Adaptation of lactic acid bacteria to butanol

Recent progress on n-butanol production by lactic acid bacteria

n-Butanol is an essential chemical intermediate produced through microbial fermentation. However, its toxicity to microbial cells has limited its production to a great extent. The anaerobe lactic acid bacteria (LAB) are the most resistant to n-butanol, so it should be the first choice for improving n-butanol production. The present article aims to review the following aspects of n-butanol production by LAB: (1) the tolerance of LAB to n-butanol, including its tolerance level and potential tolerance mechanisms; (2) genome editing tools in the n-butanol-resistant LAB; (3) methods of LAB modification for n-butanol production and the production levels after modification. This review will provide a theoretical basis for further research on n-butanol production by LAB.

Peculiar Response in the Co-Culture Fermentation of Leuconostoc mesenteroides and Lactobacillus plantarum for the Production of ABE Solvents

Two bacterial strains (CL11A and CL11D) that are capable of ABE fermentation, identified as Leuconostoc mesenteroides and Weissella cibari, were isolated from the soil surrounding the roots of bean plants. Another strain (ZM 3A), identified as Lactobacillus plantarum, which is capable of purely ethanolic fermentation was isolated from sugarcane. Glucose was used as a standard substrate to investigate the performance of these strains in mono—and co-culture fermentation for ABE production. The performance parameters employed in this study were substrate degradation rates, product and metabolite yields, pH changes and microbial growth rates. Both ABE isolates were capable of producing the three solvents but Leuconostoc mesenteroides had a higher specificity for ethanol than Weissella cibari. The co-culturing of Leuconostoc mesenteroides and Lactobacillus plantarum enhanced ethanol production at the expense of both acetone and butanol, and also influenced the final substrate consumption rate and product yield. The experiments indicated the potential of these niche environments for the isolation of ABE-producing microorganisms. This study contributes to the formulation of ideal microbial co-culture and consortia fermentation, which seeks to maximize the yield and production rates of favored products.

Effects of carbon concentration, oxygen, and controlled pH on the engineering strain Lactiplantibacillus casei E1 in the production of bioethanol from sugarcane molasses

Sugarcane molasses are considered a potential source for bioethanol's commercial production because of its availability and low market price. It contains high concentrations of fermentable sugars that can be directly metabolized by microbial fermentation. Heterofermentative lactic acid bacteria, especially Lactiplantibacillus casei, have a high potential to be a biocatalyst in ethanol production that they are characterized by strong abilities of carbohydrate metabolism, ethanol synthesis, and high alcohol tolerance. This study aimed to evaluate the feasibility of producing ethanol by Lactiplantibacillus casei used the ethanologen engineering strain L. casei E1 as a starter culture and cane molasses as substrate medium. The effects of environmental factors on the metabolism of L. casei E1 were analyzed by high-performance liquid chromatography (HPLC) system, and the gene expression of key enzymes in carbon source metabolism was detected using quantitative real-time PCR (RT-qPCR). Results showed that the strain could grow well, ferment sugar quickly in cane molasses. By fermenting this bacterium anaerobically at 37 °C for 36 h incubation in 5 °BX molasses when the fermenter's pH was controlled at 6.0, ethanol yield reached 13.77 g/L, and carbohydrate utilization percentage was 78.60%. RT-qPCR results verified the strain preferentially ferment glucose and fructose of molasses to ethanol at the molecular level. In addition, the metabolism of sugars, especially fructose, would be inhibited by elevating acidity. Our findings support the theoretical basis for exploring Lactic acid bacteria as a starter culture for converting sugarcane molasses into ethanol.

Proteomic Analysis Identifies Dysregulated Proteins in Butanol-Tolerant Gram-Positive Lactobacillus mucosae BR0713-33

Butanol can be produced biologically through fermentation of lignocellulosic biomass-derived sugars by Gram-positive Clostridium species. For cost-effective production, increased butanol fermentation titers are desired. However, the currently available butanol-fermenting microbes do not tolerate sufficiently high butanol concentrations; thus, new butanol-tolerant strains are desired. One promising strategy is to genetically modify Clostridium species by introducing stress tolerance-associated genes. This study was aimed to seek butanol tolerance genes from other Gram-positive species, which might be better suited than those from Gram-negative E. coli or eukaryotic Saccharomyces cerevisiae. Several butanol-tolerant lactobacilli were reported previously, and Lactobacillus mucosae BR0713-33, which showed the most robust anaerobic growth in 4% butanol, was used here for proteomics analyses. Cellular proteins that responded to 2, 3, and 4% butanol were characterized. Twenty-nine proteins that were identified were dysregulated in response to increased concentrations of butanol in L. mucosae . Seventeen genes involved in coding for stress-tolerant proteins GroES, GroEL, and DnaK and genes involved in substrate utilization, fatty acid metabolism, and nucleotide synthesis were induced by increased butanol, and 12 genes involving energy production (F₀F₁ATP synthases) and redox balance preservation were repressed by increased butanol. These results can help guide targeted engineering strategies to improve tolerance and production of biobutanol.

How to outwit nature: Omics insight into butanol tolerance

The energy crisis, depletion of oil reserves, and global climate changes are pressing problems of developed societies. One possibility to counteract that is microbial production of butanol, a promising new fuel and alternative to many petrochemical reagents. However, the high butanol toxicity to all known microbial species is the main obstacle to its industrial implementation. The present state of the art review aims to expound the recent advances in modern omics approaches to resolving this insurmountable to date problem of low butanol tolerance. Genomics, transcriptomics, and proteomics show that butanol tolerance is a complex phenomenon affecting multiple genes and their expression. Efflux pumps, stress and multidrug response, membrane transport, and redox-related genes are indicated as being most important during butanol challenge, in addition to fine-tuning of global regulators of transcription (Spo0A, GntR), which may further improve tolerance. Lipidomics shows that the alterations in membrane composition (saturated lipids and plasmalogen increase) are very much species-specific and butanol-related. Glycomics discloses the pleiotropic effect of CcpA, the role of alternative sugar transport, and the production of exopolysaccharides as alternative routes to overcoming butanol stress. Unfortunately, the strain that simultaneously syntheses and tolerates butanol in concentrations that allow its commercialization has not yet been discovered or produced. Omics insight will allow the purposeful increase of butanol tolerance in natural and engineered producers and the effective heterologous expression of synthetic butanol pathways in strains hereditary butanol-resistant up to 3.2 - 4.9% (w/v). Future breakthrough can be achieved by a detailed study of the membrane proteome, of which 21% are proteins with unknown functions.

Enhancing the tolerance of Clostridium saccharoperbutylacetonicum to lignocellulosic-biomass-derived inhibitors for efficient biobutanol production by overexpressing efflux pumps genes from Pseudomonas putida

Furan aldehydes and phenolic compounds generated during biomass pretreatment can inhibit fermentation for biofuel production. Efflux pumps actively transport small molecules out of cells, thus sustaining normal microbial metabolism. Pseudomonas putida has outstanding tolerance to butanol and other small molecules, and we hypothesize that its efflux pump could play essential roles for such robustness. Here, we overexpressed efflux pump genes from P. putida to enhance tolerance of hyper-butanol producing Clostridium saccharoperbutylacetonicum to fermentation inhibitors. Interestingly, overexpression of the whole unit resulted in decreased tolerance, while overexpression of the subunit (srpB) alone exerted significant enhanced robustness of the strain. Compared to the control, the engineered strain had enhanced capability to grow in media containing 17% more furfural or 50% more ferulic acid, and produced ~14 g/L butanol (comparable to fermentation under regular conditions without inhibitors). This study provided valuable reference for boosting microbial robustness towards efficient biofuel production from lignocellulosic materials.

Progress and perspectives on improving butanol tolerance

Fermentative production of butanol for use as a biofuel or chemical feedstock is regarded as a promising renewable technology that reduces greenhouse gas emissions and has the potential to become a substitute for non-sustainable chemical production route. However, butanol toxicity to the producing microbes remains a barrier to achieving sufficiently high titers for cost-effective butanol fermentation and recovery. Investigations of the external stress of high butanol concentration on butanol-producing microbial strains will aid in developing improved microbes with increased tolerance to butanol. With currently available molecular tool boxes, researchers have aimed to address and understand how butanol affects different microbes. This review will cover the individual organism's inherent responses to surrounding butanol levels, and the collective efforts by researchers to improve production and tolerance. The specific microorganisms discussed here include the native butanol producer Clostridium species, the fermentation industrial model Saccharomyces cerevisiae and the photosynthetic cyanobacteria, the genetic engineering workhorse Escherichia coli, and also the butanol-tolerant lactic acid bacteria that utilize diverse substrates. The discussion will help to understand the physiology of butanol resistance and to identify specific butanol tolerance genes that will lead to informed genetic engineering strategies for new strain development.