Lactic acid bacteria as reducing and capping agent for the fast and efficient production of silver nanoparticles

There is a growing demand for silver-based biocides, including both ionic silver forms and metallic nanosilver. The use of metallic nanosilver, typically chemically produced, faces challenges including particle agglomeration, high costs, and upscaling difficulties . Additionally, there exists a need for the development of a more eco-friendly production of nanosilver. In this study, Gram-positive and Gram-negative bacteria were utilized in the non-enzymatic production of silver nanoparticles via the interaction of silver ions and organic compounds present on the bacterial cell. Only lactic acid bacteria, Lactobacillus spp., Pediococcus pentosaceus, Enterococcus faecium, and Lactococcus garvieae, were able to reduce silver. The nanoparticles of the five best producing Lactobacillus spp. were examined more into detail with transmission electron microscopy. Particle localization inside the cell, the mean particle size, and size distribution were species dependent, with Lactobacillus fermentum having the smallest mean particle size of 11.2 nm, the most narrow size distribution, and most nanoparticles associated with the outside of the cells. Furthermore, influence of pH on the reduction process was investigated. With increasing pH, silver recovery increased as well as the reduction rate as indicated by UV–VIS analyses. This study demonstrated that Lactobacillus spp. can be used for a rapid and efficient production of silver nanoparticles.     

Short-chain fatty acids and poly-β-hydroxyalkanoates: (New) Biocontrol agents for a sustainable animal production

Because of the risk of antibiotic resistance development, there is a growing awareness that antibiotics should be used more carefully in animal production. However, a decreased use of antibiotics could result in a higher frequency of pathogenic bacteria, which in its turn could lead to a higher incidence of infections. Short-chain fatty acids (SCFAs) have long been known to exhibit bacteriostatic activity. These compounds also specifically downregulate virulence factor expression and positively influence the gastrointestinal health of the host. As a consequence, there is currently considerable interest in SCFAs as biocontrol agents in animal production. Polyhydroxyalkanoates (PHAs) are polymers of β-hydroxy short-chain fatty acids. Currently, PHAs are applied as replacements for synthetic polymers. These biopolymers can be depolymerised by many different microorganisms that produce extracellular PHA depolymerases. Interestingly, different studies provided some evidence that PHAs can also be degraded upon passage through the gastrointestinal tract of animals and consequently, adding these compounds to the feed might result in biocontrol effects similar to those described for SCFAs.     

Nitrogen removal from aquaculture pond water by heterotrophic nitrogen assimilation in lab-scale sequencing batch reactors

The potential use of sequencing batch reactors (SBRs) as an alternative bio-flocs technology (BFT) approach in aquaculture was explored. One SBR was dosed with glycerol and one with acetate for the decrease of the nitrogen concentration in simulated aquaculture water by microbial assimilation. At an optimal C/N ratio between 10 and 15, the nitrogen removal efficiency reached up to 98% (=110 mg N L(-1) reactor day(-1)) for both SBRs. The estimated biomass productivity reached 0.62-0.94 g C L(-1)r eactor day(-1) for the glycerol SBR and 0.54-0.82 g C L(-1) reactor day(-1) for the acetate SBR. The floc protein content, indicating biomass quality, reached up to 57% if grown on glycerol. With acetate, it attained a value of 61%. The highest average poly-beta-hydroxybutyrate (PHB) content was 16% on a dry weight basis for the acetate biomass.     

MenD as a versatile catalyst for asymmetric synthesis

The thiamine diphosphate (ThDP)-dependent enzyme 2-succinyl-5-enolpyruvyl-6-hydroxy-3-cyclohexene-1-carboxylate synthase (MenD) from Escherichia coli K12, formerly known as SHCHC-synthase, catalyses the decarboxylation of α-ketoglutarate and the subsequent addition of the resulting succinyl-THDP to isochorismate. Here, the enzyme is tested for unphysiologial C–C bond-forming reactions.Condensation of α-ketoglutarate after decarboxylation to a broad range of aldehydes gave α-hydroxyketones with isolated yields from 26 to 87% and 94 to 98% ee for addition to aromatic aldehydes. MenD accepts a wide range of aldehydes as acceptor substrates to produce chiral α-hydroxyketones with conserved regioselectivity where the activated succinylsemialdehyde serves selectively as the donor. Regioselectivity is inverted only for condensation of α-ketoglutarate with pyruvate (activated acetaldehyde) as donor. Besides α-ketoglutarate, pyruvate and oxalacetate are accepted as donors in combination with benzaldehyde and 2-fluorobenzaldehyde as acceptors, however with decreased activity of C–C bond formation.The physiological 1,4-addition of α-ketoglutarate to isochorismate was investigated for acceptor substrate variability. (2S,3S)-2,3-Dihydroxy-2,3-dihydrobenzoate (2,3-CHD), which lacks the pyruvyl found in isochorismate, is converted to (5S,6S)-2-succinyl-5,6-dihydroxycyclohex-2-enecarboxylate. In contrast to the addition to carbonyls, the active site of MenD does appear to impose specific constraints on the acceptor substrate for 1,4-addition with α,β-unsaturated carboxylic acids.     

The application of bioflocs technology to protect brine shrimp (Artemia franciscana) from pathogenic Vibrio harveyi

Aims: To study the potential biocontrol activity of bioflocs technology. Methods and Results: Glycerol‐grown bioflocs were investigated for their antimicrobial and antipathogenic properties against the opportunistic pathogen Vibrio harveyi. The bioflocs did not produce growth‐inhibitory substances. However, bioflocs and biofloc supernatants decreased quorum sensing‐regulated bioluminescence of V. harveyi. This suggested that the bioflocs had biocontrol activity against this pathogen because quorum sensing regulates virulence of vibrios towards different hosts. Interestingly, the addition of live bioflocs significantly increased the survival of gnotobiotic brine shrimp (Artemia franciscana) larvae challenged to V. harveyi. Conclusions: Bioflocs grown on glycerol as carbon source inhibit quorum sensing‐regulated bioluminescence in V. harveyi and protect brine shrimp larvae from vibriosis. Significance and Impact of the Study: The results presented in this study indicate that in addition to water quality control and in situ feed production, bioflocs technology could help in controlling bacterial infections within the aquaculture pond.     

Metabolic engineering provides insight into the regulation of thiamin biosynthesis in plants

Thiamin (or thiamine) is a water-soluble B-vitamin (B1), which is required, in the form of thiamin pyrophosphate, as an essential cofactor in crucial carbon metabolism reactions in all forms of life. To ensure adequate metabolic functioning, humans rely on a sufficient dietary supply of thiamin. Increasing thiamin levels in plants via metabolic engineering is a powerful strategy to alleviate vitamin B1 malnutrition and thus improve global human health. These engineering strategies rely on comprehensive knowledge of plant thiamin metabolism and its regulation. Here, multiple metabolic engineering strategies were examined in the model plant Arabidopsis thaliana. This was achieved by constitutive overexpression of the three biosynthesis genes responsible for B1 synthesis, HMP-P synthase (THIC), HET-P synthase (THI1), and HMP-P kinase/TMP pyrophosphorylase (TH1), either separate or in combination. By monitoring the levels of thiamin, its phosphorylated entities, and its biosynthetic intermediates, we gained insight into the effect of either strategy on thiamin biosynthesis. Moreover, expression analysis of thiamin biosynthesis genes showed the plant’s intriguing ability to respond to alterations in the pathway. Overall, we revealed the necessity to balance the pyrimidine and thiazole branches of thiamin biosynthesis and assessed its biosynthetic intermediates. Furthermore, the accumulation of nonphosphorylated intermediates demonstrated the inefficiency of endogenous thiamin salvage mechanisms. These results serve as guidelines in the development of novel thiamin metabolic engineering strategies.     

Evidence for direct binding of glycerol to photosystem I

The interaction between glycerol and photosystem I (PSI) was investigated using low temperature single-molecule spectroscopy. PSI complexes were dissolved in three different solutions: in buffer solution, in 66% glycerol/buffer solution, and in 66% glycerol/buffer solution that was afterwards diluted by buffer; the final glycerol concentration was <1‰. Mean fluorescence spectra and intercomplex heterogeneity of PSI complexes in 66% glycerol/buffer solution and in the re-diluted solution show high similarity, but differ from complexes in buffer solution indicating that the glycerol concentration is not the determining factor modifying the spectral properties. However, the exposure of PSI to a high glycerol concentration during sample preparation affects PSI and the effect is maintained if glycerol is removed from the solution.     

Long-chain acylhomoserine lactones increase the anoxic ammonium oxidation rate in an OLAND biofilm

The oxygen-limited autotrophic nitrification/denitrification (OLAND) process comprises one-stage partial nitritation and anammox, catalyzed by aerobic and anoxic ammonium-oxidizing bacteria (AerAOB and AnAOB), respectively. The goal of this study was to investigate whether quorum sensing influences anoxic ammonium oxidation in an OLAND biofilm, with AnAOB colonizing 13% of the biofilm, as determined with fluorescent in situ hybridization (FISH). At high biomass concentrations, the specific anoxic ammonium oxidation rate of the OLAND biofilm significantly increased with a factor of 1.5 ± 0.2 compared to low biomass concentrations. Supernatant obtained from the biofilm showed no ammonium-oxidizing activity on itself, but its addition to low OLAND biomass concentrations resulted in a significant activity increase of the biomass. In the biofilm supernatant, the presence of long-chain acylhomoserine lactones (AHLs) was shown using the reporter strain Chromobacterium violaceum CV026, and one specific AHL, N-dodecanoyl homoserine lactone (C₁₂-HSL), was identified via LC-MS/MS. Furthermore, C₁₂-HSL was detected in an AnAOB-enriched community, but not in an AerAOB-enriched community. Addition of C₁₂-HSL to low OLAND biomass concentrations resulted in a significantly higher ammonium oxidation rate (p < 0.05). To our knowledge, this is the first report demonstrating that AHLs enhance the anoxic ammonium oxidation process. Future work should confirm which species are responsible for the in situ production of C₁₂-HSL in AnAOB-based applications.     

Correlations between molecular and operational parameters in continuous lab-scale anaerobic reactors

In this study, the microbial community characteristics in continuous lab-scale anaerobic reactors were correlated to reactor functionality using the microbial resource management (MRM) approach. Two molecular techniques, denaturing gradient gel electrophoresis (DGGE) and terminal-restriction fragment length polymorphism (T-RFLP), were applied to analyze the bacterial and archaeal communities, and the results obtained have been compared. Clustering analyses showed a similar discrimination of samples with DGGE and T-RFLP data, with a clear separation between the meso- and thermophilic communities. Both techniques indicate that bacterial and mesophilic communities were richer and more even than archaeal and thermophilic communities, respectively. Remarkably, the community composition was highly dynamic for both Bacteria and Archaea, with a rate of change between 30% and 75% per 18 days, also in stable performing periods. A hypothesis to explain the latter in the context of the converging metabolism in anaerobic processes is proposed. Finally, a more even and diverse bacterial community was found to be statistically representative for a well-functioning reactor as evidenced by a low Ripley index and high biogas production.     

The host selects mucosal and luminal associations of coevolved gut microorganisms: a novel concept

Along the human gastrointestinal tract, microorganisms are confronted with multiple barriers. Besides selective physical conditions, the epithelium is regularly replaced and covered with a protective mucus layer trapping immune molecules. Recent insights into host defense strategies show that the host selects the intestinal microbiota, particularly the mucosa-associated microbial community. In this context, humans coevolved with thousands of intestinal microbial species that have adapted to provide host benefits, while avoiding pathogenic behavior that might destabilize their host interaction. While mucosal microorganisms would be crucial for immunological priming, luminal microorganisms would be important for nutrient digestion. Further, we propose that the intestinal microorganisms also coevolved with each other, leading to coherently organized, resilient microbial associations. During disturbances, functionally redundant members become more abundant and are crucial for preserving community functionality. The outside of the mucus layer, where host defense molecules are more diluted, could serve as an environment where microorganisms are protected from disturbances in the lumen and from where they can recolonize the lumen after perturbations. This might explain the remarkable temporal stability of microbial communities. Finally, commensals that become renegade or a decreased exposure to essential coevolved microorganisms may cause particular health problems such as inflammatory bowel diseases, obesity or allergies.