Alternative strategies for lignocellulose fermentation through lactic acid bacteria: the state of the art and perspectives

Abstract

Lactic acid bacteria (LAB) have a long history in industrial processes as food starters and biocontrol agents, and also as producers of high-value compounds. Lactic acid, their main product, is among the most requested chemicals because of its multiple applications, including the synthesis of biodegradable plastic polymers. Moreover, LAB are attractive candidates for the production of ethanol, polyhydroalkanoates, sweeteners and exopolysaccharides. LAB generally have complex nutritional requirements. Furthermore, they cannot directly ferment inexpensive feedstocks such as lignocellulose. This significantly increases the cost of LAB fermentation and hinders its application in the production of high volumes of low-cost chemicals. Different strategies have been explored to extend LAB fermentation to lignocellulosic biomass. Fermentation of lignocellulose hydrolysates by LAB has been frequently reported and is the most mature technology. However, current economic constraints of this strategy have driven research for alternative approaches. Co-cultivation of LAB with native cellulolytic microorganisms may reduce the high cost of exogenous cellulase supplementation. Special attention is given in this review to the construction of recombinant cellulolytic LAB by metabolic engineering, which may generate strains able to directly ferment plant biomass. The state of the art of these strategies is illustrated along with perspectives of their applications to industrial second generation biorefinery processes.

Lactic Acid Production from Lignocellulose – A Review of Major Challenges and Selected Solutions

“…Extensive research is required to uncover this mechanism as it could help to engineer single LAB strains which can secrete a large number of highly synergistic heterologous cellulases that can metabolize complex LIB directly to LA without using inducible promoters and without producing toxic substances. The focus should be on native LA producers with lower nutritional requirements, such as the Bacillus genus, including B. coagualns, B. subtilis , and B. licheniformis strains, targeting their growth on LIB without the addition of expensive yeast extract and other nutrient sources . More investigation on cellulose active proteins such as lytic polysaccharide monooxygenases (LPMOs) is needed since they have been reported to significantly aid cellulases in the depolymerization of crystalline cellulose . Further studies on genetic engineering tools for unlabeled gene integration into LAB genomic DNA, such as homologous recombination and single‐stranded DNA recombineering, needs to be studied further. They have the potential to produce recombinant LAB strains that can co‐express multiple cellulases and have also been successfully applied to improve the pentose metabolism .…”

Metabolic engineering strategies for consolidated production of lactic acid from lignocellulosic biomass

“…As far as production of LA from lignocellulose is concerned, most examples of RCSs have been targeted on LAB (for an extensive review, refer to ). LAB can produce LA with high yield, productivity, and optical purity through fermentation of several mono‐, di‐, and oligosaccharides .…”

“…As far as production of LA from lignocellulose is concerned, most examples of RCSs have been targeted on LAB (for an extensive review, refer to ). LAB can produce LA with high yield, productivity, and optical purity through fermentation of several mono‐, di‐, and oligosaccharides . Furthermore, some strains are very acid tolerant and the vast majority of them are GRAS, that is, generally recognized as safe, which avoid possible adverse health effects on either consumers or industrial production workers.…”

“…Current industrial production of LA is mainly based on fermentation by lactic acid bacteria (LAB) , but other potent natural producers of LA are bacteria belonging to the Bacillus genus and fungi of the Rhizopus genus . Unfortunately, none of these microorganisms can ferment lignocellulosic material without prior biomass saccharification . Processes featuring biomass pretreatment (through physical and/or chemical and/or enzymatic approach) followed by microbial fermentation of soluble sugars to LA can be highly efficient, with LA yields close to the theoretical maximum at nearly optical purity .…”

“…Unfortunately, none of these microorganisms can ferment lignocellulosic material without prior biomass saccharification . Processes featuring biomass pretreatment (through physical and/or chemical and/or enzymatic approach) followed by microbial fermentation of soluble sugars to LA can be highly efficient, with LA yields close to the theoretical maximum at nearly optical purity . However, biomass pre‐treatment has significant cost and, in particular, the cost of cellulases is among the highest in the entire process .…”