Influence of the growth medium,suspending liquid and measurement temperature on the physico-chemical surface properties of two enterococci strains

A knowledge of the physico-chemical surface properties of a bacterium is important to predict the first step in the adhesion process of such a microorganism to biomaterials. In this work, hydrophobicity, surface free energy and surface charge of Enterococcus faecalis ATCC29212 and E. faecalis 72 bacterial strains have been analysed under the followings conditions, which were considered as criteria that could affect the thermodynamical parameters of the cell surface: culture medium was supplemented with serum and urine and the experiments were carried out in two different buffers (KPi and PBS) and at two temperatures (22 and 37 °C). MATH hydrophobicity does not seem to be affected by temperature but water contact angle increased with temperature for both strains. Serum and urine added to the culture medium made the strains more and less hydrophobic, respectively. The zeta potential was dependent on the addition to the culture medium of serum and urine, the experimental temperature and the buffer employed and it decreased with increasing ionic strength in all cases studied. The results reveal that physico-chemical surface properties of bacteria are greatly affected by the environmental conditions in which they are measured, indicating that experiments should be carried out under experimental conditions as similar as possible to the situation of clinical interest.     

Effect of pH on distribution and adhesion of Staphylococcus aureus to glass

The adhesion of Staphylococcus aureus ATCC 25923 to glass at different pH values was investigated using scanning electron microscopy (SEM) and image analysis with the Mathlab^® program. The images obtained by SEM show that the adhesion behaviour of Staphylococcus aureus ATCC 25923 depends on the pH of the suspending medium. At highly acidic (pH 2, pH 3) and alkaline pH, the cells deposited in aggregate forms, while at pH 5 the aggregation phenomenon was absent. The quantitative adhesion (number of adhering cells to glass surface) showed that cells adhered strongly in the pH range 4 to 6 and weakly at highly acidic (pH 2, pH 3) and alkaline pH. Moreover, the surface properties of the cells were characterized by the microbial adhesion to solvents (MATS) method. A good correlation was obtained between physicochemical properties (hydrophobicity or electron donor electron acceptor character) of Staphylococcus aureus ATCC 25923 and the number of adhering cells to glass. These results show that the adhesion of Staphylococcus aureus ATCC 25923 to glass is controlled by both acid–base and hydrophobic interactions.     

Laser-Patterned Super-Hydrophobic Pure Metallic Substrates: Cassie to Wenzel Wetting Transitions

A femtosecond laser was used to create microstructures on very pure metal surfaces. The irradiated samples initially showed super-hydrophilic behavior. With time and exposure to ambient air the contact angle increased to about 160° with very low hysteresis. The surfaces supported the Cassie and Wenzel wetting states, depending on the technique used to deposit the water droplets. The created surface morphologies were idealized with a geometric model that is an assembly of densely packed cylindrical pillars with semispherical caps. Using this geometric model for calculation of the surface roughness, a theoretical Young contact angle of about 99° was calculated for all samples from the Wenzel and Cassie–Baxter equations. While the value of 99° significantly differs from the measured hydrophilic contact angles on the polished pure metallic samples, it indicates that a laser-induced surface reaction must be responsible for the evolution of contact angles to super-hydrophobic ones and that this phenomenon is independent of the type of metal.     

Wetting Transitions on Rough Substrates: General Considerations

General properties of wetting transitions for droplets on rough substrates are analyzed theoretically. The energy barrier to be surpassed for wetting transitions is much lower than the heat of evaporation of the droplet; this makes wetting transitions possible. It is shown that the energy curve of the transition state, i.e., the dependence of the interfacial part of the Gibbs free energy on the apparent contact angle in this state, as well as the energy barriers, can be expressed through the contact angles in the initial and final states without going into the geometric details of the given substrate relief. On this basis, the reason for the irreversibility of the Cassie–Wenzel transitions is elucidated: the energy barrier of the reverse transition is shown to be much higher. The scheme is also applicable to the substrates with disordered reliefs. Time-scaling arguments are important for understanding wetting transitions.     

Hygrothermal Aging of Silane-Laced Epoxy Coatings

The impact of silane on the hygrothermal stability of epoxy coatings was investigated by specular neutron reflectivity (NR). By comparing the hygrothermal degradation behavior of neat novolac epoxy coating and corresponding bis[3-(triethoxysilyl)propyl]tetrasulfide-laced epoxy coating, the role of silane was elucidated. Accelerated aging was achieved by exposing the samples to 80°C liquid water. For the pure epoxy coating, degradation occurs at the coating–substrate interface, which makes the coating vulnerable to adhesion failure. For epoxy–silane coating the addition of silane imparts resistance to the interfacial degradation observed in the neat epoxy coating.     

Bacterial adhesion to poly(vinyl chloride) films: Effect of chemical modification and water induced surface reconstruction

Escherichia coli (E. coli) and Staphyloccus aureus (S. aureus) bacteria adhesion to pure and modified poly(vinyl chloride) (PVC) was investigated through classical wettability measurements, captive bubble time dependent measurements, and static adhesion tests. Various chemical modifiers were studied, namely 4-mercaptophenol, 4-mercaptobenzylalcohol, 4-methoxybenzenethiol, 2-naphthalenethiol and 4-mercaptopyridine. The surface thermodynamics of the modified PVC films was investigated via the van Oss, Chaudhury and Good (vOCG) theory and an increase in the hydrophilic character was deduced from the increase of the contact angle of the air bubble in water with time of immersion. This is ascribed to a water induced surface reconstruction of the modified PVC surface. E. coli bacteria exhibit a hydrophilic character and strong adhesion, dependence on the nature of the PVC modifier as shown by the remaining attached bacteria. S. aureus which is hydrophobic showed no difference in its adhesion to pure or modified PVC. A slight increase in the adhesion of E. coli is observed with the water induced surface reconstruction. This work highlights the predominance of the hydrophilic/hydrophobic nature over the acid–base in the bacteria/polymer adhesion mechanism. It also provides, through chemical modification of PVC, a nice route to control the micro-organisms' adhesion to biomedical devices.     

Implications of Growth and Starvation Conditions in Bacterial Adhesion and Transport

Biofouling is synonymous with unwanted biofilms and leads to problems ranging from efficiency and resource loss to health risks. While a number of bacterial properties including biomass concentration and hydrophobicity are considered critical to biofilm development and bacterial adhesion, the variations in these properties under growth and starvation conditions are not very well known. Here, we describe the trends in these properties for four Gram-negative bacteria under growth and extended starvation conditions. A convenient and frequently-used laboratory assay, the microbial adhesion to hydrocarbons (MATH) test, was used to determine the microbial hydrophobicity based on the partitioning of cells at an aqueous-hydrocarbon interface. The bacteria tested exhibited a plateau in hydrophobicity values during the stationary growth phase and longer starvation durations (≥10 days). Starved cultures had higher hydrophobicity and lower cell sizes than growth cultures. Interestingly, hydrocarbon exposure led to an increase in cell size for starved cells as compared to control cultures, while cells under growth conditions did not show significant size changes due to hydrocarbon presence. Cells starved for short durations (up to 7–10 days) exhibited significant variations in microbial hydrophobicity, cell size, and biomass concentration (total proteins and optical density). These results show the importance of studying the bacterial properties as a function of growth and starvation phase for cell adhesion in the context of biofilm formation and biofouling.     

Application of Salting-Out Agent to Enhance the Hydrophobicity of Weakly Hydrophobic Bacterial Strains

Hydrophobicity is a vital parameter for initial cell adhesion that ultimately leads to biofouling of surfaces and loss of system performance and health issues. The efficiency of a number of biological systems could be improved by increasing the hydrophobicity of concerned bacteria. Here we used ammonium sulfate (salt) to enhance the bacterial hydrophobicity, as measured by a commonly used liquid–liquid partitioning based hydrophobicity assessment assay — the MATH test. We observed successive increases in bacterial hydrophobicity with incremental increase in salt concentration for Gram-negative bacteria. Upon addition of 2 M salt, three closely related E. coli strains were easily distinguishable from one another. Gram-positive bacteria exhibited different trends than Gram-negative strains, with no change in the hydrophobicity of S. salivarius HB cells and a sharp decline followed by an increase in hydrophobicity for D. radiodurans. Cell size measurements revealed that Gram-positive cells exhibited a change in cell size on hydrocarbon exposure, while the Gram-negative cultures remained mostly unaffected. Overall, salt addition was observed to enhance the hydrophobicity of different test strains, especially at the higher concentrations used here of 1.5 and 2 M. Salt addition in conjunction with the MATH test successfully differentiated and quantified otherwise weakly hydrophobic bacteria, thus enhancing the range of this laboratory assay. Our results demonstrate the effectiveness of salt addition in increasing the bacterial hydrophobicity, which could potentially be used in diverse areas, ranging from applied microbiology and engineering to oral care.