Are microbial biosurfactants actually only surfactants?

The term biosurfactants refers to a complex mixture of metabolites with surface-active properties produced by specific microorganisms. However, nowadays trends moves towards isolation, screening and purifying single biocompatible, biodegradable biosurfactants with high commercialization potential. Current legislation limiting petrochemicals combined with environmentally concerned consumers did not only stimulate research and development but it also promoted large-scale production of this class of molecules. However, recent data recorded on single congeners question the actual pertinence of using the word ''biosurfactant'' associated to these molecules. By evaluating the accepted characteristics of surfactants and comparing them to the actual self-assembly and bulk properties in water of molecules traditionally called ''biosurfactants'', this opinion paper aims at showing that the term ''biosurfactant'' can be somewhat reductive when applied to specific individual compounds produced by fermentation. The use of a more generic term, like bioamphiphile could probably be more pertinent and appropriate for consideration in the future.Comment: Current Opinion in Colloid & Interface Science, In pres

Advanced materials from microbial fermentation : the case of glycolipids and nanocellulose

Green chemistry is a recent discipline ruled by twelve founding principles, which include, among others, atom economy, the prevention of pollution via environmentally friendly chemical synthesis methods, such as, for example, the choice of an aqueous medium over organic solvents, but also the development of chemicals and materials derived from plant biomass. In this context, microbial synthesis is a tool to supplant, in some notable cases, syntheses by a standard organic chemistry approach. More recently, attention has begun to be given to the microbial synthesis of polymeric sugars, such as dextran or cellulose, or lipids, such as amphiphilic glycolipids. Although the microbial production of glycosylated compounds can be traced back by several decades, the development of green chemistry is encouraging teams of multidisciplinary researchers to focus on production, diversification, and applications of this class of compounds, thus going beyond the community of researchers in microbiology, historically interested in the development of fermentation products from microorganisms. This article develops the above-mentioned theme by focusing on nanocellulose, representing an important glycosylated polymer, and on biosurfactants, in regards of the glycosylated lipids. The choice of these two systems is justified by the strong development of nanocellulose-based materials but also by the need to replace in part the “conventional” surfactants, a significant source of CO2 emissions worldwide. The main classes of molecules, the classical methods of synthesis, their properties and some examples of notorious applications are presented

Measuring the bending rigidity of microbial glucolipid (biosurfactant) bioamphiphile self-assembled structures by neutron spin-echo (NSE): interdigitated vesicles, lamellae and fibers

Bending rigidity, k, is classically measured for lipid membranes to characterize their nanoscale mechanical properties as a function of composition. Widely employed as a comparative tool, it helps understanding the relationship between the lipid's molecular structure and the elastic properties of its corresponding bilayer. Widely measured for phospholipid membranes in the shape of giant unilamellar vesicles (GUVs), bending rigidity is determined here for three self-assembled structures formed by a new biobased glucolipid bioamphiphile, rather associated to the family of glycolipid biosurfactants than phospholipids. In its oleyl form, glucolipid G-C18:1 can assemble into vesicles or crystalline fibers, while in its stearyl form, glucolipid G-C18:0 can assemble into lamellar gels. Neutron spin-echo (NSE) is employed in the q-range between 0.3 nm-1 (21 nm) and 1.5 nm-1 (4.1 nm) with a spin-echo time in the range of up to 500 ns to characterize the bending rigidity of three different structures (Vesicle suspension, Lamellar gel, Fiber gel) solely composed of a single glucolipid. The low (k= 0.30 $\pm$ 0.04 kbT) values found for the Vesicle suspension and high values found for the Lamellar (k= 130 $\pm$ 40 kbT) and Fiber gels (k= 900 $\pm$ 500 kbT) are unusual when compared to most phospholipid membranes. By attempting to quantify for the first time the bending rigidity of self-assembled bioamphiphiles, this work not only contributes to the fundamental understanding of these new molecular systems, but it also opens new perspectives in their integration in the field of soft materials

Primary and Secondary Hydration Forces between Interdigitated Membranes Composed of Bolaform Microbial Glucolipids

To better understand lipid membranes in living organisms, the study of intermolecular forces using the osmotic pressure technique applied to model lipid membranes has constituted the ground knowledge in the field since four decades. However, the study of intermolecular forces in lipid systems other than phospholipids, like glycolipids, has gained a certain interest only recently. Even in this case, the work generally focus on the study of membrane glycolipids, but little is known on new forms of non-membrane functional compounds, like pH-responsive bolaform glycolipids. This works explores, through the osmotic stress method involving an adiabatic humidity chamber coupled to neutron diffraction, the short-range (< 2 nm) intermolecular forces of membranes entirely composed of interdigitated glucolipids. Experiments are performed at pH 6, when the glucolipid is partially negatively charged and for which we explore the effect of low (16 mM) and high (100 mM) ionic strength. We find that this system is characterized by primary and secondary hydration regimes, respectively insensitive and sensitive to ionic strength and with typical decay lengths of λ_H1= 0.37 ± 0.12 nm and λ_H2=1.97 ± 0.78 nm

Myelin figures from microbial glycolipid biosurfactant amphiphiles

Myelin figures (MFs) -- cylindrical lyotropic liquid crystalline structures consisting of concentric arrays of bilayers and aqueous media -- arise from the hydration of the bulk lamellar phase of many common amphiphiles. Prior efforts have concentrated on the formation, structure, and dynamics of myelin produced by phosphatidylcholine (PC)-based amphiphiles. Here, we study the myelinization of glycolipid microbial amphiphiles, commonly addressed as biosurfactants, produced through the process of fermentation. The hydration characteristics (and phase diagrams) of these biological amphiphiles are atypical (and thus their capacity to form myelin) because unlike typical amphiphiles, their molecular structure is characterized by two hydrophilic groups (sugar, carboxylic acid) on both ends with a hydrophobic moiety in the middle. We tested three different glycolipid molecules: C18:1 sophorolipids and single-glucose C18:1 and C18:0 glucolipids, all in their nonacetylated acidic form. Neither sophorolipids (too soluble) nor C18:0 glucolipids (too insoluble) displayed myelin growth at room temperature (RT, 25 C). The glucolipid C18:1 (G-C18:1), on the other hand, showed dense myelin growth at RT below pH 7.0. Examining their growth rates, we find that they display a linear L $\alpha$ t (L, myelin length; t, time) growth rate, suggesting ballistic growth, distinctly different from the L $\alpha$ t^(1/2) dependence, characterizing diffusive growth such as what occurs in more conventional phospholipids. These results offer some insight into lipidic mesophases arising from a previously unexplored class of amphiphiles with potential applications in the field of drug delivery

Sophorolipids-functionalized iron oxide nanoparticles

International audienceFunctional iron oxide nanoparticles (NP) have been synthesized in a one and a two-step method using a natural functional glycolipid belonging to the family of sophorolipids (SL). These compounds, whose open acidic form is highly suitable for nanoparticle stabilization, are readily obtained by a fermentation process of the yeast Candida bombicola (polymorph Starmerella bombicola) in large amounts. The final carbohydrate coated iron oxide nanoparticles represent interesting potentially biocompatible materials for biomedical applications. According to the synthesis strategy, magnetic properties can eventually be tuned, thus putting in evidence the direct effect of the glycolipid on the final material's structure (maghemite and ferrihydrite have been obtained here). A combination of FT-IR, Dynamic Light Scattering (DLS) and UV-Vis experiments shows that SL complex the nanoparticle surface via their accessible COOH group thus forming stable colloids, whose hydrodynamic diameter mostly varies between 10 nm and 30 nm, both in water and in KCl-containing (0.01 M and 2 M) solutions. The materials can stand multiple filtration steps (up to 10) at different extents, where the largest recorded average aggregate size is 100 nm. In general, materials synthesized at T = 80 °C display better stability and smaller size distribution than those obtained at room temperature

Shear recovery and temperature stability of Ca2+ and Ag+ glycolipid fibrillar metallogels with unusual β\beta-sheet-like domains

Low-molecular weight gelators (LMWG) are small molecules (Mw < ~1 kDa), which form self-assembled fibrillar networks (SAFiN) hydrogels in water. The great majority of SAFiN gels is described by an entangled network of self-assembled fibers, in analogy to a polymer in a good solvent. Here, fibrillation of a biobased glycolipid bolaamphiphile is triggered by Ca2+ or Ag+ ions, added to its diluted micellar phase. The resulting SAFiN, which forms hydrogel above 0.5 wt%, has a ``nano-fishnet'' structure, characterized by a fibrous network of both entangled fibers and $\beta$-sheets-like rafts, generally observed for silk fibroin, actin hydrogels or mineral imogolite nanotubes, but generally not known for SAFiN. This work focuses on the strength of the SAFIN gels, their fast recovery after applying a mechanical stimulus (strain) and their unusual resistance to temperature, studied by coupling rheology to small angle X-ray scattering (rheo-SAXS) using synchrotron radiation. The Ca2+-based hydrogel keeps its properties up to 55{\textdegree}C, while the Ag+-based gel shows a constant elastic modulus up to 70{\textdegree}C, without appearance of any gel-to-sol transition temperature. Furthermore, the glycolipid is obtained by fermentation from natural resources (glucose, rapeseed oil), thus showing that naturally-engineered compounds can have unprecedented properties, when compared to the wide range of chemically derived amphiphiles