CO2reduction to acetate in anaerobic bacteria

Acetogen and acetogenesis for biological syngas valorization

The bioconversion of syngas using (homo)acetogens as biocatalysts shows promise as a viable option due to its higher selectivity and milder reaction conditions compared to thermochemical conversion. The current bioconversion process operates primarily to produce C2 chemicals (e.g., acetate and ethanol) with sufficient technology readiness levels (TRLs) in process engineering (as midstream) and product purification (as downstream). However, the economic feasibility of this process could be improved with greater biocatalytic options in the upstream phase. This review focuses on the Wood-Ljungdahl pathway (WLP) which is a biological syngas-utilization pathway, redox balance and ATP generation, suggesting that the use of a specific biocatalysts including Eubacterium limosum could be advantageous in syngas valorization. A pertinent strategy to mainly produce chemicals with a high degree of reduction is also provided with examples of flux control, mixed cultivation and mixotrophy. Finally, this article presents future direction of industrial utilization of syngas fermentation.

Neuroimaging Patterns of Intracranial Infections: Meningitis, Cerebritis, and Their Complications

Neuroimaging provides rapid, noninvasive visualization of central nervous system infections for optimal diagnosis and management. Generalizable and characteristic imaging patterns help radiologists distinguish different types of intracranial infections including meningitis and cerebritis from a variety of bacterial, viral, fungal, and/or parasitic causes. Here, we describe key radiologic patterns of meningeal enhancement and diffusion restriction through profiles of meningitis, cerebritis, abscess, and ventriculitis. We discuss various imaging modalities and recent diagnostic advances such as deep learning through a survey of intracranial pathogens and their radiographic findings. Moreover, we explore critical complications and differential diagnoses of intracranial infections.

Electron transfer and mechanism of energy production among syntrophic bacteria during acidogenic fermentation: A review

Volatile fatty acids (VFAs) production plays an important role in the process of anaerobic digestion (AD), which is often the critical factor determining the metabolic pathways and energy recovery efficiency. Fermenting bacteria and acetogenic bacteria are in syntrophic relations during AD. Thus, clear elucidation of the interspecies electron transfer and energetic mechanisms among syntrophic bacteria is essential for optimization of acidogenic. This review aims to discuss the electron transfer and energetic mechanism in syntrophic processes between fermenting bacteria and acetogenic bacteria during VFAs production. Homoacetogenesis also plays a role in the syntrophic system by converting H₂ and CO₂ to acetate. Potential applications of these syntrophic activities in bioelectrochemical system and value-added product recovery from AD of organic wastes are also discussed. The study of acidogenic syntrophic relations is in its early stages, and additional investigation is required to better understand the mechanism of syntrophic relations.

Influence of glucose fermentation on CO₂ assimilation to acetate in homoacetogen Blautia coccoides GA-1

Fermentation of glucose influences CO2 assimilation to acetate in homoacetogens. Blautia coccoides was investigated for a better understanding of the metabolic characteristics of homoacetogens in mixotrophic cultures. Batch cultures of the strain with H2/CO2 as a sole carbon source reached an acetate yield of 5.32 g/g dry cell weight after 240 h of incubation. Autotrophic metabolism was inhibited as glucose was added into the culture: the higher the glucose concentration the lower the autotrophic ability of the bacterium. Autotrophy was inhibited by high glucose concentration, probably due to the competition for coenzyme A between the Embden-Meyerhof-Parnas pathway and the Wood-Ljungdahl carbon fixation pathway, the energy (adenosine triphosphate) allocation for synthesis of cell carbon and reduction of CO2, in combination with the low pH caused by the accumulation of acetate.

Waste activated sludge fermentation for hydrogen production enhanced by anaerobic process improvement and acetobacteria inhibition: the role of fermentation pH

In this study an efficient strategy, i.e., controlling the fermentation pH at constant pH 10, for significantly increasing hydrogen yield from waste activated sludge (WAS) via the improvement of anaerobic process (sludge solubilization, hydrolysis, and acidification) and inhibition of hydrogen consumption by acetobacteria was reported. Without addition of pure hydrogen producer and nutrient source, the effect of different constant pH in the range of pH 4-11 on hydrogen production from WAS was compared with that of different initial pH. The maximal hydrogen yield was observed respectively at constant pH 10 and initial pH 10, but the former was 47.8% higher than the latter (26.9 versus 18.2 mL per gram volatile suspended solids) and much greater than that reported in literature. Then, the mechanisms for constant pH 10 resulting in remarkably higher hydrogen production than initial pH 10 were investigated. It was observed that constant pH 10 fermentation showed much higher solubilization of sludge main particulate organic matters, hydrolysis of solubilized organic materials and acidification of hydrolyzed products, which were of benefit to the hydrogen production. Also, there was more acetic but less propionic acid in the constant pH 10 test, which was in correspondence with the theory of fermentation type affecting hydrogen production. Moreover, in the reactor of initial pH 10 the produced hydrogen was readily converted to acetic acid, but no obvious hydrogen consumption was observed in constant pH 10 reactor. Further investigation of microorganisms with enzymes analysis and fluorescence in situ hybridization (FISH) indicated that the activity and growth of acetobacteria in the reactor of constant pH 10 was much lower than those in initial pH 10 reactor.

Biochemical pathways generating post-mortem volatile compounds co-detected during forensic ethanol analyses

In this contribution are presented the fermentations of the main substrates present in a decaying corpse, namely carbohydrates, amino acids, glycerol and fatty acids, generating the post-mortem volatile compounds that could be detected along with ethanol during the forensic ethanol analysis. The available literature (preferably reviews) on microbial metabolic pathways (enzymes, substrates, conditions) that are implicated in the formation of these volatiles has been reviewed. The microbial formation of the following volatiles is supported by the presented biochemical data: ethanol, acetaldehyde, acetone, 2-propanol, 1-propanol, 1-butanol, isobutanol, isoamyl alcohol, d-amyl alcohol, acetate, propionate, butyrate, isobutyrate and ethyl esters (mainly ethyl acetate). The extracted information was correlated with the existing forensic literature on the post-mortem detected volatiles. The significance of the microbial produced volatiles on the selection of an appropriate internal standard for the ethanol analysis has been considered. Finally, the possible contribution of the presence of volatiles in the interpretation of ethanol analysis results in post-mortem cases is discussed.

Microbial CO conversions with applications in synthesis gas purification and bio-desulfurization

Recent advances in the field of microbial physiology demonstrate that carbon monoxide is a readily used substrate by a wide variety of anaerobic micro-organisms, and may be employed in novel biotechnological processes for production of bulk and fine chemicals or in biological treatment of waste streams. Synthesis gas produced from fossil fuels or biomass is rich in hydrogen and carbon monoxide. Conversion of carbon monoxide to hydrogen allows use of synthesis gas in existing hydrogen utilizing processes and is interesting in view of a transition from hydrogen production from fossil fuels to sustainable (CO2-neutral) biomass. The conversion of CO with H2O to CO2 and H2 is catalyzed by a rapidly increasing group of micro-organisms. Hydrogen is a preferred electron donor in biotechnological desulfurization ofwastewaters and flue gases. Additionally, CO is a good alternative electron donor considering the recent isolation of a CO oxidizing, sulfate reducing bacterium. Here we review CO utilization by various anaerobic micro-organisms and their possible role in biotechnological processes, with a focus on hydrogen production and bio-desulfurization.

Competition between reductive acetogenesis and methanogenesis in the pig large-intestinal flora

Washed bacterial suspensions obtained from the pig hindgut were incubated under 13CO2 in a buffer containing NaH13CO3 and carbohydrates. Incorporation of 13C into short chain fatty acids was assayed by quantitative nuclear magnetic resonance. The effects of different levels of H2 added to the gas phase (0, 20 and 80% v/v) and of the specific methanogenesis inhibitor 2-bromoethane-sulphonic acid (BES) were determined. In control incubations increasing the concentration of H2 markedly increased methane production. Single- and double-labelled acetate and butyrate were formed in all incubations. In the absence of BES, increasing H2 significantly increased the incorporation of 13CO2 into butyrate and the proportion of double-labelled acetate in total labelled acetate. The addition of BES proved to be very successful as a methane inhibitor and greatly enhanced the amount of mono- and double-labelled acetate, especially at the highest H2 partial pressure. The results suggest that methanogenesis inhibited both routes of reductive acetogenesis, i.e. the homoacetate fermentation of hexose (represented for the most part by single labelling) and the synthesis of acetate from external CO2 and H2 (represented mostly by double labelling). A highly significant interaction between BES and H2 concentration was observed. At the highest pH2 BES increased the proportion of labelled acetate in total acetate from 17.1% for the control to 50.9%. It was concluded that although acetogenesis and methanogenesis can occur simultaneously in the pig hindgut, reductive acetogenesis may become a significant pathway of acetate formation in the absence of methanogenesis.

13C NMR studies of bacterial fermentations

We describe the experimental methods used and the constraints that apply in studies of anaerobic cell metabolism by 13C NMR. We review some of the results of our recent work in this area. Clostridium neopropionicum was shown to ferment ethanol into propionate by the acrylate, non-randomizing pathway. The same metabolic route accounts for 50% of the propionate formed in the complex ecosystem that inhabits the pig's large intestine. The rest is formed via the randomizing succinate pathway. Reductive, hydrogenotrophic acetogenesis was studied in several ecosystems. Although it is usually overshadowed by methanogenesis in the competition for hydrogen, it may become an efficient electron sink when methane biosynthesis is blocked by a specific inhibitor.

Importance of tetrahydrofolate and ATP in the anaerobic O-demethylation reaction for phenylmethylethers

DL-Tetrahydrofolate (THF) and ATP were necessary for the anaerobic O-demethylation of phenylmethylethers in cell extracts of the type strain (ATCC 29683) of the homoacetogen Acetobacterium woodii. The reactants for this enzymatic activity have not been previously demonstrated in any system, nor has the mediating enzyme been studied. An assay using reaction mixtures containing 1 mM THF, 2 mM ATP, and 2 mM hydroferulate (i.e., 4-hydroxy,3-methoxyphenylpropionate) was developed and was performed under stringent anaerobic conditions. Pyridine nucleotides and several other possible cofactors were tested but had no effect on the activity. After centrifugation of disrupted cells at 27,000 x g, the activity was found primarily in the supernatant, which had a specific activity of 14.2 +/- 0.5 nmol/min/mg of protein. At saturating levels of each of the other two substrates, apparent Km values for the variable substrate were 0.65 mM hydroferulate, 0.27 mM ATP, and 0.17 mM THF. Activity was significantly decreased when extract was preincubated at 60 degrees C and was completely lost after preincubation in air for 30 min. Thus, the soluble anaerobic O-demethylating enzyme system of A. woodii is oxygen sensitive. The THF- and ATP-dependent activity measurable in the soluble fraction of cell extracts constituted about 34% of the activity seen with intact cells.