Molecular Adaptation Mechanisms Employed by Ethanologenic Bacteria in Response to Lignocellulose-derived Inhibitory Compounds

Abstract

Current international interest in finding alternative sources of energy to the diminishing supplies of fossil fuels has encouraged research efforts in improving biofuel production technologies. In countries which lack sufficient food, the use of sustainable lignocellulosic feedstocks, for the production of bioethanol, is an attractive option. In the pre-treatment of lignocellulosic feedstocks for ethanol production, various chemicals and/or enzymatic processes are employed. These methods generally result in a range of fermentable sugars, which are subjected to microbial fermentation and distillation to produce bioethanol. However, these methods also produce compounds that are inhibitory to the microbial fermentation process. These compounds include products of sugar dehydration and lignin depolymerisation, such as organic acids, derivatised furaldehydes and phenolic acids. These compounds are known to have a severe negative impact on the ethanologenic microorganisms involved in the fermentation process by compromising the integrity of their cell membranes, inhibiting essential enzymes and negatively interact with their DNA/RNA. It is therefore important to understand the molecular mechanisms of these inhibitions, and the mechanisms by which these microorganisms show increased adaptation to such inhibitors. Presented here is a concise overview of the molecular adaptation mechanisms of ethanologenic bacteria in response to lignocellulose-derived inhibitory compounds. These include general stress response and tolerance mechanisms, which are typically those that maintain intracellular pH homeostasis and cell membrane integrity, activation/regulation of global stress responses and inhibitor substrate-specific degradation pathways. We anticipate that understanding these adaptation responses will be essential in the design of 'intelligent' metabolic engineering strategies for the generation of hyper-tolerant fermentation bacteria strains.

Transcriptomic analysis of thermotolerant yeastKluyveromyces marxianusin multiple inhibitors tolerance

“…Inhibitors like acetic acid, furfural, and phenols have been reported to be related to the redox state inside cells, inducing reactive oxygen species (ROS) generation. [33][34][35] Genes related to ROS detoxication including those coding for superoxide dismutases (SODs) and their chaperones, catalases and peroxidases, glutathione and thioredoxin systems. These proteins remove excess ROS such as cOH, H 2 O 2 , and O 2 c À etc.…”

Modulation of the Acetone/Butanol Ratio during Fermentation of Corn Stover-Derived Hydrolysate by Clostridium beijerinckii Strain NCIMB 8052

“…The issue is that of how SLCs consume NADH in C. beijerinckii fermentation. It has been reported that lignocellulose-derived inhibitors can generate oxidative stress, including in C. beijerinckii, such as was seen after adding furfural to the medium, leading to increased expression of genes related to perturbations in redox balance (37,42,43). Moreover, furan aldehydes inhibit yeast fermentation by causing oxidative cellular damage (44).…”

Thermophilic fermentations of lignocellulosic substrates and economics of biofuels: prospects in Pakistan

“…[1.9 mol of ethanol/mole of hexose, higher ethanol tolerance i.e. [12 %, higher robustness and higher resistance to the toxic inhibitors [5,12,19,97]. Another highly efficient ethanologenic microbe is Zymomonas mobilis which is a mesophilic bacterium and tolerates approximately up to 12 % ethanol and has the potential of 2.5 times faster growth than yeasts [5,33].…”