Microbial Cellulose Utilization: Fundamentals and Biotechnology

Abstract

Fundamental features of microbial cellulose utilization are examined at successively higher levels of aggregation encompassing the structure and composition of cellulosic biomass, taxonomic diversity, cellulase enzyme systems, molecular biology of cellulase enzymes, physiology of cellulolytic microorganisms, ecological aspects of cellulase-degrading communities, and rate-limiting factors in nature. The methodological basis for studying microbial cellulose utilization is considered relative to quantification of cells and enzymes in the presence of solid substrates as well as apparatus and analysis for cellulose-grown continuous cultures. Quantitative description of cellulose hydrolysis is addressed with respect to adsorption of cellulase enzymes, rates of enzymatic hydrolysis, bioenergetics of microbial cellulose utilization, kinetics of microbial cellulose utilization, and contrasting features compared to soluble substrate kinetics. A biological perspective on processing cellulosic biomass is presented, including features of pretreated substrates and alternative process configurations. Organism development is considered for “consolidated bioprocessing” (CBP), in which the production of cellulolytic enzymes, hydrolysis of biomass, and fermentation of resulting sugars to desired products occur in one step. Two organism development strategies for CBP are examined: (i) improve product yield and tolerance in microorganisms able to utilize cellulose, or (ii) express a heterologous system for cellulose hydrolysis and utilization in microorganisms that exhibit high product yield and tolerance. A concluding discussion identifies unresolved issues pertaining to microbial cellulose utilization, suggests approaches by which such issues might be resolved, and contrasts a microbially oriented cellulose hydrolysis paradigm to the more conventional enzymatically oriented paradigm in both fundamental and applied contexts

Patterns of seasonality and group membership characterize the gut microbiota in a longitudinal study of wild Verreaux's sifakas (Propithecus verreauxi)

“…Members of the Ruminococcaceae and Lachnospiraceae are efficient fermenters of fiber, such as cellulose or xylan, producing short‐chain fatty acids (SCFAs) in the process (Flint et al. 2012, Lynd, Weimer, van Zyl, & Pretorius, 2002). Lachnospiraceae were significantly positively correlated to fiber intake in our dataset, as were Sphaerochaetaceae, a family from the phylum Spirochaetes, which contains genera highly enriched in fermentation and carbohydrate metabolism genes.…”

A functionally based model for hydrolysis of cellulose by fungal cellulase

“…However, such effects are expected to be relatively unimportant at the low enzyme loadings usually used for laboratory characterization of cellulase enzymes, which are the focus of this study. In addition, the Langmuir adsorption based on a single affinity constant has been repeatedly found to be quite good for descriptive purposes (Lynd et al, 2002;Zhang and Lynd, 2004).…”

“…In particular, biologically mediated cellulose hydrolysis could be widely used to sustainably produce fuels and chemicals. The classical mechanism for enzymatic cellulose hydrolysis involves synergistic action by endoglucanases and cellobiohydrolases to hydrolyze solid cellulose on the surface of cellulose to soluble cellodextrins in the liquid phase, followed by bglucosidase-mediated hydrolysis of cellobiose to glucose in the liquid phase (Lynd et al, 2002;Zhang and Lynd, 2004). The primary depolymerization reactions from long solid cellulose chains to short soluble cellodextrins are a ratelimiting step for the overall hydrolysis process (Lynd et al, 2002;Zhang and Lynd, 2004).…”

Dynamic flux balance modeling of microbial co‐cultures for efficient batch fermentation of glucose and xylose mixtures

“…Although strains that express the ethanol dehydrogenase and pyruvate decarboxylase enzymes from the bacterium Zymomonas mobilis overproduce ethanol, diauxic consumption of pentose and hexose sugars is still observed (Lawford and Rousseau, 2002).While metabolic engineering offers the possibility of combining versatile substrate utilization and high product yields into a single recombinant strain, alternative strategies should be explored. The unique properties of microbes currently envisioned for biofuels production suggest that microbial consortia consisting of synergistic combinations of pentose and hexose consuming species may offer distinct advantages over single microbe strategies (Lynd et al, 2002). Rather than attempting to engineer cells to deviate from their natural metabolic behavior, these consortia take advantage of the native capabilities of different species to metabolize different sugars and direct available carbon to targeted biofuels.…”