Role of acid–base interactions on the adhesion of oral streptococci and actinomyces to hexadecane and chloroform—influence of divalent cations and comparison between free energies of partitioning and free energies obtained by extended DLVO analysis

Calcium is an abundantly present, divalent cation in the oral cavity and plays a crucial role in the adhesion of oral microorganisms to tooth surfaces as well as in coaggregation and coadhesion among the oral microflora. The aim of this study was to determine the effects of divalent cation (Ca²⁺, Mg²⁺, Ba²⁺) adsorption on the adhesion of two actinomyces and two streptococcal strains to hexadecane (MATH) and chloroform (MATS) in order to detect changes in acid–base character of the cell surfaces. Initial removal rates of the organisms by hexadecane, lacking an acid–base interaction with the organisms, were always smaller than those by chloroform. Furthermore, adsorption of divalent cations generally increased the initial removal rates of the microorganisms, but no statistically significant differences among different cations were observed. Gibbs energies of partitioning calculated from the stationary end-point adhesion of the organisms ranged from −2 to −4 kT for adhesion to hexadecane and were about twofold more negative for adhesion to chloroform. Contact angles on lawns of microorganisms with and without adsorbed divalent cations were similar. Zeta potentials of all microorganisms were slightly negative under the conditions of MATH and MATS and became only 4 mV more positive upon divalent cation adsorption. Hexadecane had a zeta potentials of −21 mV in the potassium phosphate solution used, which became 13 mV less negative upon Ca²⁺ adsorption. An extended DLVO approach of microbial adhesion to hexadecane, based on microbial contact angles and zeta potentials, taking into account Lifshitz–van der Waals, acid–base and electrostatic interactions did not show any potential energy barrier and demonstrated a deep primary interaction minimum at close approach due to acid–base attraction. As the Gibbs energy of partioning was only −2 to −4 kT, it is concluded that for the collection of organisms studied here, the final contactable surface area is small and structural features on the cell surfaces like fibrils and fimbriae, maintain a distance of ca. 10–15 nm between the hexadecane and the overall cell surface and therewith prevent acid–base interactions to become operative to a significant extend. Furthermore, from the lack of influence of divalent cations on macroscopic cell surface contact angles and zeta potentials, it is suggested that cation adsorption is minor and localized to the fibrils and fimbriae.     

Contact of oil with solid surfaces in aqueous media probed using sum frequency generation spectroscopy

We have studied the interface between hexadecane droplets and sapphire substrates in water using infrared-visible sum frequency generation spectroscopy (SFG). At high pH and above the isoelectric point of the sapphire substrate, the hexadecane drop is repelled due to electrostatic forces. The SFG measurements are consistent with the observation that a thick layer of water is present between the oil and the sapphire substrate. Below the isoelectric point of the sapphire substrate, the hexadecane drops stick to the sapphire surface. Surprisingly, the SFG results show the presence of a thin layer of water between hexadecane drop and the sapphire substrate. At this contact interface, we observe contributions to the SFG signal from both the hexadecane/water and water/sapphire interfaces. The reasons for the presence of a thin water layer with adhesive contact can be explained due to weaker repulsive double layer and the attractive van der Waals interactions.     

Surface thermodynamics and adhesion forces governing bacterial transmission in contact lens related microbial keratitis

Contact lens induced microbial keratitis results from bacterial transmission from one surface to another. We investigated the adhesion forces of Pseudomonas aeruginosa, Staphylococci and Serratia to different contact lenses, lens cases and corneal surfaces using AFM, and applied a Weibull analysis on these adhesion forces to calculate bacterial transmission probabilities from lens case to corneas with a contact lens as an intermediate. Also a new surface thermodynamic parameter was introduced, the interfacial free energy of transmission, which in essence compares the interfacial free energies of bacterial adhesion, calculated from measured contact angles with liquids on the donating and receiving surfaces in the transmission process. Bacterial adhesion forces were generally strongest among all eight strains for the lens case (-6.5 to -12.0 nN) and corneas (-3.5 to -11.5 nN), while contact lenses (-0.6 to -13.1 nN) exerted slightly smaller adhesion forces. Consequently, bacterial transmission from lens case to contact lens yielded a smaller contribution in the final transmission than from contact lens to cornea. Bacterial transmission probabilities as derived from force analyses were higher when the interfacial free energies of transmission were more negative, which is in line with surface thermodynamic principles. Therewith this parameter could provide useful in analyzing other bacterial transmission phenomena between donating and receiving surfaces as well.     

Bacterial adhesion to silica sand as related to Gibbs energy variations

Bacterial adhesion to silica sand was related to variations in system Gibbs energy DeltaG(adh). Two typical Gram-positive bacterial strains of Streptococcus mitis and Lactobacillus casei were used as the model bacteria in this research. Impacts of solution chemistry and goethite coating of silica sand on bacterial adhesion were also explored. S. mitis and L. casei had negative DeltaG(adh) with both uncoated and goethite-coated silica sand, demonstrating their adhesion potentials to these substrate. After goethite coating, DeltaG(adh) decreased (negatively increased) for both S. mitis and L. casei. In the presence of rhamnolipid biosurfactant, DeltaG(adh) increased (negatively decreased) in answer to the increase of the rhamnolipid biosurfactant concentration. Bacterial percentage adhesion to silica sand corresponded to DeltaG(adh). This study demonstrated that bacterial adhesion to substrate could be explained in terms of bacterial, substratum and intervening medium physicochemical surface properties, which can be independently determined based on contact angle measurements.     

THE SURFACE PROPERTIES OF A HYDROPHOBIC TRANSPOSON (Tn-5) MUTANT OF RHIZOBIUM ETLI

The physicochemical surface characteristics of a Tn-5 induced hydrophobic mutant (CE3003) of Rhizobium etli CE3 were investigated. The wild type CE3 was very hydrophilic with low contact angles for polar liquids, while the Tn-5 induced mutant had a surface that was moderately hydrophobic, with polar liquid contact angles in the 50–60° range. As a result, the polar surface free energy components (γ ⁺ and γ ^-) that constitute the acid-base component (γ ^AB) of surface tension, were greatly reduced on THE surface of the hydrophobic mutant. This decreased electron donicity of the mutant' surface caused an almost five fold increase in the magnitude of the acid-base component of the interfacial interaction free energy between the mutant and hexadecane. The increased adhesion to hexadecane reported earlier is probably attributable to this interaction free energy change and not to any alteration in zeta potential, which was similar for CE3 and CE3003 at pH 7. X-ray photoelectron spectroscopy showed CE3OO3 to have less surface carbon and nitrogen and more surface oxygen than CE3 with alterations in the (C-C,H) and (C-O,N) components being observed.     

Atomic force microscopy measurement of heterogeneity in bacterial surface hydrophobicity

The structure and physicochemical properties of microbial surfaces at the molecular level determine their adhesion to surfaces and interfaces. Here, we report the use of atomic force microscopy (AFM) to explore the morphology of soft, living cells in aqueous buffer, to map bacterial surface heterogeneities, and to directly correlate the results in the AFM force-distance curves to the macroscopic properties of the microbial surfaces. The surfaces of two bacterial species, Acinetobacter venetianus RAG-1 and Rhodococcus erythropolis 20S-E1-c, showing different macroscopic surface hydrophobicity were probed with chemically functionalized AFM tips, terminating in hydrophobic and hydrophilic groups. All force measurements were obtained in contact mode and made on a location of the bacterium selected from the alternating current mode image. AFM imaging revealed morphological details of the microbial-surface ultrastructures with about 20 nm resolution. The heterogeneous surface morphology was directly correlated with differences in adhesion forces as revealed by retraction force curves and also with the presence of external structures, either pili or capsules, as confirmed by transmission electron microscopy. The AFM force curves for both bacterial species showed differences in the interactions of extracellular structures with hydrophilic and hydrophobic tips. A. venetianus RAG-1 showed an irregular pattern with multiple adhesion peaks suggesting the presence of biopolymers with different lengths on its surface. R. erythropolis 20S-E1-c exhibited long-range attraction forces and single rupture events suggesting a more hydrophobic and smoother surface. The adhesion force measurements indicated a patchy surface distribution of interaction forces for both bacterial species, with the highest forces grouped at one pole of the cell for R. erythropolis 20S-E1-c and a random distribution of adhesion forces in the case of A. venetianus RAG-1. The magnitude of the adhesion forces was proportional to the three-phase contact angle between hexadecane and water on the bacterial surfaces.     

The influence of ionic strength, nutrients and pH on bacterial adhesion to metals

Bacteria-metal interactions in aqueous solutions are important in biofilm formation, biofouling and biocorrosion problems in the natural environment and engineered systems. In this study, the adhesion forces of two anaerobes (Desulfovibrio desulfuricans and Desulfovibrio singaporenus) and an aerobe (Pseudomonas sp.) to stainless steel 316 in various aqueous systems were quantified using atomic force microscopy (AFM) with a cell probe. Results show that the nutrient and ionic strength of the solutions influence the bacteria-metal interactions. The bacteria-metal adhesion force was reduced in the presence of the nutrients in the solution, because a trace organic film was formed and thus decreased the metal surface wettability. Stronger ionic strength in the solution results in a larger bacteria-metal adhesion force, which is due to the stronger electrostatic attraction force between the positively charged metal surface and negatively charged bacterial surface. Solution pH also influences the interaction between the bacterial cells and the metal surface; the bacteria-metal adhesion force reached its highest value when the pH of the solution was near the isoelectric point of the bacteria, i.e. at the zero point charge. The adhesion forces at pH 9 were higher than at pH 7 due to the increase in the attraction between Fe ions and negative carboxylate groups.     

Amphiphile grafted membranes for the separation of oil-in-water dispersions

Perfluorinated end-capped polyethylene glycol surfactants were covalently attached to fritted glass membranes as a means to improve the separation of oil-in-water emulsions. Hexadecane was used as representative oil for the oil-in-water emulsions; membrane pore size was varied between 10 and 174 microm. Membranes were characterized with respect to contact angle, permeability of bulk fluids, and separation efficiency. Performance was compared to similar metrics applied to unmodified membranes. Modified membranes demonstrated static hexadecane contact angles which were higher than static water contact angles converse to their unmodified counterparts. The relative hydrophilicity and corresponding oleophobicity of the modified membranes resulted in greater water permeability as compared to hexadecane permeability. The presence of the perfluorinated constituent of the amphiphile retarded the flow of hexadecane. For modified membranes, suspended hexadecane coalesced at the membrane surface, was undercut by water, and floated to the surface such that only trace amounts of oil were present in the permeate. Therefore, modified membranes resisted fouling from oil due to the self-cleaning properties of the attached amphiphile.     

Physicochemical aspects of microbial adhesion — Influence of antibody adsorption on the deposition ofStreptococcus sobrinus in a parallel-plate flow chamber

Deposition of the oral bacteriumStreptococcus sobrinus HG977 onto glass (water contact angle 0°) and onto FEP-Teflon (fluoroethylenepropylene; water contact angle 110°) was studied in a parallel-plate flow chamber in the presence and absence of polyclonal antibodies (pAb) and monoclonal antibodies (mAbs) adsorbed onto the cells. The zeta potentials of the bacteria ranged from −7.1 to −8.5 mV at pH 6.8 and were not affected by the presence of pAb or mAbs. Hydrophobicity (by water contact angles) increased from 30° (no antibodies) to 88° in the presence of pAb adsorbed onto the bacterial cell surface. The untreatedS. sobrinus had a greater tendency to adhere to glass (44.5 × 10⁶ cm⁻²) than to FEP-Teflon (18.3 × 10⁶ cm⁻²), in accordance with thermodynamic modelling. After preincubation ofS. sobrinus with pAb, its clear preference for adhesion to glass disappeared as expected from its increased hydrophobicity. Although forS. sobrinus preincubated with OMVU10 no difference was found in hydrophobicity in comparison to the untreated bacteria, the number of bacteria adhering to glass decreased to 10.2 ¢ 10⁶ cm⁻². Formation of bacterial aggregates was found whenS. sobrinus, preincubated with pAb or OMVU10, adhered to glass and FEP. This was also observed when untreated bacteria adhered to glass coated with OMVU10, or to FEP coaled with OMVU10 or pAb. Adhesion in these experiments is therefore thought to occur via near-neighbour collection induced by the presence of pAb or mAbs. Low numbers of bacteria were removed from glass after draining the flow cell, whereas high numbers of untreated bacteria and bacteria preincubated with OMVU10 were removed from FEP.S. sobrinus cells preincubated with pAb were not removed but piled up. It was concluded that the adhesion of untreatedS. sobrinus andS. sobrinus preincubated with pAb is in accordance with thermodynamic modelling, based on the overall wettability of the cell surfaces, whereas the adhesion ofS. sobrinus preincubated with OMVU10 may be through localized interactions, not expressed in overall surface properties.     

Significance of cell electrokinetic properties determined by soft-particle analysis in bacterial adhesion onto a solid surface

The influence of extracellular polymeric substances (EPSs) on bacterial cell electrokinetic properties and on cell adhesion onto glass beads in connection with bacterial cell electrokinetic properties was investigated using 12 heterotrophic bacterial strains. Bacterial cell surface properties such as the softness 1/lambda and charge density ZN were determined by Ohshima's soft-particle analysis using the measured electrophoretic mobility as a function of ionic strength. In 10 of 12 strains, when EPSs covering the cell surface were removed, the softness of the cell decreased, indicating that EPS adsorption enhanced the ease of liquid fluid in the ion-penetrable layer on the cell surface. On the other hand, the negative charge density of the cell surface increased for 9 of 12 strains, suggesting that EPSs covering the cell surface decreased the negative charge density of the cell surface layer. In addition, the characteristics of bacterial cell adhesion onto glass beads were evaluated by the packed-bed method and the data were interpreted to indicate cell adhesiveness. As a result, the efficiency of cell adhesion onto glass beads increased as negative cell surface potential psi0 decreased, whereas there seemed to be no correlation between zeta potential and cell adhesiveness. Cell surface potential psi0, which was derived by taking the bacterial polymer layer with EPSs into consideration, provided a more detailed understanding of the electrokinetic properties of bacterial cells.     

Contact angle hysteresis of microbead suspensions

Microbead suspensions are often used in microfluidic devices for transporting biomolecules. An experimental investigation on the wettability of microbead suspension is presented in this study. The variation in the surface tension and the equilibrium contact angle with the change in the volume fraction of the microbead is presented here. The surface tension of the microbead suspension is measured with the pendant drop technique, whereas the dynamic contact angle measurements, i.e., advancing and receding contact angles, are measured with the sessile drop technique. An equilibrium contact angle of a suspension with particular volume fraction is determined by computing an average over the measured advancing and receding contact angles. It is observed that the surface tension and the equilibrium contact angle determined from advancing and receding contact angles vary with the magnitude of the microbeads volume fraction in the suspension. A decrease in the surface tension with an increase in the volume fraction of the microbead suspension is observed. The advancement and the recession in contact line for dynamic contact angle measurements are achieved with the motorized dosing mechanism. For microbead suspensions, the advancement of the contact line is faster as compared to the recession of the contact line for the same flow rate. The presence of microbeads assists in the advancement and the recession of the contact line of the suspension. A decrease in the equilibrium contact angles with an increase in the microbead suspension volume fraction is observed. Inclusion of microbeads in the suspension increases the wetting capability for the considered combination of the microbead suspension and substrate. Finally, empirical correlations for the surface tension and the contact angle of the suspension as a function of microbead volume fraction are proposed. Such correlations can readily be used to develop mechanistic models for the capillary transport of microbead suspensions related to LOC applications.     

Superhydrophobic surfaces prepared by microstructuring of silicon using a femtosecond laser

We present a simple method for fabricating superhydrophobic silicon surfaces. The method consists of irradiating silicon wafers with femtosecond laser pulses and then coating the surfaces with a layer of fluoroalkylsilane molecules. The laser irradiation creates a surface morphology that exhibits structure on the micro- and nanoscale. By varying the laser fluence, we can tune the surface morphology and the wetting properties. We measured the static and dynamic contact angles for water and hexadecane on these surfaces. For water, the microstructured silicon surfaces yield contact angles higher than 160 degrees and negligible hysteresis. For hexadecane, the microstructuring leads to a transition from nonwetting to wetting.     

BHK cells behaviour on laser treated polydimethylsiloxane surface

The surface of polydimethylsiloxane (PDMS) was modified using a CO2-pulsed laser to evaluate the changes in physical and biological properties of the treated surface. Attachment of anchorage dependent cells, namely baby hamster kidney (BHK) fibroblastic cells, on PDMS surface was investigated in stationary culture conditions. BHK cell adhesion and growth on the PDMS surfaces were studied using scanning electron microscopy (SEM) and optical microscopy. To evaluate the surface wettability, water drop contact angles were determined. The laser treated PDMS surfaces showed high hydrophobicity and low cell adhesion, no spreading and growth in comparison with the unmodified PDMS. It was found that both the wettability and surface structure of the PDMS surface control cell attachment and growth.     

Hydrophobic and oleophobic surface modification using fluorinated bis-urea and bis-amide gelators

A series of fluorinated bis-urea and bis-amide derivatives were synthesized from fluorinated amines and explored as surface modifiers for nonwoven substrates. A majority of these derivatives showed excellent gelation properties both in organic solvents as well as in supercritical carbon dioxide (scCO₂) at concentrations ranging from 0.3 to 3 wt%. Gelation in the presence of a nonwoven substrate led to a gel-impregnated surface, which upon drying produced a composite with porous microstructure morphology on the surface. The composites thus produced showed high water and hexadecane contact angles, indicative of excellent hydrophobic and lyophobic properties. The superior hydrophobic and oleophobic behaviors observed in these composites are attributed to a combination of increased surface roughness and the presence of fluoroalkyl functionalities in the gelator backbone.     

A self-loading microfluidic device for determining the minimum inhibitory concentration of antibiotics

This article describes a portable microfluidic technology for determining the minimum inhibitory concentration (MIC) of antibiotics against bacteria. The microfluidic platform consists of a set of chambers molded in poly(dimethylsiloxane) (PDMS) that are preloaded with antibiotic, dried, and reversibly sealed to a second layer of PDMS containing channels that connect the chambers. The assembled device is degassed via vacuum prior to its use, and the absorption of gas by PDMS provides the mechanism for actuating and metering the flow of fluid in the microfluidic channels and chambers. During the operation of the device, degas driven flow introduces a suspension of bacterial cells, dissolves the antibiotic, and isolates cells in individual chambers without cross contamination. The growth of bacteria in the chambers in the presence of a pH indicator produces a colorimetric change that can be detected visually using ambient light. Using this device we measured the MIC of vancomycin, tetracycline, and kanamycin against Enterococcus faecalis 1131, Proteus mirabilis HI4320, Klebsiella pneumoniae, and Escherichia coli MG1655 and report values that are comparable to standard liquid broth dilution measurements. The device provides a simple method for MIC determination of individual antibiotics against human pathogens that will have applications for clinical and point-of-care medicine. Importantly, this device is designed around simplicity: it requires a single pipetting step to introduce the sample, no additional components or external equipment for its operation, and provides a straightforward visual measurement of cell growth. As the device introduces a novel approach for filling and isolating dead-end microfluidic chambers that does not require valves and actuators, this technology should find applications in other portable assays and devices.     

Determination of the van der Waals, electron donor and electron acceptor surface tension components of static Gram-positive microbial biofilms

A large number of studies have shown the influence of the physico-chemical properties of a surface on microbial adhesion phenomenon. In this study, we considered that the presence of a bacterial biofilm may be regarded as a “conditioning film” that may modify the physico-chemical characteristics of the support, and thus the adhesion capability of planktonic micro-organisms coming into contact with this substratum. In this context, we adapted a protocol for biofilm formation that allows, under our experimental conditions, contact angle measurements, the reference method to determine the energetic surface properties of a substratum. This made it possible to determine the van der Waals, electron acceptor and electron donor properties of static biofilms grown at 25°C on stainless-steel slides with six Gram-positive bacteria isolated in dairy plants. A variance analysis indicated significant effects (P<0.05) of the bacterial strains and of the physiological state of the micro-organisms (planktonic or sessile) on the contact angles. To link the energetic properties of the six biofilms with direct adhesion experiments, we measured the affinity of fluorescent carboxylate-modified polystyrene beads for the different biofilm surfaces. The results correlated best with the electron-acceptor components of the biofilm surface energies, stressing the importance of Lewis acid–base interactions in adhesion mechanisms.     

Wettability and antifouling behavior on the surfaces of superhydrophilic polymer brushes

The surface wettabilities of polymer brushes with hydrophobic and hydrophilic functional groups were discussed on the basis of conventional static and dynamic contact angle measurements of water and hexadecane in air and captive bubble measurements in water. Various types of high-density polymer brushes with nonionic and ionic functional groups were prepared on a silicon wafer by surface-initiated atom-transfer radical polymerization. The surface free energies of the brushes were estimated by Owens-Wendt equation using the contact angles of various probe liquids with different polarities. The decrease in the water contact angle corresponded to the polarity of fluoroalkyl, hydroxy, ethylene oxide, amino, carboxylic acid, ammonium salt, sulfonate, carboxybetaine, sulfobetaine, and phosphobetaine functional groups. The poly(2-perfluorooctylethyl acrylate) brush had a low surface free energy of approximately 8.7 mN/m, but the polyelectrolyte brushes revealed much higher surface free energies of 70-74 mN/m, close to the value for water. Polyelectrolyte brushes repelled both air bubbles and hexadecane in water. Even when the silicone oil was spread on the polyelectrolyte brush surfaces in air, once they were immersed in water, the oil quickly rolled up and detached from the brush surface. The oil detachment behavior observed on the superhydrophilic polyelectrolyte brush in water was explained by the low adhesion force between the brush and the oil, which could contribute to its excellent antifouling and self-cleaning properties.     

Wetting study of imidazolium ionic liquids

In this work, we present a systematic contact angles study of a series of 1-alkyl, 3-methyl-imidazolium ionic liquids (ILs) on well-defined polar and nonpolar monolayer surfaces supported on Si wafers. The advancing and receding contact angles of ILs were used to determine the surface energy of the monolayer surfaces using Neumann's equation-of-state and Zisman's critical surface tension approaches. In parallel, the contact angles of conventional probe fluids (molecular liquids) including water, formamide, methylene iodide, ethylene glycol, and hexadecane were determined on the same surfaces. The results obtained showed a great deal of similarity in wetting behavior of ionic vs molecular probe fluids: the contact angles of both sets of liquids followed the same patterns in accord with the surface tension of the fluid. A good agreement was found between the surface energy determined by different sets of liquids.