Rhamnolipid Research Articles

Application of Rhamnolipids as Dispersing Agents for the Fabrication of Composite MnO2-Carbon Nanotube Electrodes for Supercapacitors.

The high theoretical capacitance of MnO2 renders it a promising material for the cathodes of asymmetric supercapacitors. The good dispersion of MnO2 and conductive additives in a nanocomposite electrode is a key factor for efficient electrode performance. This article describes, for the first time, the application of rhamnolipids (RL) as efficient natural biosurfactants for the fabrication of nanocomposite MnO2-carbon nanotube electrodes for supercapacitors. RL act as co-dispersants for MnO2 and carbon nanotubes and facilitate their efficient mixing, which allows for advanced capacitive properties at an active mass of 40 mg cm−2 in Na2SO4 electrolytes. The highest capacitance obtained from the cyclic voltammetry data at a scan rate of 2 mV s−1 is 8.10 F cm−2 (202.6 F g−1). The highest capacitance obtained from the galvanostatic charge–discharge data at a current density of 3 mA cm−2 is 8.65 F cm−2 (216.16 F g−1). The obtained capacitances are higher than the capacitances of MnO2-based electrodes of the same active mass reported in the literature. The approach developed in this investigation is simple compared to other techniques used for the fabrication of electrodes with high active mass. It offers advantages of using a biocompatible RL biosurfactant.

Colloidal Processing of Mn3O4-Carbon Nanotube Nanocomposite Electrodes for Supercapacitors.

This investigation addresses the challenges in the development of efficient nanostructured Mn3O4 cathodes for supercapacitors. A high areal capacitance and the ability to avoid a time-consuming activation procedure for electrodes with high active mass loading of 40 mg cm−2 are reported. This facilitates practical applications of Mn3O4 based electrodes. The highest capacitance of 6.11 F cm−2 (153 F g−1) is obtained from cyclic voltammetry at a scan rate of 2 mV s−1 and 6.07 F cm−2 (151.9 F g−1) from the chronopotentiometry at a current density of 3 mA cm−2 in a potential window of 0.9 V in a neutral Na2SO4 electrolyte. The new approach is based on the application of rhamnolipids (RL) as a capping agent for the synthesis of Mn3O4 particles and a co-dispersant for Mn3O4 and carbon nanotubes, which are used as conductive additives. The size and shape of the Mn3O4 particles are influenced by RL. The enhanced performance of the electrodes is linked to the chemical structure and properties of RL molecules, which exert influence on Mn3O4 particle size and shape during synthesis, reduce agglomeration, facilitate RL adsorption on Mn3O4 and carbon nanotubes, and influence their co-dispersion and mixing at the nanometric scale.

Carbohydrate-carbohydrate interaction drives the preferential insertion of dirhamnolipid into glycosphingolipid enriched membranes

Rhamnolipids (RLs) are among the most important biosurfactants produced by microorganisms, and have been widely investigated because of their multiple biological activities. Their action appears to depend on their structural interference with lipid membranes, therefore several studies have been performed to investigate this aspect. We studied by X-ray scattering, neutron reflectometry and molecular dynamic simulations the insertion of dirhamnolipid (diRL), the most abundant RL, in model cellular membranes made of phospholipids and glycosphingolipids. In our model systems the affinity of diRL to the membrane is highly promoted by the presence of the glycosphingolipids and molecular dynamics simulations unveil that this evidence is related to sugar-sugar attractive interactions at the membrane surface. Our results improve the understanding of the plethora of activities associated with RLs, also opening new perspectives in their selective use for pharmaceutical and cosmetics formulations. Additionally, they shed light on the still debated role of carbohydrate-carbohydrate interactions as driving force for molecular contacts at membrane surface.

Multi-Omics Revealed the Protective Effects of Rhamnolipids in Lipopolysaccharide Challenged Broilers.

Rhamnolipid (RL) is a glycolipid biosurfactant and exhibits the following outstanding characteristics: strong antibacterial properties, low toxicity, and high biodegradability. The present research was conducted to explore the protective effects and mechanisms of rhamnolipids as an alternative to antibiotics in LPS (lipopolysaccharide)-challenged broilers. 16S rRNA gene sequencing and metabolomics were used for analyzing the cecal microbial composition and serum metabolites. Dietary antibiotics and RLS supplementation decreased the weight loss rate, enhanced serum immunoglobulin levels, reduced serum diamine oxidase and D-lactate acid concentration, and improved the symptoms of intestinal bleeding and villus height, when broilers were challenged with LPS. The addition of RLS in the diet enhanced serum interleukin-4 and interleukin-10 contents and reduced serum interleukin-6 and tumor necrosis factor-α levels in LPS-challenged broilers compared with the antibiotics group. Spearman’s correlation analysis revealed that RLS may alleviate LPS-induced inflammatory responses through altering the 6-methoxymellein level in broilers. The genus Bacteroides may contribute to the decreased weight loss rate via regulating the serum lysoPC [20:5(5Z,8Z,11Z,14Z,17Z)] secretion. RLS alleviates LPS-induced intestinal injury, enhances the growth and immunity, ameliorates intestinal microflora, and improves serum metabolites in LPS-challenged broilers. RLS exhibited better protective effect than antibiotic supplementation in the diet of LPS-challenged broilers. These findings provide potential regulation strategies and novel insights for RLS enhancing its protective effect in LPS-challenged broilers.

Effect of Rhamnolipid Amidation on Biosurfactant Adsorption Loss and Oil-Washing Efficiency.

Surfactant adsorption loss seriously hinders the economy of surfactant binary flooding technology for enhancing oil recovery, especially for biosurfactants with higher manufacturing costs. Here, biosurfactant rhamnolipid (RL) is chemically modified to develop a more efficient surfactant, rhamnolipid monoethanol amide (RL-MEA), which is characterized by decreased adsorption loss and increased oil-washing efficiency for enhanced oil recovery at a laboratory scale. Synthesis and characterization of the rhamnolipid monoethanol amide are carried out using high-performance liquid chromatography (HPLC), HPLC/MS, 1H nuclear magnetic resonance (NMR), and Fourier transform infrared (FTIR) spectroscopy. The aggregation behavior is disclosed by surface tension, dynamic light scattering, and fluorescence spectra with pyrene as the probe. The applied performances of RL-MEA in the simulated enhanced oil recovery are researched, including the efficiency of oil washing, wettability to crude oil, and adsorption isotherms on silicates. Compared with the critical micelle concentration (CMC) of rhamnolipid of 14.23 × 10-5 M in pure water and 9.02 × 10-5 M in 0.2 M NaCl solution, the modified RL-MEA shows a significantly lower CMC of 7.15 × 10-5 M in pure water and 5.34 × 10-5 M in 0.2 M NaCl solution. More importantly, the modified RL-MEA reduces adsorption loss by 20% and enhanced oil-washing efficiency at higher temperatures and salt concentrations compared with the parent RLs. These findings would provide valuable information for developing efficient surfactant flooding agents for harsh reservoir geological conditions.

Rhamnolipid Biosurfactants for Oil Recovery: Salt Effects on the Structural Properties Investigated by Mesoscale Simulations.

Rhamnolipids (RLs) are biosurfactants produced by Pseudomonas. The biodegradability and the variety of their functionality make them suitable for environmental remediation and oil recovery. We use dissipative particle dynamics simulations to investigate the aggregation behaviors of ionic RL congeners with nonane in various operating conditions. Under zero-salinity conditions, all RL congeners studied here form small ellipsoidal clusters with detectable free surfactants. When salt ions are present, the electrostatic repulsion between the ionized heads is overcome, resulting in the formation of larger aggregates of unique structures. RLs with C10-alkyl tails tend to form elongated wormlike micelles, while RLs with C16-alkyl tails tend to form clusters in spherical symmetry, including vesicles. Di-rhamnolipids (dRLs) require stronger solvation than monorhamnolipids (mRLs) to form clusters, and the resulting size of micelles is decreased. The morphology of the mixed dRL/mRL/oil systems is controlled based on the type of the congeners and the oil contents. In addition, the divalent calcium ions are found to be influential to the structure of the micelles through different mechanisms. For 5 wt % salinity, the ionic RLs can form oil-swollen micelles up to a 1:1 surfactant-to-oil ratio, suggesting that ionic RLs are superb to act as cleaning agents for petroleum hydrocarbons in the marine area. These key findings may guide the design for RL-based washing techniques in enhanced oil recovery.

Local delivery of usnic acid loaded Rhamnolipid vesicles by gelatin / tragacanth gum / montmorillonite/ vanillin cryogel scaffold for expression of osteogenic biomarkers and antimicrobial activity

Treatment of osteomyelitis as a destructive disease with strong, systemic, and high-dose antibiotics always faces tough challenges and severe side effects. The present study aimed to design dual-purpose cryogel nanocomposite scaffolds with antibacterial and bone regenerative properties. The gelatin/tragacanth gum/montmorillonite/vanillin cryogel scaffold (GTM/V) was prepared and enriched with Usnic acid (UA) encapsulated in Rhamnolipid (RH) biosurfactant nanoparticles as a dispersing agent with antioxidant and antimicrobial properties. The physicochemical properties of drug loaded nanoparticles, mechanical, rheological, and biological properties of the cryogel were evaluated. Biological evaluations, including cell proliferation (by MTT assay), cell attachment (by SEM), osteogenic activity (by Alizarin Red staining and alkaline phosphatase assay), and osteogenic gene expression (by quantitative real-time polymerase chain reaction) were studied using MG-63 cells. The antibiofilm effect of the cryogel was studied on Staphylococcus aureus. The results showed the mechanical profile of UA/RH loaded GTM/V scaffold was 3.08 ± 0.20 MPa compressive strength at 70% strain and the elastic modulus was 19.69 ± 1.57 MPa. Biocompatible UA/RH containing cryogel scaffolds showed osteogenic effect through increasing the ALP activity, and the Alizarin-red staining on days 7 and 14 compared to the control group (P < 0.05). The relative increase in expression of bone repair markers, including COL1A1, BGLAP, and SPP1F were shown by RT-PCR. The scaffold had strong antibacterial effects on S. aureus, with an inhibition zone of 1.40 ± 0.21 cm and antibiofilm activity of 43.76 ± 1.65%. In conclusion, our findings support the application of UA/RH loaded GTM/V scaffold for the treatment of osteomyelitis and bone regeneration.

Using plasma-mediated covalent functionalization of rhamnolipids on polydimethylsiloxane towards the antimicrobial improvement of catheter surfaces

Controlling bacterial biofilm formation on silicone-based bloodstream catheters is of great concern to prevent related-infections. In this study, rhamnolipids (RLs), glycolipid biosurfactants, specifically a RLs mixture and the purified di-RL (RhaRhaC10:0C10:0) were covalently bonded to silicone with the intention of reaching long-lasting antibiofilm surfaces.RLs mixture and di-RL were identified by an UHPLC-MS method that also allowed the confirmation of compound isolation by automated flash chromatography. Silicone surfaces underwent air-plasma treatment, inducing reactive oxygen radicals able to promote the RLs grafting that was confirmed by contact angle, FTIR-ATR and AFM measurements. The antibiofilm activity towards different Gram positive strains was evaluated by colony forming units (CFU) count and confocal laser microscopy. In addition, protein adsorption and biocompatibility were also investigated.RLs were successfully grafted onto silicone and RLs mixture and RhaRhaC10C10:0 functionalized specimens reduced the biofilm formation over 2.3 log units against methicillin sensitive Staphylococcus aureus. Additionally, a decrease of 1 log unit was observed against methicillin resistant S. aureus and S. epidermidis. Functionalized samples showed cytocompatibility towards human dermal fibroblasts, hemocompatibility and no vascular irritation potential. The results mentioned above revealed a synergy between the antimicrobial and the anti-adhesive properties of RLs, making these compounds good candidates for the improvement of the medical devices antibiofilm properties.

Identification of putative producers of rhamnolipids/glycolipids and their transporters using genome mining

• A total of 71 new putative rhamnolipid (RL)-producing species were discovered with genome mining. • Industrially interesting new RL producers included non-pathogenic and anaerobic species. • The identified RhlABC orthologs categorized into 11 distinct phylogenetic clades. • Most RL producers harbored a conserved putative RL transport mechanism, RhlDEF. • Transport genes (rhlDEF) co-localized primarily with biosynthetic gene clusters of RL. Rhamnolipids (RLs) are microbial glycolipids (GLs) with interesting structure-dependent bioactivities and physicochemical properties making them suitable for diverse medical and industrial applications. The discovery of RLs with more interesting bioactivities and properties has relied on laborious screening of new RL producers isolated from the environment, and has resulted in the redundant identification of already known RL producers and structures. Here, we present a genome mining approach that enabled the identification of 80 RL-producing species (including the two reference species), 71 of which were previously unreported. Distance trees of two of their RL biosynthetic enzymes, RhlAB, allowed for the identification of 11 distinct clades. Preliminary experimental validation with thin layer chromatography on one non-pathogenic RL/GL producer, Nevskia soli , confirmed its putative production of RLs. Additionally, this study led to the discovery of the putative RL transport mechanism involving three transmembrane proteins whose coding genes are highly conserved and clustered with one of the RL biosynthetic gene clusters in most RL/GL producers identified in this study.

Enhancement of co-conversion of endogenous carbon and nitrogen of dairy wastewater in mesophilic hydrolysis-acidification coupled microalgae culture system by rhamnolipid

In order to evaluate the efficiency of co-conversion of endogenous carbon and nitrogen of dairy wastewater in mesophilic hydrolysis-acidification coupled microalgae culture system by rhamnolipid (RL), the effects of RL dosages on the coupled system were investigated. RL can significantly increase volatile fatty acids (VFAs) production in hydrolysis-acidification liquor (HAL) (maximum VFAs production reached 11,484.38 mg COD/L in 2.5 g/L RL test at 144 h). A macrogenome sequencing revealed that the proportion of acidobacteria increased with the increasing dosage of RL. Furthermore, the effect of RL enhanced HAL on microalgae (Chlorella pyrenoidosa) growth, nutrient removal of the microalgae culture system were investigated. The harvested biomass concentration, the removal rates of ammonia nitrogen (NH4+-N), chemical oxygen demand (COD) were positively correlated with RL dosage. The maximum removal rates of NH4+-N (98.10%), COD (76.17%) which higher than those of the control (77.27% and 59.44%, respectively) were recorded. Furthermore, protein, carbohydrates, and higher heating value (HHV) outputs were also enhanced by RL treatment. More in-depth analysis found that, compared with the control test, RL can exactly enhance the endogenous C, N assimilation, the maximum promotion rates of C and N assimilation yield can reach 156% and 145%.

Pseudomonas aeruginosa rhamnolipid micelles deliver toxic metabolites and antibiotics into Staphylococcus aureus

SummaryEfficient delivery of toxic compounds to bacterial competitors is essential during interspecies microbial warfare. Rhamnolipids (RLPs) are glycolipids produced by Pseudomonas and Burkholderia species involved in solubilization and uptake of environmental aliphatic hydrocarbons and perform as biosurfactants for swarming motility. Here, we show that RLPs produced by Pseudomonas aeruginosa associate to form micelles. Using high-resolution microscopy, we found that RLP micelles serve as carriers for self-produced toxic compounds, which they deliver to Staphylococcus aureus cells, thereby enhancing and accelerating S. aureus killing. RLPs also potentiated the activity of lincosamide antibiotics, suggesting that RLP micelles may transport not only self-produced but also heterologous compounds to target competing bacterial species

Antimicrobial Applications of Rhamnolipid Biosurfactant Produced from Achromobacter sp. (PS1) Isolate Using Lignocellulosic Hydrolysate

Heterogeneous mixture of partially purified rhamnolipid (RL) produced from Achromobacter sp. (PS1) using lignocellulosic rice straw (RS) sugar hydrolysate medium revealed six different congeners- Rha- C10-C10, Rha-C8-C10/Rha-C10-C8, Rha- C12-C10 / Rha- C10-C12, referring mono-rhamnolipids amounting to total 68.23 % and Rha-Rha-C10-C10, Rha-Rha-C8-C10/Rha-Rha-C10-C8, Rha-Rha-C10-C12/Rha-Rha-C12-C10, referring di-rhamnolipids amounting to 31.73 %, with Mono to Di- RL in the ratio of 2.1:1. This mixture's antimicrobial action containing more mono-rhamnolipids analyzed using broth macro-dilution method exhibited a broad-spectrum antibacterial activity showing ≥ 90 % growth inhibition of both Gram-positive and Gram-negative pathogenic bacteria at MIC ranging from 1.25 mg/mL to 10 mg/mL of total rhamnolipids. This might be due to the more hydrophobic character of mono-rhamnolipids containing a single rhamnosyl group and showing high surface activities. On the other hand, the non-antifungal activity may be attributed to the lower percentage of di-rhamnolipids in the partially purified mixture.

Antiviral Activity of the Rhamnolipids Mixture from the Antarctic Bacterium Pseudomonas gessardii M15 against Herpes Simplex Viruses and Coronaviruses.

Emerging and re-emerging viruses represent a serious threat to human health at a global level. In particular, enveloped viruses are one of the main causes of viral outbreaks, as recently demonstrated by SARS-CoV-2. An effective strategy to counteract these viruses could be to target the envelope by using surface-active compounds. Rhamnolipids (RLs) are microbial biosurfactants displaying a wide range of bioactivities, such as antibacterial, antifungal and antibiofilm, among others. Being of microbial origin, they are environmentally-friendly, biodegradable, and less toxic than synthetic surfactants. In this work, we explored the antiviral activity of the rhamnolipids mixture (M15RL) produced by the Antarctic bacteria Pseudomonas gessardii M15 against viruses belonging to Coronaviridae and Herpesviridae families. In addition, we investigated the rhamnolipids’ mode of action and the possibility of inactivating viruses on treated surfaces. Our results show complete inactivation of HSV-1 and HSV-2 by M15RLs at 6 µg/mL, and of HCoV-229E and SARS-CoV-2 at 25 and 50 µg/mL, respectively. Concerning activity against HCoV-OC43, 80% inhibition of cytopathic effect was recorded, while no activity against naked Poliovirus Type 1 (PV-1) was detectable, suggesting that the antiviral action is mainly directed towards the envelope. In conclusion, we report a significant activity of M15RL against enveloped viruses and demonstrated for the first time the antiviral effect of rhamnolipids against SARS-CoV-2.

Milk phospholipids-based nanostructures functionalized with rhamnolipids and bacteriocin: Intrinsic and synergistic antimicrobial activity for cheese preservation

Milk fat globule membrane (MFGM) phospholipids-based nanostructures were developed and their functionalization with rhamnolipids (RLs) was carried out to enhance the preservation of cheese against resistant foodborne pathogens i.e., Listeria monocytogenes and Escherichia coli. Void (without nisin) and nisin-loaded RLs functionalized MFGM nanostructures (RLs-MFGM-NS) were fabricated by ultrasonication-assisted self-assembly method. Cubic morphology of void and loaded RLs-MFGM-NS and pristine MFGM nanostructures (MFGM-NS) was observed under scanning electron microscopy (SEM), which indicated uniform size ranging from 43 nm (void RLs-MFGM-NS) to 194 nm (loaded RLs-MFGM-NS). FTIR analyses confirmed the electrostatic interaction, predominantly H-bonding and linkage of carboxyl ester group of MFGM with C–H group in RLs after functionalization of NS. Furthermore, quantitative antimicrobial assay on cheese slices confirmed the broad-spectrum potential of intrinsically active nanostructures (due to RLs) having synergistic activity with nisin against L. monocytogenes and E. coli. Hence, nisin-loaded RLs-MFGM-NS can be applied as promising bioactive additives for sustained preservation of cheese.

Glucose metabolism in Pseudomonas aeruginosa is cyclic when producing Polyhydroxyalkanoates and Rhamnolipids

Pseudomonas aeruginosa is an important chassis for production of polyhydroxyalkanoates (PHA) and rhamnolipids (RHL). Advances in the understanding of the biosynthesis metabolism of these biocompounds are crucial for increasing yield. 13C-Metabolic Flux Ratio Analysis (13C-MFA) is a technique to estimate in vivo metabolic fluxes ratios. PHA and RHL are essentially non-growth associated products of biotechnological interest and both contain hydroxyalkanoates (HAs), whose labeling patterns could be accessed by GC-MS. In this study, to reveal the relative contributions of the Entner-Doudoroff (ED) pathway and the non-oxidative Pentose Phosphate (PP) pathway to PHA and RHL production, 13C-MFA was performed in Pseudomonas aeruginosa LFM634 when supplied with labeled glucose. This bacterial strain lacks both functional EMP and the oxidative PP branch. Labeling patterns in HAs were measured. Experiments with [U-13C] glucose indicated a low flux though PP pathway. An optimal design of labeling experiment showed that [6-13C] glucose would be the best substrate to enable an estimation of the ED flux with high accuracy. Results of experiments performed with this isotope indicated that about two-thirds of glyceraldehyde 3-phosphate is recycled through a cyclic ED architecture, suggesting that P. aeruginosa utilizes that cycle to regulate the NADPH/Acetyl-CoA ratio for PHA and RHL biosynthesis.

Evaluation of the Antimicrobial Efficacy of N-Acetyl-l-Cysteine, Rhamnolipids, and Usnic Acid-Novel Approaches to Fight Food-Borne Pathogens.

In the food industry, the increasing antimicrobial resistance of food-borne pathogens to conventional sanitizers poses the risk of food contamination and a decrease in product quality and safety. Therefore, we explored alternative antimicrobials N-Acetyl-l-cysteine (NAC), rhamnolipids (RLs), and usnic acid (UA) as a novel approach to prevent biofilm formation and reduce existing biofilms formed by important food-borne pathogens (three strains of Salmonella enterica and two strains of Escherichia coli, Listeria monocytogenes, Staphylococcus aureus). Their effectiveness was evaluated by determining minimum inhibitory concentrations needed for inhibition of bacterial growth, biofilm formation, metabolic activity, and biofilm reduction. Transmission electron microscopy and confocal scanning laser microscopy followed by image analysis were used to visualize and quantify the impact of tested substances on both planktonic and biofilm-associated cells. The in vitro cytotoxicity of the substances was determined as a half-maximal inhibitory concentration in five different cell lines. The results indicate relatively low cytotoxic effects of NAC in comparison to RLs and UA. In addition, NAC inhibited bacterial growth for all strains, while RLs showed overall lower inhibition and UA inhibited only the growth of Gram-positive bacteria. Even though tested substances did not remove the biofilms, NAC represents a promising tool in biofilm prevention.

Effect of ethanol on wetting and adhesion properties of rhamnolipid

The wetting and adhesion properties of the surfactants and biosurfactants play a very important role in the processes occurring in practice. These properties can be changed by the addition to the aqueous solution of the surfactants and biosurfactants different kinds of the substances influencing the structure of water and the hydration of the head and tail of the surfactants molecules. Therefore, we studied effect of the ethanol on the wetting and adhesion properties of rhamnolipid by the measurements of the contact angle of the aqueous solution of rhamnolipid in the presence of ethanol on the polytetrafluoroethylene (PTFE), polymethyl methacrylate (PMMA) and quartz surface. The obtained results of the contact angle were analyzed in the light of the surface tension components of the solids, water, ethanol as well as the head and tail of the rhamnolipid surface tension. Based on these analyses the adsorption of the ethanol and rhamnolipid at the solid-solution, the dependence between adhesion and surface tension, the dependence between the adhesion work of the solution to the solid surfaces and solution surface tension as well as the critical surface tension of solid wetting were considered. The critical surface tension was compared to the surface tension of the solid. From above mentioned consideration, it appeared, among other things, that on the basis of the PTFE-aqueous solution of ethanol (ET) and PTFE-aqueous solution of rhamnolipid (RL) interface tension, it is possible to predict the contribution of ET and RL adsorption at the PTFE-solution of RL and ET mixture. It also appeared that it is possible to determine the Gibbs standard free energy of ET and RL adsorption at the solid-air and solid-solution interfaces, by using the Gibbs excess concentration of ET and RL at these interfaces determined on the basis of contact angle in Langmuir equation modified by de Boer. • Replacement of RL by ET molecules in interfaces layer can cause contact angle increase. • Above ET CAC contact angle depends practically on its adsorption at interfaces but not RL. • Parameter changes linearly as a function of logarithm from sum of and but not • Solution of ET and RL mixture spreads complete over PMMA and quartz at C of ET higher than its CAC. • For PMMA and quartz there is linear dependence between adhesion work and

Surfactant rhamnolipid promotes anaerobic codigestion of excess sludge and plant waste.

In order to solve the bottleneck of low methane production in anaerobic codigestion of excess sludge (ES) and plant waste (PW), a new strategy of enhancing hydrolysis and acidification by rhamnolipid (RL) was proposed under thermophilic condition. The results showed that the optimal dosage of RL was 50 g/kg total suspended solids, and the maximum yield of methane was 198.5 mL/g volatile suspended solids (VSS), which was 2.3 times of that in the control. RL promoted the dissolution of organic matter in the codigestion process of ES and PW, and the higher the dosage of RL, the higher the concentration of soluble chemical oxygen demand (SCOD) in the fermentation broth. When RL was 100 g/kg, the maximum content of SCOD in fermentation broth was 2,451 mg/L, and the contents of soluble protein and polysaccharide were 593 mg/L and 419 mg/L on 10 d, respectively, which were significantly higher than other groups. In addition, the yield of VFA in RL group was also significantly increased, and acetate and propionate were the main components of VFAs. This research work provides data support for the resource utilization of ES and PW, and expands the application field of RL.

Revealing the mechanisms of rhamnolipid enhanced hydrogen production from dark fermentation of waste activated sludge

Rhamnolipid (RL), as an environmentally compatible biosurfactant, has been used to enhance waste activated sludge (WAS) fermentation. However, the effect of RL on hydrogen accumulation in anaerobic fermentation remains unclear. Therefore, this work targets to investigate the mechanism of RL-based dark fermentation system on hydrogen production of WAS. It was found that the maximum yield of hydrogen increased from 1.76 ± 0.26 to 11.01 ± 0.30 mL/g VSS (volatile suspended solids), when RL concentration increased from 0 to 0.10 g/g TSS (total suspended solids). Further enhancement of RL level to 0.12 g/g TSS slightly reduced the production to 10.80 ± 0.28 mL/g VSS. Experimental findings revealed that although RL could be degraded to generate hydrogen, it did not play a major role in enhancing hydrogen accumulation. Mechanism analysis suggested that RL decreased the surface tension between sludge liquid and hydrophobic compounds, thus accelerating the solubilization of WAS, improving the proportion of biodegradable substances which could be used for subsequent hydrogen production. Regardless of the fact that adding RL suppressed all the fermentation processes, the inhibition effect of processes associated with hydrogen consumption was much severer than that of hydrogen production. Further investigations of microbial community revealed that RL enriched the relative abundance of hydrogen producers e.g., Romboutsia but reduced that of hydrogen consumers like Desulfobulbus and Caldisericum.

Bacterial rhamnolipids and their 3-hydroxyalkanoate precursors activate Arabidopsis innate immunity through two independent mechanisms

Plant innate immunity is activated upon perception of invasion pattern molecules by plant cell-surface immune receptors. Several bacteria of the genera Pseudomonas and Burkholderia produce rhamnolipids (RLs) from l-rhamnose and (R)-3-hydroxyalkanoate precursors (HAAs). RL and HAA secretion is required to modulate bacterial surface motility, biofilm development, and thus successful colonization of hosts. Here, we show that the lipidic secretome from the opportunistic pathogen Pseudomonas aeruginosa, mainly comprising RLs and HAAs, stimulates Arabidopsis immunity. We demonstrate that HAAs are sensed by the bulb-type lectin receptor kinase LIPOOLIGOSACCHARIDE-SPECIFIC REDUCED ELICITATION/S-DOMAIN-1-29 (LORE/SD1-29), which also mediates medium-chain 3-hydroxy fatty acid (mc-3-OH-FA) perception, in the plant Arabidopsis thaliana HAA sensing induces canonical immune signaling and local resistance to plant pathogenic Pseudomonas infection. By contrast, RLs trigger an atypical immune response and resistance to Pseudomonas infection independent of LORE. Thus, the glycosyl moieties of RLs, although abolishing sensing by LORE, do not impair their ability to trigger plant defense. Moreover, our results show that the immune response triggered by RLs is affected by the sphingolipid composition of the plasma membrane. In conclusion, RLs and their precursors released by bacteria can both be perceived by plants but through distinct mechanisms.