Patrick Bouriat

Cytoplasmic wax ester accumulation during biofilm-driven substrate assimilation at the alkane--water interface by Marinobacter hydrocarbonoclasticus SP17.

During growth on n-alkanes, the marine bacterium Marinobacter hydrocarbonoclasticus SP17 formed a biofilm at the alkane-water interface. We showed that hexadecane degradation was correlated with biofilm development and that alkane uptake is localized in the biofilm but not in the bulk medium. Biofilms were observed in cultures on metabolizable n-alkanes (C8-C28) and n-alcohols (C12 and C16), but were formed neither on non-metabolizable alkanes (pristane, heptamethylnonane and n-C32) nor on inert substrata (glass, polystyrene and Permanox). This substratum specificity indicates that biofilm formation is determined by the presence of an interface between an insoluble substrate and the aqueous phase. Simultaneously with biofilm growth, planktonic cells were released from the biofilm. Detached cells were in a non-growing state, implying that the growing population was exclusively located within the biofilm. Planktonic and sessile cells exhibited differences in their ultrastructure and lipid content. Biofilm cells contained a large amount of wax esters (0.47mg/mg protein) in rounded or irregularly shaped cytoplasmic inclusions, whereas detached cells displayed rod-shaped inclusions and contained 5 times fewer wax esters (0.10mg/mg protein) than their sessile counterparts. This study points out the inter-relationship between biofilm formation, insoluble substrate uptake and lipid storage.

Behavior of Marinobacter hydrocarbonoclasticus SP17 Cells During Initiation of Biofilm Formation at the Alkane-Water Interface

Hexadecane assimilation by Marinobacter hydrocarbonoclasticus SP17 occurs through the formation of a biofilm at the alkane–water interface. In this study we focused on the interactions of cells with the alkane–water interface occurring during initiation of biofilm development. The behavior of cells at the interface was apprehended by investigating alterations of the mechanical properties of the interface during cell adsorption, using dynamic drop tensiometry measurements. It was found that after having reached the hexadecane–water interface, by a purely thermal diffusion process, cells released surface‐active compounds (SACs) resulting in the formation of an interfacial visco‐elastic film. Release of SACs was an active process requiring protein synthesis. This initial interaction occurred on metabolizable as well as non‐metabolizable alkanes, indicating that at this stage cells are not affected by the nature of the alkane forming the interface. In contrast, at a later stage, the nature of the interface turned out to exert control over the behavior of the cells. The availability of a metabolizable alkane at the interface influenced cell activity, as revealed by cell cluster formation and differences in the interfacial elasticity. Biotechnol. Bioeng. 2010; 105: 461–468.

A study of insoluble monolayers by deposition at a bubble interface

We propose an experimental approach, alternative to Langmuir trough, pendant or sessile drop experiments, to study insoluble monolayers at the air–water interface. The method is based on the direct deposition of an insoluble surfactant at the interface of an air bubble, measuring the surface tension according to the classical axisymmetric drop shape analysis (ADSA) technique. This bubble configuration, in contrast to the classical ones for studying Langmuir monolayers have several remarkable advantages like the easy control of the surrounding bulk composition (opening new potential research applications), the fast experimental time for a monolayer to be ready (<20 min), the small bulk volume (10 mL), and mostly the simple way to carry out dilatational rheology. The experiment consists of performing compression of an insoluble monolayer recording the Π–AB curve (Π is the interfacial pressure and AB the bubble area) and obtaining dilatational rheology over the compression range establishing the E–Π curve (with E is the elastic modulus). We showed that the experimental results can be satisfactorily fitted using the Volmers equation of state including the two-dimensional compressibility factor e, offering access to the excluded area per molecule ω0 and to the number N of molecules at the interface, without initially knowing the amount of deposited material. This proof of concept study was carried out on dioleoyl-sn-glycerophosphatidylcholine (DOPC), dipalmitoyl-sn-glycerophosphatidylcholine (DPPC), and cholesterol at 20 °C, systems chosen to show qualitative differences in their thermodynamic behavior upon monolayer compression. Likewise, dilatational rheology of these insoluble monolayers allows evidencing the compressibility of the DOPC monolayer in contrast to the DPPC monolayer, and finally, the compression domains where the interface loses the surfactant through a comparison of the dilatational elasticity with the Gibbs elasticity calculated from the compression curves. Finally, we propose an example of the new application offered by the possibility to exchange the fluid phase surrounding the bubble, herein to study mixed monolayers made with soluble/insoluble surfactants (Tween 80/Span 65).

Dilatational rheology of a gel point network formed by nonionic soluble surfactants at the oil-water interface

This study presents new aspects of the dilatational rheology of soluble nonionic surfactants at the water–oil interface giving a two-dimensional rheological gel-point behavior. We illustrate that these surprising results can be described by coupling the Lucassen and van den Tempel model to a power function describing the relaxation times distribution, involving the presence at the interface of two-dimensional aggregates with a wide distribution of sizes. The principal interfacial relaxation mechanism is then attributed to adsorption–desorption of surfactant clusters of various sizes. The main hypothesis for the use of the van den Tempel model is that clusters do not interact with each other while surfactants do interact within each cluster. Using the hyperscaling relation and assuming that the cluster relaxation times are related to the cluster masses by a power law, we propose a new expression of the fractal dimension df for interfacial aggregating systems in function of the relaxation exponent n of the form df = 2/(1 + n). This original study contributes to the understanding of the two-dimensional rheology of soluble monolayers with attractive surfactants, thus opening new doors for the characterization of their behavior at the interface.

Help from a Hindrance: Using Astigmatism in Round Capillaries To Study Contact Angles and Wetting Layers

Round glass capillaries are a basic tool in soft-matter science, but often are shunned due to the astigmatism they introduce in micrographs. Here, we show how refraction in a capillary can be a help instead of a hindrance to obtain precise and sensitive information on two important interfacial properties: the contact angle of two immiscible fluids and the presence of thin films on the capillary wall. Understanding optical cusps due to refraction allows direct mesurement of the inner diameter of a capillary at the meniscus, which, with the height of the meniscus cap, determines the contact angle. The meniscus can thus be measured without intrusive additives to enhance visibility, such as dyes or calibrated particles, in uniform, curved, or even tapered capillaries or under demanding conditions not accessible by conventional methods, such as small volumes (μL), high temperatures, or high pressures. We further elicit the conditions for strong internal reflection on the inner capillary wall, involving the wall and fluid refractive indices and the wall thickness, and show how to choose the capillary section to detect thin (submicron) layers on the wall, by the contribution of total internal reflection to the cusps. As examples, we report the following: (i) CO2-water or -brine contact angles at glass interfaces, measured at temperatures and pressures up to 200 °C and 600 bar, revealing an effect apparently so far unreported-the decrease in the water-wet character of glass, due to dissolved salts in brine, is strongly reduced at high temperatures, where contact angles converge toward the values in pure water; (ii) A tenuous gas hydrate layer growing from the water-guest contact line on glass, invisible in transmission microscopy but prominent in the cusps due to total internal reflection.