Development of a reversible binding process for in situ removal of 3-hydroxypropionaldehyde during biotechnological conversion of glycerol

Application of the Reuterin System as Food Preservative or Health-Promoting Agent: A Critical Review

The reuterin system is a complex multi-component antimicrobial system produced by Limosilactobacillus reuteri by metabolizing glycerol. The system mainly includes 3-hydroxypropionaldehyde (3-HPA, reuterin), 3-HPA dimer, 3-HPA hydrate, acrolein and 3-hydroxypropionic acid, and has great potential to be applied in the food and medical industries due to its functional versatility. It has been reported that the reuterin system possesses regulation of intestinal flora and anti-infection, anti-inflammatory and anti-cancer activities. Typically, the reuterin system exerts strong broad-spectrum antimicrobial properties. However, the antimicrobial mechanism of the reuterin system remains unclear, and its toxicity is still controversial. This paper presents an updated review on the biosynthesis, composition, biological production, antimicrobial mechanisms, stability, toxicity and potential applications of the reuterin system. Challenges and opportunities of the use of the reuterin system as a food preservative or health-promoting agent are also discussed. The present work will allow researchers to accelerate their studies toward solving critical challenges obstructing industrial applications of the reuterin system.

Sensory Relevance of Strecker Aldehydes in Wines. Preliminary Studies of Its Removal with Different Type of Resins

The orthonasal quality of two synthetic contexts of wine (young wine and oaked wine) spiked with six different levels of the Strecker aldehydes (isobutyraldehyde, 2-methylbutanal, 3-methylbutanal, methional and phenylacetaldehyde) was evaluated by a panel of wine experts. The aldehyde levels simulated the concentrations present in wines protected from oxidation during production and storage and after severe oxidation. Significant quality detriments were observed at concentrations of 13 µg/L of methional, 49 µg/L of phenylacetaldehyde, 17 µg/L of isobutyraldehyde, 12 µg/L of 2-methylbutanal and 24 µg/L of 3-methylbutanal. The presence of these levels of aldehyde concentrations induced the reduction of fruitiness in young wines and of woody notes in oaked wines as well as the appearance of the typical attributes that define wine oxidation. More than 75% of recently opened commercial wines contain total levels of Strecker aldehydes higher than those, however their effect is not always noticeable as they are forming inodorous adducts with SO₂. Nevertheless, this content is a potential risk for the shelf life of the wine, as once SO₂ is depleted, these aldehydes could release back into their odour-active forms. Thus, in order to reduce the presence of Strecker aldehydes, eight different resins were studied (two scavengers, four mixed-mode anion exchange and two pure anion exchange) in white wine at two levels of SO₂. After 24-h contact, the mixed mode Strata X-A resin was able to significantly reduce aldehydes’ percentages: between 11% for isobutyraldehyde and 86% for phenylacetaldehyde. On the other hand, wine colour was affected and therefore the applicability of the treatment should be further studied. However, this work can be considered a starting point to solve the technological challenge involved in the elimination of aldehydes from wine.

In Vitro Antimicrobial Activity and Downregulation of Virulence Gene Expression on Helicobacter pylori by Reuterin

Helicobacter pylori is an infectious agent commonly associated with gastrointestinal diseases. The use of probiotics to treat this infection has been documented, however, their potential antimicrobial metabolites have not yet been investigated. In the present study, the effect of reuterin produced by Lactobacillus reuteri on H. pylori growth and virulence gene expression was evaluated. It was observed that reuterin caused significant (P < 0.05) H. pylori growth inhibition at concentrations from 0.08 to 20.48 mM, with minimal inhibitory concentrations (MICs) of 20.48 mM for H. pylori ATCC700824 and 10.24 mM for H. pylori ATCC43504. In a reuterin bacterial killing assay, it was observed that half of the MIC value for H. pylori (ATCC700824) significantly (P < 0.01) reduced colony numbers from 5.65 ± 0.35 to 3.78 ± 0.35 Log₁₀ CFU/mL after 12 h of treatment and then increased them to 5.25 ± 0.23 Log₁₀ CFU/mL at 24 h; at its MIC value (20.48 mM), reuterin abrogated (P < 0.01) H. pylori (ATCC700824) growth after 20 h of culture. In addition, reuterin significantly (P < 0.01) reduced H. pylori (ATCC 43504) colony numbers from 5.65 ± 0.35 to 4.1 ± 0.12 Log10 CFU/mL from 12 to 24 h of treatment and abrogated its growth at its MIC value (10.24 mM), after 20 h of treatment. Reuterin did not alter normal human gastric Hs738.St/Int cell viability at the concentrations tested for H. pylori strains. Furthermore, 10 μM reuterin was shown to significantly (P < 0.01) reduce mRNA relative expression levels of H. pylori virulence genes vacA and flaA at 3 h post-treatment, whose effect was higher at 6 h post-treatment, as measured by RT-qPCR. The observed direct antimicrobial effect and the downregulation of expression of virulence genes on H. pylori by reuterin may contribute to the understanding of the mechanisms of action of probiotics against H. pylori.

Disruption of the Reductive 1,3-Propanediol Pathway Triggers Production of 1,2-Propanediol for Sustained Glycerol Fermentation by Clostridium pasteurianum

Crude glycerol, the major by-product of biodiesel production, is an attractive bioprocessing feedstock owing to its abundance, low cost, and high degree of reduction. In line with the advent of the biodiesel industry, Clostridium pasteurianum has gained prominence as a result of its unique capacity to convert waste glycerol into n-butanol, a high-energy biofuel. However, no efforts have been directed at abolishing the production of 1,3-propanediol (1,3-PDO), the chief competing product of C. pasteurianum glycerol fermentation. Here, we report rational metabolic engineering of C. pasteurianum for enhanced n-butanol production through inactivation of the gene encoding 1,3-PDO dehydrogenase (dhaT). In spite of current models of anaerobic glycerol dissimilation, culture growth and glycerol utilization were unaffected in the dhaT disruption mutant (dhaT::Ll.LtrB). Metabolite characterization of the dhaT::Ll.LtrB mutant revealed an 83% decrease in 1,3-PDO production, encompassing the lowest C. pasteurianum 1,3-PDO titer reported to date (0.58 g liter(-1)). With 1,3-PDO formation nearly abolished, glycerol was converted almost exclusively to n-butanol (8.6 g liter(-1)), yielding a high n-butanol selectivity of 0.83 g n-butanol g(-1) of solvents compared to 0.51 g n-butanol g(-1) of solvents for the wild-type strain. Unexpectedly, high-performance liquid chromatography (HPLC) analysis of dhaT::Ll.LtrB mutant culture supernatants identified a metabolite peak consistent with 1,2-propanediol (1,2-PDO), which was confirmed by nuclear magnetic resonance (NMR). Based on these findings, we propose a new model for glycerol dissimilation by C. pasteurianum, whereby the production of 1,3-PDO by the wild-type strain and low levels of both 1,3-PDO and 1,2-PDO by the engineered mutant balance the reducing equivalents generated during cell mass synthesis from glycerol.

IMPORTANCE

Organisms from the genus Clostridium are perhaps the most notable native cellular factories, owing to their vast substrate utilization range and equally diverse variety of metabolites produced. The ability of C. pasteurianum to sustain redox balance and glycerol fermentation despite inactivation of the 1,3-PDO pathway is a testament to the exceptional metabolic flexibility exhibited by clostridia. Moreover, identification of a previously unknown 1,2-PDO-formation pathway, as detailed herein, provides a deeper understanding of fermentative glycerol utilization in clostridia and will inform future metabolic engineering endeavors involving C. pasteurianum To our knowledge, the C. pasteurianum dhaT disruption mutant derived in this study is the only organism that produces both 1,2- and 1,3-PDOs. Most importantly, the engineered strain provides an excellent platform for highly selective production of n-butanol from waste glycerol.

Coenzyme A-acylating propionaldehyde dehydrogenase (PduP) from Lactobacillus reuteri: kinetic characterization and molecular modeling

3-Hydroxypropionic acid (3-HP), an important C3 chemical for a bio-based industry, is natively produced by Lactobacillus reuteri from glycerol. Conversion of glycerol occurs via the intermediate 3-hydroxypropionaldehyde (3-HPA), followed by an ATP-producing pathway initiated by the CoA-acylating propionaldehyde dehydrogenase (PduP). The pduP gene of L. reuteri was cloned and expressed in Escherichia coli and the recombinant enzyme was purified to homogeneity for characterization of its activity and properties. Kinetic studies with propionaldehyde as substrate showed a maximum specific activity of 28.9 U/mg, which is 80-fold higher than that reported previously. Maximum activity of 18 U/mg was obtained at 3-HPA concentration of 7 mM, above which substrate inhibition was observed. Substrate inhibition was also seen with coenzyme A at a concentration above 0.5mM and with NADP(+) above 9 mM. A structure of PduP is proposed based on homology modeling. In silico docking of the co-factors coenzyme A and NAD(+), respectively, showed a common binding site consisting of amino acids Thr145, Ile275, Cys277 and Ser417, which through site-directed mutagenesis to alanine and kinetic studies, were confirmed as essential for the catalytic activity of PduP.

A novel cell modification method used in biotransformation of glycerol to 3-HPA by Lactobacillus reuteri

The aim of the present study was to develop a new cell modification method to facilitate the cell separation from broth. In order to reduce the transfer limitation of substrate and product caused by general immobilization methods in the following biotransformation of glycerol, the carboxyl-functioned superparamagnetic nanoparticle (MNP) was directly attached to the surface of Lactobacillus reuteri for 3-hydroxypropionealdehyde producing. The modification process could be finished in several minutes by just adding MNP fluid into the bulk fermentation broth. The modified cells could be rapidly separated from the solution with the aid of magnetic field. The interaction between cell and MNP was shown by electron microscopy. The efficiency of the cells attached by MNPs for transformation of various concentrations of glycerol (100-400 mM) was studied at various temperatures (25-40 °C) and pH levels (5.8-7.5) with different cell concentrations (7.5-30 g/L). The 3- hydroxypropionealdehyde (HPA)/glycerol molar conversion under optimal condition (30 °C and pH 7) reached 70 %. The inactive modified cell could be reactivated easily by fresh medium and recovered the ability of glycerol conversion. MNPS distributing on cell surface had little adverse effect on cell activity. The modification method simplified the two-step production of 3-HPA by resting L. reuteri. The method of MNPs attached to cell surface is totally different from the traditional immobilization method in which the cell is attached to or entrapped in big carrier. The results obtained in this study showed that carboxyl-functioned MNP could be directly used as cell modification particle and realized cell recycle with the aid of magnetic field in bioprocess.