Multicomponent biosurfactants — A “Green Toolbox” extension

Production and Characterization of Rhamnolipids Produced by Pseudomonas aeruginosa DBM 3774: Response Surface Methodology Approach

Rhamnolipids are extensively studied biosurfactants due to their potential in many industrial applications, eco-friendly production and properties. However, their availability for broader application is severely limited by their production costs, therefore the optimization of efficacy of their cultivation gains significance as well as the information regarding the physio-chemical properties of rhamnolipids resulting from various cultivation strategies. In this work, the bioprocess design focused on optimization of the rhamnolipid yield of Pseudomonas aeruginosa DBM 3774 utilizing the response surface methodology (RSM). Six carbon sources were investigated for their effect on the rhamnolipid production. The RSM prediction improved the total rhamnolipid yield from 2.2 to 13.5 g/L and the rhamnolipid productivity from 11.6 to 45.3 mg/L/h. A significant effect of the carbon source type, concentration and the C/N ratio on the composition of the rhamnolipid congeners has been demonstrated for cultivation of P. aeruginosa DBM 3774 in batch cultivation. Especially, changes in presence of saturated fatty acid in the rhamnolipid congeners, ranging from 18.8% of unsaturated fatty acids (carbon source glycerol; 40 g/L) to 0% (sodium citrate 20 g/L) were observed. This demonstrates possibilities of model based systems as basis in cultivation of industrially important compounds like biosurfactants rhamnolipids and the importance of detailed study of interconnection between cultivation conditions and rhamnolipid mixture composition and properties.

Unraveling the Differential Aggregation of Anionic and Nonionic Monorhamnolipids at Air-Water and Oil-Water Interfaces: A Classical Molecular Dynamics Simulation Study

The molecular structure of a surfactant molecule is known to have a great effect on the interfacial properties. We employ molecular dynamics simulations for a detailed atomistic study of monolayers of the nonionic and anionic form of the most common congener of monorhamnolipids, α-rhamnopyranosyl-β-hydroxydecanoyl-β-hydroxydecanoate (( R, R)-Rha-C10-C10), at the air-water and oil-water interfaces. An atomistic-level understanding of monolayer aggregation is necessary to explain a recent experimental observation indicating that nonionic and anionic Rha-C10-C10 show surprisingly different surface area per molecule at the critical micelle concentration. Surface-pressure analysis, interface formation energy calculations, and mass density profiles of the monolayers at the air-water interface show similar properties between nonionic and anionic Rha-C10-C10 aggregation. It is found that there is a significant difference in the headgroup conformations of Rha-C10-C10 in the nonionic and anionic monolayers. Hydrogen bonding interactions between the Rha-C10-C10 molecules in the monolayers is also significantly different between nonionic and anionic forms. Representative snapshots of the simulated system at different surface concentrations show the segregation of molecular aggregates from the interface into the bulk water in the anionic Rha-C10-C10 monolayer at higher concentrations, whereas in the nonionic Rha-C10-C10 monolayer, the molecules are still located at the interface. The present work provides insight into the different aggregation properties of nonionic and anionic Rha-C10-C10 at the air-water interface. Further analyses were carried out to understand the aggregation behavior of nonionic and anionic Rha-C10-C10 at the oil-water interface. It is observed that the presence of oil molecules does not significantly influence the aggregation properties of Rha-C10-C10 as compared to those of the air-water interface.

Evolution of Aggregate Structure in Solutions of Anionic Monorhamnolipids: Experimental and Computational Results

The evolution of solution aggregates of the anionic form of the native monorhamnolipid (mRL) mixture produced by Pseudomonas aeruginosa ATCC 9027 is explored at pH 8.0 using both experimental and computational approaches. Experiments utilizing surface tension measurements, dynamic light scattering, and both steady-state and time-resolved fluorescence spectroscopy reveal solution aggregation properties. All-atom molecular dynamics simulations on self-assemblies of the most abundant monorhamnolipid molecule, l-rhamnosyl-β-hydroxydecanoyl-β-hydroxydecanoate (Rha-C10-C10), in its anionic state explore the formation of aggregates and the role of hydrogen bonding, substantiating the experimental results. At pH 8.0, at concentrations above the critical aggregation concentration of 201 μM but below ∼7.5 mM, small premicelles exist in solution; above ∼7.5 mM, micelles with hydrodynamic radii of ∼2.5 nm dominate, although two discrete populations of larger lamellar aggregates (hydrodynamic radii of ∼10 and 90 nm) are also present in solution in much smaller number densities. The critical aggregation number for the micelles is determined to be ∼26 monomers/micelle using fluorescence quenching measurements, with micelles gradually increasing in size with monorhamnolipid concentration. Molecular dynamics simulations on systems with between 10 and 100 molecules of Rha-C10-C10 indicate the presence of stable premicelles of seven monomers with the most prevalent micelle being ∼25 monomers and relatively spherical. A range of slightly larger micelles of comparable stability can also exist that become increasing elliptical with increasing monomer number. Intermolecular hydrogen bonding is shown to play a significant role in stabilization of these aggregates. In total, the computational results are in excellent agreement with the experimental results.

Structural Properties of Nonionic Monorhamnolipid Aggregates in Water Studied by Classical Molecular Dynamics Simulations

Molecular dynamics simulations were carried out to investigate the structure and stabilizing factors of aggregates of the nonionic form of the most common congener of monorhamnolipids, α-rhamnopyranosyl-β-hydroxydecanoyl-β-hydroxydecanoate (Rha-C10-C10), in water. Aggregates of size ranging from 5 to 810 monomers were observed in the simulation forming spherical and ellipsoidal structures, a torus-like structure, and a unilamellar vesicle. The effects of the hydrophobic chain conformation and alignment in the aggregate, role of monomer···monomer and monomer···water H-bonds, and conformations of monomers in the aggregate were studied in detail. The unilamellar vesicle is highly stable due to the presence of isolated water molecules inside the core adding to the binding energy. Dissociation of a monomer from a larger micellar aggregate is relatively easy compared to that from smaller aggregates as seen from potential of mean force calculations. This analysis also shows that monomers are held more strongly in aggregates of Rha-C10-C10 than the widely used surfactant sodium dodecyl sulfate. Comparisons between the aggregation behavior of nonionic and anionic forms of Rha-C10-C10 are presented.

Recent Advances in the Utilization of Natural Emulsifiers to Form and Stabilize Emulsions

Consumer concern about human and environmental health is encouraging food manufacturers to use more natural and sustainable food ingredients. In particular, there is interest in replacing synthetic ingredients with natural ones, and in replacing animal-based ingredients with plant-based ones. This article provides a review of the various types of natural emulsifiers with potential application in the food industry, including phospholipids, biosurfactants, proteins, polysaccharides, and natural colloidal particles. Increased utilization of natural emulsifiers in food products may lead to a healthier and more sustainable food supply. However, more research is needed to identify, isolate, and characterize new sources of commercially viable natural emulsifiers suitable for food use.

Formation and stabilization of nanoemulsions using biosurfactants: Rhamnolipids

Nanoemulsions are used in the food, cosmetics, personal care and pharmaceutical industries to provide desirable optical, textural, stability, and delivery characteristics. In many industrial applications, it is desirable to formulate nanoemulsions using natural ingredients so as to develop label-friendly products. Rhamnolipids are biosurfactants isolated from certain microorganisms using fermentation processes. They are glycolipids that have a polar head consisting of rhamnose units and a non-polar tail consisting of a hydrocarbon chain. In this study, the interfacial characteristics of this natural surfactant at medium chain triglyceride (MCT) oil-water interfaces were characterized, and its ability to form nanoemulsions was compared to that of another natural surfactant (quillaja saponins). The influence of rhamnolipid concentration, homogenization pressure, and oil type on the mean droplet diameter of emulsions produced by microfluidization was determined. Rhamnolipids were highly effective at forming small droplets (d32<0.15μm) at low surfactant-to-oil ratios (SOR<1:10) for MCT oil. Rhamnolipids could also be used to form small droplets using long chain triglyceride oils, such as corn and fish oil. Rhamnolipid-coated droplets were stable to aggregation over a range of pH values (5-9), salt concentrations (<100mM NaCl) and temperatures (20-90°C). However, droplet aggregation was observed at highly acidic (pH 2-4) and high ionic strength (200-500mM NaCl) conditions. These effects were attributed to a reduction in electrostatic repulsion at low pH and high salt levels. Rhamnolipid-coated droplets had a high negative charge at neutral pH that decreased in magnitude with decreasing pH. These results indicate that rhamnolipids are effective natural surfactants that may be able to replace synthetic surfactants in certain commercial applications.

Biosurfactant Produced by Salmonella Enteritidis SE86 Can Increase Adherence and Resistance to Sanitizers on Lettuce Leaves (Lactuca sativa L., cichoraceae)

Salmonella Enteritidis SE86 is an important foodborne pathogen in Southern Brazil and it is able to produce a biosurfactant. However, the importance of this compound for the microorganism is still unknown. This study aimed to investigate the influence of the biosurfactant produced by S. Enteritidis SE86 on adherence to slices of lettuce leaves and on resistance to sanitizers. First, lettuce leaves were inoculated with S. Enteritidis SE86 in order to determine the amount of biosurfactant produced. Subsequently, lettuce leaves were inoculated with S. Enteritidis SE86 with and without the biosurfactant, and the adherence and bacterial resistance to different sanitization methods were evaluated. S. Enteritidis SE86 produced biosurfactant after 16 h (emulsification index of 11 to 52.15 percent, P < 0.05) and showed greater adherence capability and resistance to sanitization methods when the compound was present. The scanning electron microscopy demonstrated that S. Enteritidis was able to adhere, form lumps, and invade the lettuce leaves' stomata in the presence of the biosurfactant. Results indicated that the biosurfactant produced by S. Enteritidis SE86 contributed to adherence and increased resistance to sanitizers when the microorganism was present on lettuce leaves.