Surface Hydrophobicity of Culture and Water Biofilm of <Emphasis Type="Italic">Penicillium</Emphasis> spp.

Fungal surface hydrophobicity is involved in several functions in fungal growth and development. Water contact angles measurement has been used as a direct and simple approach for its characterisation in solid cultures. Microsphere adhesion assay is said to be the best method to assess cell hydrophobicity of filamentous fungi. This study aimed to apply these two methods to study hydrophobicity of Penicillium expansum and Penicillium brevicompactum grown as mycelial mats in solid culture, liquid culture and water biofilms. As result, both species in solid cultures were classified as hydrophobic with contact angles ≥90o, but in liquid cultures and water biofilms showed different levels of hydrophobicity when microsphere adhesion assay was applied. In addition, was found that biofilms have specific hydrophobic hyphae which may be involved in fungal ecological functions.     

Contact Angle Measurement and Cell Hydrophobicity of Granular Sludge from Upflow Anaerobic Sludge Bed Reactors

The contact angle, which is generally used to evaluate the hydrophobicities of pure bacterial strains and solid surfaces, was used to study mixed cell cultures of bacteria involved in anaerobic digestion. Previously published data and data from this study showed that most acidogens are hydrophilic (contact angle, <45(deg)) but most of the acetogens and methanogens isolated from granular sludge are hydrophobic (contact angle, >45(deg)). The hydrophobicities of mixtures of hydrophilic and hydrophobic cells were found to be linearly correlated with the cell mixing ratio. The hydrophobicities of cells present in effluents from upflow anaerobic sludge bed reactors which were treating different types of substrates were different depending on the reactor conditions. When the reactor liquid had a high surface tension, cells sloughing off from sludge granules, as well as cells present on the outer surfaces of the granules, were hydrophobic. Short-term batch enrichment cultures revealed that proteins selected for highly hydrophilic cells. Long-term in-reactor enrichment cultures revealed that sugars selected for hydrophilic acidogens on the surfaces of the granules, while fatty acids tended to enrich for hydrophobic methanogens. When linear alkylbenzenesulfonate was added, the cells on the surfaces of granules became more hydrophilic. Control tests performed with pure cultures revealed that there was no change in the surface properties due to linear alkylbenzenesulfonate; hence, the changes in the wash-out observed probably reflect changes in the species composition of the microbial association. A surface layer with moderate hydrophobicity, a middle layer with extremely high hydrophobicity, and a core with high hydrophobicity could be distinguished in the grey granules which we studied.     

Study on the Hydrophobic Property of Shark-Skin-Inspired Micro-Riblets

This paper aims to characterize the hydrophobic property of shark-skin-inspired riblets with potential engineering appli- cations. Based on the hydrophobic theory, a new hydrophobic model which is consistent with the special structure of shark-skin-inspired micro-riblets was proposed. Then, the contact angles of different droplets were measured by optical contact angle measuring device on the shark-skin-inspired micro-riblets and the smooth surface, respectively. The results show that the surface of micro-riblets possesses obvious hydrophobicity, and the actual contact angles of different droplets residing on the riblets decrease with the increase in the droplet volume. According to the new hydrophobic model and the measurement of contact angle, it was found that the arrangement and structure of the shark-skin-inspired micro-riblets significantly affect the surface hydrophobic property. Using the new hydrophobic model, the prediction error of contact angle can be less than 3% compared with the measured one. The research on hydrophobic property of biomimetic micro-riblets is proved to be necessary and important to well explain drag reduction and microbe-resistant property of micro-riblets.     

The role of bacterial cell wall hydrophobicity in adhesion

In this study, the adhesion of bacteria differing in surface hydrophobicity was investigated. Cell wall hydrophobicity was measured as the contact angle of water on a bacterial layer collected on a microfilter. The contact angles ranged from 15 to 70 degrees. This method was compared with procedures based upon adhesion to hexadecane and with the partition of cells in a polyethylene glycol-dextran two-phase system. The results obtained with these three methods agreed reasonably well. The adhesion of 16 bacterial strains was measured on sulfated polystyrene as the solid phase. These experiments showed that hydrophobic cells adhered to a greater extent than hydrophilic cells. The extent of adhesion correlated well with the measured contact angles (linear regression coefficient, 0.8).     

The role of bacterial cell wall hydrophobicity in adhesion.

In this study, the adhesion of bacteria differing in surface hydrophobicity was investigated. Cell wall hydrophobicity was measured as the contact angle of water on a bacterial layer collected on a microfilter. The contact angles ranged from 15 to 70 degrees. This method was compared with procedures based upon adhesion to hexadecane and with the partition of cells in a polyethylene glycol-dextran two-phase system. The results obtained with these three methods agreed reasonably well. The adhesion of 16 bacterial strains was measured on sulfated polystyrene as the solid phase. These experiments showed that hydrophobic cells adhered to a greater extent than hydrophilic cells. The extent of adhesion correlated well with the measured contact angles (linear regression coefficient, 0.8).     

Effect of Surface-Active Pseudomonas spp. on Leaf Wettability

Different strains of Pseudomonas putida and P. fluorescens isolated from the rhizosphere and phyllosphere were tested for surface activity in droplet cultures on polystyrene. Droplets of 6 of the 12 wild types tested spread over the surface during incubation, and these strains were considered surface active; strains not showing this reaction were considered non-surface active. Similar reactions were observed on pieces of wheat leaves. Supernatants from centrifuged broth cultures behaved like droplets of suspensions in broth; exposure to 100°C destroyed the activity. Average contact angles of the supernatants of surface-active and non-surface-active strains on polystyrene were 24° and 72°, respectively. The minimal surface tension of supernatants of the surface-active strains was about 46 mN/m, whereas that of the non-surface-active strains was 64 mN/m (estimations from Zisman plots). After 6 days of incubation, wheat flag leaves sprayed with a dilute suspension of a surface-active strain of P. putida (WCS 358RR) showed a significant increase in leaf wettability, which was determined by contact angle measurements. Increasing the initial concentration of bacteria and the amount of nutrients in the inoculum sprayed on leaves reduced the contact angles from 138° on leaves treated with antibiotics (control) to 43° on leaves treated with surface-active bacteria. A closely related strain with no surface activity on polystyrene did not affect leaf wettability, although it was present in densities similar to those of the surface-active strain. Nutrients alone could occasionally also increase leaf wettability, apparently by stimulating naturally occurring surface-active bacteria. When estimating densities of Pseudomonas spp. underneath droplets with low contact angles, it appeared that populations on leaves treated with a surface-active strain could vary from about 10⁴ to 10⁶ CFU cm⁻², suggesting that the surface effect may be prolonged after a decline of the population. The possible ecological implications are discussed.     

Use of water to evaluate hydrophobicity of organically-modified xerogel enzyme supports

Silica xerogels are a new class of materials suitable for the immobilization of enzymes for various applications including biotransformations and biosensors. The physicochemical properties of xerogels, such as hydrophobicity, can be manipulated by the introduction of organically-modified silicates. This allows the immobilization matrix to be engineered to suit the enzyme and its application. Interfacial activation of lipase is a phenomenon in which the enzyme displays increased activity when it is bound to a hydrophobic interface. Lipase was entrapped in organically-modified xerogels in which the hydrophobicity of the enzyme support was modulated by the selection of different alkyltrimethoxysilane co-precursors and the ratio in which they were combined with tetramethyl orthosilicate. Interaction between the enzyme support and water was investigated with two methods to quantitatively assess the hydrophobicity of the entrapment matrix. The contact angle formed between the xerogel and water was used to determine hydrophobicity on a macroscopic level. Temperature-controlled water desorption was used to determine hydrophobicity on a microscopic level. Both methods were suitable for quantitatively discriminating between hydrophobic and hydrophilic materials. Further, the hydrophobicity of the enzyme support influenced the hydrolytic activity of the entrapped lipase under non-aqueous conditions. The specific activity of lipase increased only when entrapped in xerogels which could be classified as hydrophobic materials, that is with contact angles greater than 90 degrees or hydrophobicity values as determined by water desorption greater than 0.65.     

Comparison of contact angles and adhesion to hexadecane of urogenital, dairy, and poultry lactobacilli: effect of serial culture passages.

The aim of this study was to examine the hydrophobicities of 23 urogenital, dairy, poultry, and American Type Culture Collection isolates of lactobacilli and to determine the effect on hydrophobicity of serially passaging the strains in liquid medium. To this end, strains were grown after isolation and identification and then serially passaged up to 20 times. Hydrophobicity was assessed through contact angle measurements on lawns of cells by using water, formamide, methylene iodide, 1-bromonaphthalene, and hexadecane as wetting agents and through measurement of their partitioning in a hexadecane-water system. The hydrophobicities of these strains varied widely, with Lactobacillus casei strains being predominantly hydrophilic and L. acidophilus strains being mostly hydrophobic. For some isolates, serial passaging was accompanied by a clear loss of hydrophobic surface properties, whereas for other strains, cultures became heterogeneous in that some cells had already lost their hydrophobic surface properties while others were still hydrophobic. Adhesion of this collection of lactobacilli to hexadecane droplets in microbial adhesion to hexadecane (MATH) tests was driven by their aversion to water rather than by their affinity for hexadecane, as concluded from the fact that hexadecane contact angles were zero for all strains. Furthermore, adhesion of the lactobacilli to hexadecane in MATH tests occurred only when the water contact angle on the cells was above 60 degrees.     

Comparison of contact angles and adhesion to hexadecane of urogenital, dairy, and poultry lactobacilli: effect of serial culture passages.

The aim of this study was to examine the hydrophobicities of 23 urogenital, dairy, poultry, and American Type Culture Collection isolates of lactobacilli and to determine the effect on hydrophobicity of serially passaging the strains in liquid medium. To this end, strains were grown after isolation and identification and then serially passaged up to 20 times. Hydrophobicity was assessed through contact angle measurements on lawns of cells by using water, formamide, methylene iodide, 1-bromonaphthalene, and hexadecane as wetting agents and through measurement of their partitioning in a hexadecane-water system. The hydrophobicities of these strains varied widely, with Lactobacillus casei strains being predominantly hydrophilic and L. acidophilus strains being mostly hydrophobic. For some isolates, serial passaging was accompanied by a clear loss of hydrophobic surface properties, whereas for other strains, cultures became heterogeneous in that some cells had already lost their hydrophobic surface properties while others were still hydrophobic. Adhesion of this collection of lactobacilli to hexadecane droplets in microbial adhesion to hexadecane (MATH) tests was driven by their aversion to water rather than by their affinity for hexadecane, as concluded from the fact that hexadecane contact angles were zero for all strains. Furthermore, adhesion of the lactobacilli to hexadecane in MATH tests occurred only when the water contact angle on the cells was above 60 degrees.     

Relations between hydrophobicity tested by three methods and surface chemical composition of Escherichia coli

The cell surface hydrophobicity of three strains of Escherichia coli cultured in liquid medium and on solid medium was measured using various methods including adsorption to pxylene, partition of cells in a polyethylene glycol/dextran (PEG/DEX) two phase system and contact angle measurements. The percentage adsorbed to pxylene ranged from 1.6% to 67% and the percentage of cells in polyethylene glycol phase ranged from 19% to 64%. The contact angle data of less than 40 degrees C revealed a hydrophylic character of the E. coli strains studied here. No relations were found between paraxylene/water partitioning, PEG/DEX partioning and water contact angles. The linear correlation coefficients between the results of the three hydrophobicity assays and the elemental concentration ratios obtained by X-ray photoelectron spectroscopy (XPS) were calculated. A linear correlation was found between the contact angles and the O/C ratios (r=0.91) and the N/C ratios (0.67). The adsorption to pxylene correlates better with N/C ratios (0.88) but does not correlate with O/C ratios (0.46). However, this test correlates with N/P ratios (0.79). No relation was obtained between partition in PEG/DEX system and any elemental concentration ratios. The surface composition determined by XPS was converted into a molecular composition in terms of proteins, polysaccharides, and hydrocarbon-like compounds. The proteins/polysaccharides and the hydrocarbons/polysaccharides seems to determine the contact angle of E. coli but not the adsorption to paraxylene or partition in the PEG/DEX system.     

Increased water resistance of CTMP fibers by oat (Avena sativa L.) husk lignin

The insertion of oat husk lignin onto chemithermomechanical pulp (CTMP) fibers was studied to increase fiber hydrophobicity. The pretreated pulp samples were subsequently used for preparation of handsheets for characterization. Treatment of CTMP with laccase in the presence of oat husk lignin resulted in a significant increase in hydrophobicity of the handsheet surface, as indicated by dynamic contact angle analysis. Water absorption time of 8 s was obtained with initial contact angle of 118°. Although the handsheet's brightness was reduced by 33%, tensile index was only subtly decreased. Neither laccase nor oat husk lignin alone gave much improved water absorption times. Therefore, handsheets made of laccase-treated pulp with and without oat husk lignin were further examined by XPS, which suggested that both laccase and oat husk lignin were inserted onto CTMP fibers. The oat husk lignin was distributed as heterogeneous aggregates on the handsheet surface whereas laccase was uniformly distributed. Evidence was obtained that the adsorbed laccase layer formed a noncovalent base for the insertion of oat husk lignin onto fiber surfaces.     

Effects of Methanol on Wettability of the Non-Smooth Surface on Butterfly Wing

The contact angles of distilled water and methanol solution on the wings of butterflies were determined by a visual contact angle measuring system. The scale structures of the wings were observed using scanning electron microscopy, The influence of the scale micro- and ultra-structure on the wettability was investigated. Results show that the contact angle of distilled water on the wing surfaces varies from 134.0° to 159.2°. High hydrophobicity is found in six species with contact angles greater than 150°. The wing surfaces of some species are not only hydrophobic but also resist the wetting by methanol solution with 55% concentration. Only two species in Parnassius can not resist the wetting because the micro-structure （spindle-like shape） and ultra-structure （pinnule-like shape） of the wing scales are remarkably different from that of other species. The concentration of methanol solution for the occurrence of spreading/wetting on the wing surfaces of different species varies from 70% to 95%. After wetting by methanol solution for 10 min, the distilled water contact angle on the wing surface increases by 0.8°-2.1°, showing the promotion of capacity against wetting by distilled water.     

Changes in stability of the DPPC monolayer during its contact with the liquid phase

The Langmuir-Blodgett (LB) method was applied and a few series of advancing and receding contact angles measurements as a function of time were performed to examine stability of model phospholipid monolayers during their contact with water, formamide and diiodomethane droplets. The studied monolayer was single-component saturated phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) transferred onto mica surface. When the time of the contact angle measurements is prolonged in these systems, some changes in the DPPC layer structure occur due to the contact with probing liquid, especially water, which is reflected in the changes of measured contact angle. Generally, with increasing time of the droplet contact with DPPC monolayer the contact angle decreases. Some correlation between the contact angle decrease and molecular rearrangements of initially hydrophobic DPPC monolayer is observed if it comes into contact with water. On the other hand, the contact angle completed within the first few seconds can faithfully reflect the original structure of the layer, and thus its energetic state, because during this time the structure changes are insignificant. Basing on the measured contact angles the monolayer's apparent surface free energy and its components, corresponding to different contact times of the droplets, were calculated. These results are helpful for better characterization of the processes taking place in the phospholipid layers being in contact with polar (water and formamide) and nonpolar (diiodomethane) liquids.     

Characterization of physicochemical forces involved in adhesion of Listeria monocytogenes to surfaces

This study investigated the physicochemical forces involving the adhesion of Listeria monocytogenes to surfaces. A total of 22 strains of L. monocytogenes were compared for relative surface hydrophobicity with the salt aggregation test. Cell surface charges and hydrophobicity of L. monocytogenes Scott A were also determined by electrophoretic mobility, hydrophobic-interaction chromatography, and contact angle measurements. Electrokinetic measurements indicated that the strain Scott A has a negative electrophoretic mobility. Physicochemical characterization of L. monocytogenes by various methods indicates that this microorganism is hydrophilic. All L. monocytogenes strains tested with the salt aggregation test method aggregated a at very high ammonium sulfate molarities. The hydrophobicity-interaction chromatography results show that L. monocytogenes Scott A cells do not adhere to octyl-Sepharose unless the pH is low. Results from contact angle measurements showed that the surface free energy of strain Scott A was 65.9 mJ.m-2, classifying this microorganism as a hydrophilic bacterium. In addition, the interfacial free energy of adhesion of L. monocytogenes Scott A estimated for polypropylene and rubber was lower than that for glass and stainless steel. However, these theoretical implications could not be correlated with the attachment capabilities of L. monocytogenes.     

Effect of electrode surface properties on enhanced electron transfer activity in microbial fuel cells

The influence of electrode surface chemistry over biofilm growth was evaluated for photo‐bioelectrocatalytic fuel cell. A consortium of photosynthetic bacteria was grown onto different electrodes designed with polyethylenimine (PEI) and multiwall carbon nanotubes as hydrophilic and hydrophobic modifier, respectively. The designed electrodes were loaded with 0.08, 0.17, and 0.33 μg/cm² of PEI to change the hydrophilicity. However, 0.56, 0.72, and 0.83 mg/cm² of multiwall carbon nanotubes were used to alter the hydrophobicity of the electrodes. The surface chemistry of electrode and bio‐interaction was evaluated as a function of contact angle and biofilm formation. The results were compared with those obtained with a carbon paper electrode. The contact angle on the untreated electrode (carbon paper) was 118°, whereas for hydrophobic and hydrophilic electrodes, the maximum and minimum contact angles were 170° and 0°, respectively. Interestingly, the maximum biofilm growth (0.2275 g, wet basis) was observed on highly hydrophobic surface; however, the maximum electrochemical performance (246 mV) was shown by the most hydrophilic electrode surface. PEI‐based electrode with good biofilm formation showed comparatively higher electrogenic activity.     

Use of Self-Assembled Monolayers of Different Wettabilities To Study Surface Selection and Primary Adhesion Processes of Green Algal (Enteromorpha) Zoospores

We investigated surface selection and adhesion of motile zoospores of a green, macrofouling alga (Enteromorpha) to self-assembled monolayers (SAMs) having a range of wettabilities. The SAMs were formed from alkyl thiols terminated with methyl (CH₃) or hydroxyl (OH) groups or mixtures of CH₃- and OH-terminated alkyl thiols and were characterized by measuring the advancing contact angles and by X-ray photoelectron spectroscopy. There was a positive correlation between the number of spores that attached to the SAMs and increasing contact angle (hydrophobicity). Moreover, the sizes of the spore groups (adjacent spores touching) were larger on the hydrophobic SAMs. Video microscopy of a patterned arrangement of SAMs showed that more zoospores were engaged in swimming and “searching” above the hydrophobic sectors than above the hydrophilic sectors, suggesting that the cells were able to “sense” that the hydrophobic surfaces were more favorable for settlement. The results are discussed in relation to the attachment of microorganisms to substrata having different wettabilities.     

Superhydrophobicity of Bionic Alumina Surfaces Fabricated by Hard Anodizing

Bionic alumina samples were fabricated on convex dome type aluminum alloy substrate using hard anodizing technique.The convex domes on the bionic sample were fabricated by compression molding under a compressive stress of 92.5 MPa.The water contact angles of the as-anodized bionic samples were measured using a contact angle meter (JC2000A) with the 3 μL water drop at room temperature.The measurement of the wetting property showed that the water contact angle of the unmodified as-anodized bionic alumina samples increases from 90° to 137° with the anodizing time.The increase in water contract angle with anodizing time arises from the gradual formation of hierarchical structure or composite structure.The structure is composed of the micro-scaled alumina columns and pores.The height of columns and the depth of pores depend on the anodizing time.The water contact angle increases significantly from 96° to 152° when the samples were modified with self-assembled monolayer of octadecanethiol (ODT),showing a change in the wettability from hydrophobicity to super-hydrophobicity.This improvement in the wetting property is attributed to the decrease in the surface energy caused by the chemical modification.     

Surface thermodynamics of phagocytic ingestion of non-opsonized bacteria by granulocytes in liquids of different surface tensions

The free energy of engulfment of four bacterial species by human granulocytes is calculated from contact angle data as a function of the surface tension γ_LV of the suspending liquid. The resulting curves predict that at low liquid surface tensions γ_LV, the phagocytic ingestion increases with decreasing hydrophobicity of the bacteria while at high surface tensions γ_LV, it increases with increasing hydrophobicity. Furthermore, these curves reach a minimum at values of γ_LV equal to the surface tension γ_LV of the bacteria. The experimental results support these predictions. Thus, the determination of the surface tension of the suspending medium at which phagocytic ingestion becomes minimum represents a novel technique to establish the surface tension of ingested particles, such as bacteria. The results obtained in this fashion for the four bacterial species are in good agreement with those obtained from contact angles, as well as those derived from bacterial adhesion experiments.     

Effect of culture media onLactobacillus hydrophobicity and electrophoretic mobility

The hydrophobicity of six strains representing three species ofLactobacillus was measured using dextran-polyethylene glycol contact angle measurements. These ranged from 123.6° forLactobacillus casei douche to 26.2° forL. casei RC-17 under identical growth conditions. The results indicated that the nutritional environment affected bacterial hydrophobicity. Electrophoretic mobilities of the lactobacilli were also determined and found to be negative for all specimens, and to vary with growth media, especially when sugars were added to urine. The electrophoretic mobility histograms showed one main peak for all strains, exceptLactobacillus acidophilus T-13 which had two peaks, suggesting two morphological sizes or types within its population. In addition, strain T-13 was more positively charged than the other five strains after growth in agar, urine, and supplemented urine. The use of contact angle and electrophoretic mobility techniques allows examination of cell surface properties of lactobacilli that may have importance in the colonization of mucosal epithelia.     

Adhesion of Chlorella vulgaris to solid surfaces, as mediated by physicochemical interactions

Although adhesion of bacteria and yeast have been extensively studied by a wide range of experimental and theoretical approaches, significantly less attention has been focused on microalgae adhesion to solid materials. This work is focused on physicochemical aspects of microalgae adhesion. The results are based on experimental characterization of surface properties of both microalgae and solids by contact angle and zeta potential measurements. These data are used in modeling the surface interactions (thermodynamic and colloidal models) resulting in quantitative prediction of the interaction intensities. Finally, the model predictions are compared with experimental adhesion tests of microalgae onto model solids in order to identify the physicochemical forces governing the microalgae–solid interaction. The model solids were prepared in order to cover a wide range of properties (hydrophobicity and surface charge). The results revealed that, in low ionic strength environment, the adhesion was influenced mostly by electrostatic attraction/repulsion between surfaces, while with increasing ionic strength grew the importance of apolar (hydrophobic) interactions. The impact of solid surface properties on the degree of colonization by microlagae was statistically more significant than the influence of medium composition on cell surface of Chlorella vulgaris.