in biomass recycling in nature and corresponding potential as sources of enzymes for biotechnological applications. Although Cellulomonas species are more commonly associated with the cleavage and utilization of plant cell wall polysaccharides, here, we show that C. flavigena produces a unique lytic polysaccharide monooxygenase with activity on β-chitin, which is found, for example, in arthropods. The limited distribution of orthologous chitinolytic LPMOs suggests adaptation of individual cellulomonads to specific nutrient niches present in soil ecosystems. This research provides new insight into the biochemical specificity of LPMOs in Cellulomonas species and related bacteria, and it raises new questions about the physiological function of these enzymes.

A review presenting production, characterization, and applications of biopolymer curdlan in food and pharmaceutical sectors

Curdlan is an exopolysaccharide, specifically a homopolysaccharide, with a high molecular weight that is made up entirely of monomeric glucose molecules connected by β-1,3-glycosidic bonds. Curdlan was first isolated in 1962 by Harada and his colleagues from Alcaligenes faecalis var myxogenes 10C3. Microbial synthesis of this curdlan is mainly associated with soil bacteria. Preliminary screening of curdlan-producing microorganisms is done on aniline blue media. The aniline blue positive microorganisms are subjected to submerged fermentation for the production of curdlan. To improve the yield of curdlan produced, various optimization techniques are employed such as Plackett–Burman, response surface methodology, and others. Curdlan can be characterized by its morphology, gel strength, its infrared, and magnetic resonances among many other characteristics. Due to its distinctive physicochemical and rheological properties, it has gained immense popularity in the food, biomedical, and pharmaceutical sectors. However, curdlan’s functionality can be improved by chemically modifying curdlan to obtain grafted curdlan, hydrogels, and nanocomposites which are discussed in detail herewith. Curdlan was authorized to be used in the food industry by the United States Food and Drug Administration in 1996 and also in 1989 in Taiwan, Japan, and Korea. Over the years, many patents using curdlan have also been filed from different parts of the world. This review provides information about its structure, biosynthesis, production strategies, optimization, characterization, applications, and patents.

Graphic abstract

Linear and branched β-Glucans degrading enzymes from versatile Bacteroides uniformis JCM 13288T and their roles in cooperation with gut bacteria

β-glucans are the dietary nutrients present in oats, barley, algae, and mushrooms. The macromolecules are well known for their immune-modulatory activity; however, how the human gut bacteria digest them is vaguely understood. In this study, Bacteroides uniformis JCM 13288 ^T was found to grow on laminarin, pustulan, and porphyran. We sequenced the genome of the strain, which was about 5.05 megabase pairs and contained 4868 protein-coding genes. On the basis of growth patterns of the bacterium, two putative polysaccharide utilization loci for β-glucans were identified from the genome, and associated four putative genes were cloned, expressed, purified, and characterized. Three glycoside hydrolases (GHs) that were endo-acting enzymes (BuGH16, BuGH30, and BuGH158), and one which was an exo-acting (BuGH3) enzyme. The BuGH3, BuGH16, and BuGH158 can cleave linear exo/endo- β- 1-3 linkages while BuGH30 can digest endo- β- 1-6 linkages. BuGH30 and BuGH158 were further explored for their roles in digesting β- glucans and generation of oligosaccharides, respectively. The BuGH30 predominately found to cleave long chain β- 1-6 linked glucans, and obtained final product was gentiobiose. The BuGH158 used for producing oligosaccharides varying from degree of polymerization 2 to 7 from soluble curdlan. We demonstrated that these oligosaccharides can be utilized by gut bacteria, which either did not grow or poorly grew on laminarin. Thus, B. uniformis JCM 13288 ^T is not only capable of utilizing β-glucans but also shares these glycans with human gut bacteria for potentially maintaining the gut microbial homeostasis.

Chemistry and microbial sources of curdlan with potential application and safety regulations as prebiotic in food and health

Curdlan - a homopolysaccharide is comprised of glucose using β-1,3-glycosidic bond and produced by different types of microorganisms as exopolysaccharide. Curdlan gel is stable during freezing and thawing processes which find several applications in food and pharmaceutical industries. It acts as a prebiotic, stabilizer and water-holding, viscosifying and texturing agent. Additionally, curdlan gel is used as a food factor to develop the new products e.g. milk fat substitute, non-fat whipped cream, retorting (freeze-drying) process of Tofu, low-fat sausage, and low-fat hamburger. However, a great variation exists among different countries regarding the regulatory aspects of curdlan as food additives, dietary components or prebiotic substances. Therefore, the present review paper aims to discuss safety issues and the establishment of common guidelines and legislation globally, focusing on the use the applications of curdlan in the food sector including the development of noodles, meat-based products, and fat-free dairy products. This review analyzes and describes in detail the potential of curdlan as a sustainable alternative additive in health and food industries, emphasizing on the chemical composition, production, properties, and potential applications.

Biofilm formation as a microbial strategy to assimilate particulate substrates

In most ecosystems, a large part of the organic carbon is not solubilized in the water phase. Rather, it occurs as particles made of aggregated hydrophobic and/or polymeric natural or man-made organic compounds. These particulate substrates are degraded by extracellular digestion/solubilization implemented by heterotrophic bacteria that form biofilms on them. Organic particle-degrading biofilms are widespread and have been observed in aquatic and terrestrial natural ecosystems, in polluted and man-driven environments and in the digestive tracts of animals. They have central ecological functions as they are major players in carbon recycling and pollution removal. The aim of this review is to highlight bacterial adhesion and biofilm formation as central mechanisms to exploit the nutritive potential of organic particles. It focuses on the mechanisms that allow access and assimilation of non-dissolved organic carbon, and considers the advantage provided by biofilms for gaining a net benefit from feeding on particulate substrates. Cooperative and competitive interactions taking place in biofilms feeding on particulate substrates are also discussed.

Comparative Study of Extracellular Proteolytic, Cellulolytic, and Hemicellulolytic Enzyme Activities and Biotransformation of Palm Kernel Cake Biomass by Lactic Acid Bacteria Isolated from Malaysian Foods

Biotransformation via solid state fermentation (SSF) mediated by microorganisms is a promising approach to produce useful products from agricultural biomass. Lactic acid bacteria (LAB) that are commonly found in fermented foods have been shown to exhibit extracellular proteolytic, β-glucosidase, β-mannosidase, and β-mannanase activities. Therefore, extracellular proteolytic, cellulolytic, and hemicellulolytic enzyme activities of seven Lactobacillus plantarum strains (a prominent species of LAB) isolated from Malaysian foods were compared in this study. The biotransformation of palm kernel cake (PKC) biomass mediated by selected L. plantarum strains was subsequently conducted. The results obtained in this study exhibited the studied L. plantarum strains produced versatile multi extracellular hydrolytic enzyme activities that were active from acidic to alkaline pH conditions. The highest total score of extracellular hydrolytic enzyme activities were recorded by L. plantarum RI11, L. plantarum RG11, and L. plantarum RG14. Therefore, they were selected for the subsequent biotransformation of PKC biomass via SSF. The hydrolytic enzyme activities of treated PKC extract were compared for each sampling interval. The scanning electron microscopy analyses revealed the formation of extracellular matrices around L. plantarum strains attached to the surface of PKC biomass during SSF, inferring that the investigated L. plantarum strains have the capability to grow on PKC biomass and perform synergistic secretions of various extracellular proteolytic, cellulolytic, and hemicellulolytic enzymes that were essential for the effective biodegradation of PKC. The substantial growth of selected L. plamtraum strains on PKC during SSF revealed the promising application of selected L. plantarum strains as a biotransformation agent for cellulosic biomass.

Effects of Different Carbon Sources on Enzyme Production and Ultrastructure of Cellulosimicrobium cellulans

The secretomes of the strain Cellulosimicrobium cellulans F16 grown on different carbon sources were analyzed by zymography, and the subcellular surface structures were extensively studied by electron microscope. The exo-cellulase and xylanase systems were sparse when cells were grown on soluble oligosaccharides, but were significantly increased when grown on complex and water-insoluble polysaccharides, such as Avicel, corn cob, and birchwood xylan. The cellulosome-like protuberant structures were clearly observed on the cell surfaces of strain F16 grown on cellulose, with diameters of 15-20 nm. Fibrous structures that connected the adjacent cells can be seen under microscope. Moreover, protuberances that adsorbed the cell to cellulose were also observed. As the adhesion of Cellulosimicrobium cellulans cells onto cellulose surfaces occurs via thick bacterial curdlan-type exopolysaccharides (EPS), such surface layer is potentially important in the digestion of insoluble substrates such as cellulose or hemicellulose, and the previously reported xylanosomes are part of such complex glycocalyx layer on the surface of the bacterial cell.

Functional and structural properties of a novel cellulosome-like multienzyme complex: efficient glycoside hydrolysis of water-insoluble 7-xylosyl-10-deacetylpaclitaxel

Cellulosome is a kind of multienzyme complex that displays high activity, selectivity, and stability. Here, we report a novel, non-cellulolytic, cellulosome-like multienzyme complex that produced by the Cellulosimicrobium cellulans wild-type strain F16 isolated from soil microflora. This multienzyme complex, with excellent catalytic efficiency of kcat 13.2 s(-1) to remove the C-7 xylosyl group from 7-xylosyl-10-deacetylpaclitaxel (10-DAXP), has an outstanding tolerance against organic solvents and an excellent general stability, with the long half-life of 214 hours. This cellulosome-like multienzyme complex has a novel structure distinct from the well-documented ones. The key catalytic subunit responsible for the β-xylosidase activity against 10-DAXP is identified to be a novel protein, indicating a new glycoside hydrolase (GH) family. The pioneering work described here offers a novel nanoscale biocatalyst for the production of biofuels and chemicals from renewable plant-based natural resources.

Determination of the cellulase activity distribution in Clostridium thermocellum and Caldicellulosiruptor obsidiansis cultures using a fluorescent substrate

This study took advantage of resorufin cellobioside as a fluorescent substrate to determine the distribution of cellulase activity in cellulosic biomass fermentation systems. Cellulolytic biofilms were found to express nearly four orders greater cellulase activity compared to planktonic cultures of Clostridium thermocellum and Caldicellulosiruptor obsidiansis, which can be primarily attributed to the high cell concentration and surface attachment. The formation of biofilms results in cellulases being secreted close to their substrates, which appears to be an energetically favorable stategy for insoluble substrate utilization. For the same reason, cellulases should be closely associated with the surfaces of suspended cell in soluble substrate-fed culture, which has been verified with cellobiose-fed cultures of C. thermocellum and C. obsidiansis. This study addressed the importance of cellulase activity distribution in cellulosic biomass fermentation, and provided theoretical foundation for the leading role of biofilm in cellulose degradation. System optimization and reactor designs that promote biofilm formation in cellulosic biomass hydrolysis may promise an improved cellulosic biofuel process.

Effective Immobilization of Agrobacterium sp. IFO 13140 Cells in Loofa Sponge for Curdlan Biosynthesis

Curdlan production by Agrobacterium sp. IFO13140 immobilized on loofa sponge, alginate and loofa sponge with alginate was investigated. There was no statistically-significant difference in curdlan production when the microorganism was immobilized in different matrices. The loofa sponge was chosen because of its practical application and economy and because it provides a high stability through its continued use. The best conditions for immobilization on loofa sponge were 50 mg of cell, 200 rpm and 72 h of incubation, which provided a curdlan production 1.50-times higher than that obtained by free cells. The higher volumetric productivity was achieved by immobilized cells (0.09 g/L/h) at 150 rpm. The operating stability was evaluated, and until the fourth cycle, immobilized cells retained 87.40% of the production of the first cycle. The immobilized cells remained active after 300 days of storage at 4 °C. The results of this study demonstrate success in immobilizing cells for curdlan biosynthesis, making the process potentially suitable for industrial scale-up. Additional studies may show a possible contribution to the reduction of operating costs.

Insights into Clostridium phytofermentans biofilm formation: aggregation, microcolony development and the role of extracellular DNA

Biofilm formation is a critical component to the lifestyle of many naturally occurring cellulose-degrading microbes. In this work, cellular aggregation and biofilm formation of Clostridium phytofermentans, a cellulolytic anaerobic bacterium, was investigated using a combination of microscopy and analytical techniques. Aggregates included thread-like linkages and a DNA/protein-rich extracellular matrix when grown on soluble cellobiose. Similar dense biofilms formed on the surface of the model cellulosic substrate Whatman no. 1 filter paper. Following initially dispersed attachment, microcolonies of ~500 µm diameter formed on the filter paper after 6 days. Enzymic treatment of both the biofilm and cellular aggregates with DNase and proteinase resulted in significant loss of rigidity, pointing to the key role of extracellular DNA and proteins in the biofilm structure. A high-throughput biofilm assay was adapted for studying potential regulators of biofilm formation. Various media manipulations were shown to greatly impact biofilm formation, including repression in the presence of glucose but not the β(1→4)-linked disaccharide cellobiose, implicating a balance of hydrolytic activity and assimilation to maintain biofilm integrity. Using the microtitre plate biofilm assay, DNase and proteinase dispersed ~60 and 30 % of mature biofilms, respectively, whilst RNase had no impact. This work suggests that Clostridium phytofermentans has evolved a DNA/protein-rich biofilm matrix complementing its cellulolytic nature. These insights add to our current understanding of natural ecosystems as well as strategies for efficient bioprocess design.

The genome sequences of Cellulomonas fimi and "Cellvibrio gilvus" reveal the cellulolytic strategies of two facultative anaerobes, transfer of "Cellvibrio gilvus" to the genus Cellulomonas, and proposal of Cellulomonas gilvus sp. nov

Actinobacteria in the genus Cellulomonas are the only known and reported cellulolytic facultative anaerobes. To better understand the cellulolytic strategy employed by these bacteria, we sequenced the genome of the Cellulomonas fimi ATCC 484(T). For comparative purposes, we also sequenced the genome of the aerobic cellulolytic "Cellvibrio gilvus" ATCC 13127(T). An initial analysis of these genomes using phylogenetic and whole-genome comparison revealed that "Cellvibrio gilvus" belongs to the genus Cellulomonas. We thus propose to assign "Cellvibrio gilvus" to the genus Cellulomonas. A comparative genomics analysis between these two Cellulomonas genome sequences and the recently completed genome for Cellulomonas flavigena ATCC 482(T) showed that these cellulomonads do not encode cellulosomes but appear to degrade cellulose by secreting multi-domain glycoside hydrolases. Despite the minimal number of carbohydrate-active enzymes encoded by these genomes, as compared to other known cellulolytic organisms, these bacteria were found to be proficient at degrading and utilizing a diverse set of carbohydrates, including crystalline cellulose. Moreover, they also encode for proteins required for the fermentation of hexose and xylose sugars into products such as ethanol. Finally, we found relatively few significant differences between the predicted carbohydrate-active enzymes encoded by these Cellulomonas genomes, in contrast to previous studies reporting differences in physiological approaches for carbohydrate degradation. Our sequencing and analysis of these genomes sheds light onto the mechanism through which these facultative anaerobes degrade cellulose, suggesting that the sequenced cellulomonads use secreted, multidomain enzymes to degrade cellulose in a way that is distinct from known anaerobic cellulolytic strategies.

Factors affecting accumulation and degradation of curdlan, trehalose and glycogen in cultures of Cellulomonas flavigena strain KU (ATCC 53703)

Cellulomonas flavigena strain KU (ATCC 53703) is a cellulolytic, Gram-positive bacterium which produces large quantities of an insoluble exopolysaccharide (EPS) when grown in minimal media with a high carbon-to-nitrogen (C/N) ratio. Earlier studies proved the EPS is structurally identical to the linear β-1,3-glucan known as curdlan and provided evidence that the EPS functions as a carbon and energy reserve compound. We now report that C. flavigena KU also accumulates two intracellular, glucose-storage carbohydrates under conditions of carbon and energy excess. These carbohydrates were partially purified and identified as the disaccharide trehalose and a glycogen/amylopectin-type polysaccharide. A novel method is described for the sequential fractionation and quantitative determination of all three carbohydrates from culture samples. This fractionation protocol was used to examine the effects of C/N ratio and osmolarity on the accumulation of cellular carbohydrates in batch culture. Increasing the C/N of the growth medium caused a significant accumulation of curdlan and glycogen but had a relatively minor effect on accumulation of trehalose. In contrast, trehalose levels increased in response to increasing osmolarity, while curdlan levels declined and glycogen levels were generally unaffected. During starvation for an exogenous source of carbon and energy, only curdlan and glycogen showed substantial degradation within the first 24 h. These results support the conclusion that extracellular curdlan and intracellular glycogen can both serve as short-term reserve compounds for C. flavigena KU and that trehalose appears to accumulate as a compatible solute in response to osmotic stress.

Curdlan-like exopolysaccharide production by Cellulomonas flavigena UNP3 during growth on hydrocarbon substrates

Cellulomonas flavigena UNP3, a natural isolate from vegetable oil contaminated soil sample has been studied for growth associated exopolysaccharide (EPS) production during growth on glucose, groundnut oil and naphthalene. The EPS showed matrix formation surrounding the cells during scanning electron microscopy. Cell surface hydrophobicity and emulsifying activity studies confirmed the role of EPS as bioemulsifier. Emulsifying activity was found to increase with time (0.2 U/mg for 10 min to 0.27 U/mg for 30 min). Emulsification index, E24 value increased with the increase in EPS concentration. Degradation of polyaromatic hydrocarbons was confirmed using gas chromatography analysis. FTIR analysis showed presence of characteristic absorbance at 895.10 cm(-1) for β-configuration of glucan. NMR studies also revealed EPS produced by C. flavigena UNP3 as a linear β-1, 3-D-glucan, and a curdlan like polysaccharide.

Complete genome sequence of Cellulomonas flavigena type strain (134)

Cellulomonas flavigena (Kellerman and McBeth 1912) Bergey et al. 1923 is the type species of the genus Cellulomonas of the actinobacterial family Cellulomonadaceae. Members of the genus Cellulomonas are of special interest for their ability to degrade cellulose and hemicellulose, particularly with regard to the use of biomass as an alternative energy source. Here we describe the features of this organism, together with the complete genome sequence, and annotation. This is the first complete genome sequence of a member of the genus Cellulomonas, and next to the human pathogen Tropheryma whipplei the second complete genome sequence within the actinobacterial family Cellulomonadaceae. The 4,123,179 bp long single replicon genome with its 3,735 protein-coding and 53 RNA genes is part of the Genomic Encyclopedia of Bacteria and Archaea project.

Potential of biofilm-based biofuel production

Biofilm technology has been extensively applied to wastewater treatment, but its potential application in biofuel production has not been explored. Current technologies of converting lignocellulose materials to biofuel are hampered by costly processing steps in pretreatment, saccharification, and product recovery. Biofilms may have a potential to improve efficiency of these processes. Advantages of biofilms include concentration of cell-associated hydrolytic enzymes at the biofilm-substrate interface to increase reaction rates, a layered microbial structure in which multiple species may sequentially convert complex substrates and coferment hexose and pentose as hydrolysates diffuse outward, and the possibility of fungal-bacterial symbioses that allow simultaneous delignification and saccharification. More importantly, the confined microenvironment within a biofilm selectively rewards cells with better phenotypes conferred from intercellular gene or signal exchange, a process which is absent in suspended cultures. The immobilized property of biofilm, especially when affixed to a membrane, simplifies the separation of biofuel from its producer and promotes retention of biomass for continued reaction in the fermenter. Highly consolidated bioprocessing, including delignification, saccharification, fermentation, and separation in a single reactor, may be possible through the application of biofilm technology. To date, solid-state fermentation is the only biofuel process to which the advantages of biofilms have been applied, even though it has received limited attention and improvements. The transfer of biofilm technology from environmental engineering has the potential to spur great innovations in the optimization of biofuel production.

Impact of Mycobacterium ulcerans biofilm on transmissibility to ecological niches and Buruli ulcer pathogenesis

The role of biofilms in the pathogenesis of mycobacterial diseases remains largely unknown. Mycobacterium ulcerans, the etiological agent of Buruli ulcer, a disfiguring disease in humans, adopts a biofilm-like structure in vitro and in vivo, displaying an abundant extracellular matrix (ECM) that harbors vesicles. The composition and structure of the ECM differs from that of the classical matrix found in other bacterial biofilms. More than 80 proteins are present within this extracellular compartment and appear to be involved in stress responses, respiration, and intermediary metabolism. In addition to a large amount of carbohydrates and lipids, ECM is the reservoir of the polyketide toxin mycolactone, the sole virulence factor of M. ulcerans identified to date, and purified vesicles extracted from ECM are highly cytotoxic. ECM confers to the mycobacterium increased resistance to antimicrobial agents, and enhances colonization of insect vectors and mammalian hosts. The results of this study support a model whereby biofilm changes confer selective advantages to M. ulcerans in colonizing various ecological niches successfully, with repercussions for Buruli ulcer pathogenesis.

Studies of the extracellular glycocalyx of the anaerobic cellulolytic bacterium Ruminococcus albus 7

Anaerobic cellulolytic bacteria are thought to adhere to cellulose via several mechanisms, including production of a glycocalyx containing extracellular polymeric substances (EPS). As the compositions and structures of these glycocalyces have not been elucidated, variable-pressure scanning electron microscopy (VP-SEM) and chemical analysis were used to characterize the glycocalyx of the ruminal bacterium Ruminococcus albus strain 7. VP-SEM revealed that growth of this strain was accompanied by the formation of thin cellular extensions that allowed the bacterium to adhere to cellulose, followed by formation of a ramifying network that interconnected individual cells to one another and to the unraveling cellulose microfibrils. Extraction of 48-h-old whole-culture pellets (bacterial cells plus glycocalyx [G] plus residual cellulose [C]) with 0.1 N NaOH released carbohydrate and protein in a ratio of 1:5. Boiling of the cellulose fermentation residue in a neutral detergent solution removed almost all of the adherent cells and protein while retaining a residual network of adhering noncellular material. Trifluoroacetic acid hydrolysis of this residue (G plus C) released primarily glucose, along with substantial amounts of xylose and mannose, but only traces of galactose, the most abundant sugar in most characterized bacterial exopolysaccharides. Linkage analysis and characterization by nuclear magnetic resonance suggested that most of the glucosyl units were not present as partially degraded cellulose. Calculations suggested that the energy demand for synthesis of the nonprotein fraction of EPS by this organism represents only a small fraction (<4%) of the anabolic ATP expenditure of the bacterium.

Polysaccharide biological response modifiers

Biological response modifiers (BRMs) are substances which augment immune response. BRMs can be cytokines which are produced endogenously in our body by immune cells or derivatives of bacteria, fungi, brown algae, Aloe vera and photosynthetic plants. Such exogeneous derivatives (exogeneous BRMs) can be nucleic acid (CpG), lipid (lipotechoic acid), protein or polysaccharide in nature. The receptors for these exogeneous BRMs are pattern recognition receptors. The binding of exogeneous BRMs to pattern recognition receptors triggers immune response. Exogenous BRMs have been reported to have anti-viral, anti-bacterial, anti-fungal, anti-parasitic, and anti-tumor activities. Among different exogeneous BRMs, polysaccharide BRMs have the widest occurrence in nature. Some polysaccharide BRMs have been tested for their therapeutic properties in human clinical trials. An overview of current understandings of polysaccharide BRMs is summarized in this review.