Carbon catabolite repression during the simultaneous utilization of lignocellulose-derived sugars in lactic acid production: Influencing factors and mitigation strategies

Lignocellulose is the most abundant, sustainable, and comparatively economical renewable biomass containing ample fermentable sugars for bio-based chemical production, such as lactic acid (LA). LA is a versatile chemical with substantial global demand. However, the concurrent utilization of mixed sugars derived from lignocellulose, including glucose, xylose, and arabinose, remains a formidable challenge because of the metabolic regulation of carbon catabolite repression (CCR), in which glucose is preferentially utilized over non-glucose sugars, resulting in the loss of carbon resources and a decrease in biorefinery efficacy. Most current studies on CCR have concentrated on elucidating the principles and their impact on specific bacterial species using mixed carbon sources. However, there remains a notable dearth of comprehensive reviews summarizing the underlying principles and corresponding mitigation strategies across other bacterial strains encountering similar challenges. In light of this, this article delineates the possible factors that lead to CCR, including signal transduction and metabolic pathways. Additionally, the fermentation conditions and nutrients are described. Finally, this study proposes appropriate mitigation strategies to overcome the aforementioned obstacles and presents new insights into the rapid and simultaneous consumption of mixed sugars to bolster the production yields of biofuels and chemicals in the future.

Enhanced production of lactic acid from pretreated rice straw using co-cultivation of Bacillus licheniformis and Bacillus sonorenesis

Lactic acid (LA) production from sugar mixture derived from lignocellulosic rice straw employing co- culture system of thermotolerant and inhibitor tolerant Bacillus licheniformis DGB and Bacillus sonorenesis DGS15 was carried out. In minimal media, both the strains of Bacillus DGB and DGS15 worked together by efficiently utilising glucose and xylose respectively. Response Surface Methodology (RSM) was used for optimisation of pretreatment of rice straw to achieve maximum yield of 50.852 g/L total reducing sugar (TRS) from 100 gm of rice straw biomass. Pretreatment of rice straw resulted in its delignification, as confirmed by FTIR spectroscopy, since the peak at 1668 cm-1 disappeared due to removal of lignin and scanning electron microscopy (SEM) revealed disruption in structural and morphological features. Crystallinity index (CrI) of treated rice straw increased by 15.54% in comparison to native biomass. DGB and DGS15 individually yielded 0.64 g/g and 0.82 g/g lactic acid respectively, where as their co-cultivation led to effective utilisation of both glucose and xylose within 15 h (70%) and complete utilisation in 48 h, producing 49.75 g/L LA with a yield of 0.98 g/g and productivity of 1.036 g/L/h, and resulting in reduction in fermentation time. Separate hydrolysis of rice straw and co-fermentation (SHCF) of hydrolysates by Bacillus spp. enhanced the production of lactic acid, can circumvent challenges in biorefining of lignocellulosic biomass.

Microbial Fermentation Processes of Lactic Acid: Challenges, Solutions, and Future Prospects

The demand for lactic acid and lactic acid-derived products in the food, pharmaceutical, and cosmetic industries is increasing year by year. In recent decades, the synthesis of lactic acid by microbials has gained much attention from scientists due to the superior optical purity of the product, its low production costs, and its higher production efficiency compared to chemical synthesis. Microbial fermentation involves the selection of feedstock, strains, and fermentation modes. Each step can potentially affect the yield and purity of the final product. Therefore, there are still many critical challenges in lactic acid production. The costs of feedstocks and energy; the inhibition of substrates and end-product; the sensitivity to the inhibitory compounds released during pretreatment; and the lower optical purity are the main obstacles hindering the fermentation of lactic acid. This review highlights the limitations and challenges of applying microbial fermentation in lactic acid production. In addition, corresponding solutions to these difficulties are summarized in order to provide some guidance for the industrial production of lactic acid.

Integrated lactic acid production from lignocellulosic agricultural wastes under thermal conditions

The production of lactic acid (LA) from agricultural wastes attracts great attention because of the sustainability and abundance of lignocellulosic feedstocks, as well as the increasing demand for biodegradable polylactic acid. In this study, we isolated a thermophilic strain Geobacillus stearothermophilus 2H-3 for use in robust production of L-(+)LA under the optimal conditions of 60 °C, pH 6.5, which were consistent with the whole-cell-based consolidated bio-saccharification (CBS) process. Sugar-rich CBS hydrolysates derived from various agricultural wastes, including corn stover, corncob residue, and wheat straw, were used as the carbon sources for 2H-3 fermentation by directly inoculating 2H-3 cells into the CBS system, without intermediate sterilization, nutrient supplementation, or adjustment of fermentation conditions. Thus, we successfully combined two whole-cell-based steps into a one-pot successive fermentation process to efficiently produce LA with high optical purity (99.5%), titer (51.36 g/L), and yield (0.74 g/g_biomass). This study provides a promising strategy for LA production from lignocellulose through CBS and 2H-3 fermentation integration.

Biotechnological enhancement of lactic acid conversion from pretreated palm kernel cake hydrolysate by Actinobacillus succinogenes 130Z

The aim of this study was to establish an improved pretreatment and fermentation method i.e. immobilized cells for high recovery of fermentable sugars from palm kernel cake (PKC) and its effects on fermentability performance by Actinobacillus succinogenes 130Z in the conversion of the fermentable sugar to lactic acid. The effects of oxalic acid concentrations (1-6% w/v) and residence times (1-5 h) on the sugar recovery were initially investigated and it was found that the highest mannose concentration was 25.1 g/L at the optimum hydrolysis conditions of 4 h and 3% (w/v) oxalic acid. The subsequent enzymatic saccharification of the pretreated PKC afforded the highest enzymatic digestibility with the recovered sugars amounting to 25.18 g/L and 9.14 g/L of mannose and glucose, respectively. Subsequently, the fermentability performance of PKC hydrolysate was evaluated and compared in terms of cultivation phases (i.e. mono and dual-phases), carbonate loadings (i.e. magnesium and sodium carbonates), and types of sugars (i.e. glucose and mannose). The highest titer of 19.4 g/L lactic acid was obtained from the fermentation involving A. succinogenes 130Z in dual-phase cultivation supplemented with 30 g/L of magnesium carbonate. Lactic acid production was further enhanced by using immobilized cells with coconut shell-activated carbon (CSAC) of different sizes (A, B, C, and D) in the repeated batch cultivation of dual-phase fermentation producing 31.64 g/L of lactic acid. This work sheds light on the possibilities to enhance the utilization of PKC for lactic acid production via immobilized A. succinogenes 130Z.

An efficient CRISPR/Cas9-based genome editing system for alkaliphilic Bacillus sp. N16-5 and application in engineering xylose utilization for D-lactic acid production

Alkaliphiles are considered more suitable chassis than traditional neutrophiles due to their excellent resistance to microbial contamination. Alkaliphilic Bacillus sp. N16-5, an industrially interesting strain with great potential for the production of lactic acid and alkaline polysaccharide hydrolases, can only be engineered genetically by the laborious and time-consuming homologous recombination. In this study, we reported the successful development of a CRISPR/Cas9-based genome editing system with high efficiency for single-gene deletion, large gene fragment deletion and exogenous DNA chromosomal insertion. Moreover, based on a catalytically dead variant of Cas9 (dCas9), we also developed a CRISPRi system to efficiently regulate gene expression. Finally, this efficient genome editing system was successfully applied to engineer the xylose metabolic pathway for the efficient bioproduction of D-lactic acid. Compared with the wild-type Bacillus sp. N16-5, the final engineered strain with XylR deletion and AraE overexpression achieved 34.3% and 27.7% increases in xylose consumption and D-lactic acid production respectively. To our knowledge, this is the first report on the development and application of CRISPR/Cas9-based genome editing system in alkaliphilic Bacillus, and this study will significantly facilitate functional genomic studies and genome manipulation in alkaliphilic Bacillus, laying a foundation for the development of more robust microbial chassis.

Corynebacterium glutamicum as an Efficient Omnivorous Microbial Host for the Bioconversion of Lignocellulosic Biomass

Corynebacterium glutamicum has been successfully employed for the industrial production of amino acids and other bioproducts, partially due to its native ability to utilize a wide range of carbon substrates. We demonstrated C. glutamicum as an efficient microbial host for utilizing diverse carbon substrates present in biomass hydrolysates, such as glucose, arabinose, and xylose, in addition to its natural ability to assimilate lignin-derived aromatics. As a case study to demonstrate its bioproduction capabilities, L-lactate was chosen as the primary fermentation end product along with acetate and succinate. C. glutamicum was found to grow well in different aromatics (benzoic acid, cinnamic acid, vanillic acid, and p-coumaric acid) up to a concentration of 40 mM. Besides, ¹³C-fingerprinting confirmed that carbon from aromatics enter the primary metabolism via TCA cycle confirming the presence of β-ketoadipate pathway in C. glutamicum. ¹³C-fingerprinting in the presence of both glucose and aromatics also revealed coumarate to be the most preferred aromatic by C. glutamicum contributing 74 and 59% of its carbon for the synthesis of glutamate and aspartate respectively. ¹³C-fingerprinting also confirmed the activity of ortho-cleavage pathway, anaplerotic pathway, and cataplerotic pathways. Finally, the engineered C. glutamicum strain grew well in biomass hydrolysate containing pentose and hexose sugars and produced L-lactate at a concentration of 47.9 g/L and a yield of 0.639 g/g from sugars with simultaneous utilization of aromatics. Succinate and acetate co-products were produced at concentrations of 8.9 g/L and 3.2 g/L, respectively. Our findings open the door to valorize all the major carbon components of biomass hydrolysate by using C. glutamicum as a microbial host for biomanufacturing.

Enterococcus faecium s6 Enabled Efficient Homofermentative Lactic Acid Production from Xylan-Derived Sugars

A thermotolerant Enterococcus faecium s6 strain exhibited homoferementative lactic acid (LA) production at high xylose concentration (≥50 g/L). In batch fermentation at 45 °C and controlled pH of 6.5, strain E. faecium s6 produced up to 72.9 g/L of LA with a yield of 0.99 g/g-consumed xylose and productivity of 1.74 g/L.h from 75 g/L xylose. An increased LA concentration and productivities with high yields were obtained in repeated batch fermentation that was conducted for 24 runs. In this mode, the strain could produce LA up 81.1 g/L within 5 h fermentation at a high productivity of 13.5 g/L.h and a yield of 1.02 g/g-consumed xylose. The strain was unable to consume birchwood xylan as sole carbon source. However, it could efficiently utilize xylooligosaccharides of xylobiose, xylotriose, xylotetraose, xylopentaose, and xylohexaose. The intracellular xylosidase activity was induced by xylose. Using xylooligosaccharide (50 g/L)/xylose (5 g/L) mixtures, the strain was able to produce maximum LA at 48.2 g/L within 120 h at a yield of 1.0 g/g-consumed sugar and productivity of 0.331 g/L.h. These results indicate that E. faecium s6 is capable of directly utilizing xylan-hydrolyzate and will enable the development of a feasible and economical approach to the production of LA from hemicellulosic hydrolysate.

Cyclic L-lactide synthesis from lignocellulose biomass by biorefining with complete inhibitor removal and highly simultaneous sugars assimilation

Cyclic chiral lactide is the monomer chemical for polymerization of high molecular weight polylactic acid (PLA). The synthesis of cyclic L-lactide starts from poly-condensation of L-lactic acid to a low molecular weight pre-polymer and then depolymerized to cyclic L-lactide. Lignocellulose biomass is the most promising carbohydrate feedstock for lactic acid production, but the synthesis of cyclic L-lactide from L-lactic acid produced from lignocellulose has so far not been successful. The major barriers are the impurities of residual sugars and inhibitors in the crude cellulosic L-lactic acid product. Here we show a successful cyclic L-lactide synthesis from cellulosic L-lactic acid by lignocellulose biorefining with complete inhibitor removal and coordinated sugars assimilation. The removal of inhibitors from lignocellulose pretreatment was accomplished by biodetoxification using a unique fungus Amorphotheca resinae ZN1. The non-glucose sugars were completely and simultaneously assimilated at the same rate with glucose by the engineered L-lactic acid bacterium Pediococcus acidilactici. The L-lactic acid production from wheat straw was comparable to that from corn starch with high optical pure (99.6%), high L-lactic acid titer (129.4 g/L), minor residual total sugars (~2.2 g/L), and inhibitors free. The cyclic L-lactide was successfully synthesized from the regularly purified L-lactic acid and verified by detailed characterizations. This study paves the technical foundation of carbon neutral production of biodegradable PLA from lignocellulose biomass. This article is protected by copyright. All rights reserved.

Preparation of Oil Palm Empty Fruit Bunch Hydrolysate

Malaysia is the second largest palm oil producer and exporter globally. When crude palm oil is produced in both plantations and oil processing mills, a large amount of oil palm empty fruit bunch (OPEFB) is simultaneously produced as a waste product. Here, we describe the preparation of hydrolysate from OPEFB. After OPEFB was hydrothermally treated at 180–200 °C, the resultant liquid phase was subjected to high-performance liquid chromatography analysis, while the solid phase was used for acidic and enzymatic hydrolysis. Hemicellulose yield from the acid-treated solid phase decreased from 153 mg/g-OPEFB to 27.5 mg/g-OPEFB by increasing the hydrothermal treatment temperature from 180 to 200 °C. Glucose yield from the enzyme-treated solid phase obtained after hydrothermal treatment at 200 °C was the highest (234 ± 1.90 mg/g-OPEFB, 61.7% production efficiency). In contrast, xylose, mannose, galactose, and arabinose yields in the hydrolysate prepared from the solid phase hydrothermally treated at 200 °C were the lowest. Thus, we concluded that the optimum temperature for hydrothermal pretreatment was 200 °C, which was caused by the low hemicellulose yield. Based on these results, we have established an effective method for preparing OPEFB hydrolysates with high glucose content.