Hydrophobicity of biosurfaces — Origin, quantitative determination and interaction energies

Effects of calcium ions and polysaccharides type on transparent exopolymer particle formation and the related fouling mechanisms

Organics and divalent cations are the primary barriers constraining the performance of membrane technology, while the interactions between them and the detailed mechanisms of their impacts are still lacking in-depth analysis. In this study, sodium alginate and xanthan gum were selected as polysaccharides models, and the formation of transparent extracellular polymer particles (TEP) was assessed to examine the effect of Ca2+ and polysaccharides type on membrane fouling from both qualitative and quantitative perspectives. The results revealed that higher Ca2+ concentrations led to a greater abundance of TEP, and the transformation of TEP microstructure is a key factor for the membrane fouling change indicated by specific filtration resistance (SFR). TEP formed by sodium alginate underwent a transformation from amorphous-TEP (a-TEP) form to particle-TEP (p-TEP), corresponding to a unimodal pattern of SFR variation. With increasing Ca2+ concentration, the molecular interactions of xanthan gum became stronger, resulting in larger fibrous a-TEP and a continuous SFR increase. According to the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, TEP formed by xanthan gum exhibited higher adhesion energy, thus causing more severe membrane fouling. The SFR variation of the TEP system can be satisfactorily explained by the conception of chemical potential change in the filtration process depicted in Flory-Huggins theory. This study is the first work to introduce models regarding chemical potential and TEP microstructure, linking the system chemical potential and TEP microstructure with membrane fouling indicated by SFR. As all, this study provided a new perspective for analyzing the polysaccharide fouling behavior via TEP determination and further enhanced the understanding through thermodynamic analysis.

Enhanced sludge dewatering using a novel synergistic iron/peroxymonosulfate-polyacrylamide method

In the sludge dewatering process, a formidable challenge arises due to the robust interactions between extracellular polymeric substances (EPS) and bound water. This study introduces a novel, synergistic conditioning method that combines iron (Fe2+)/peroxymonosulfate (PMS) and polyacrylamide (PAM) to significantly enhance sludge dewatering efficiency. The application of the Fe2+/PMS-PAM conditioning method led to a substantial reduction in specific filtration resistance (SFR) by 82.75% and capillary suction time (CST) by 80.44%, marking a considerable improvement in dewatering performance. Comprehensive analyses revealed that pre-oxidation with Fe2+/PMS in the Fe2+/PMS-PAM process effectively degraded EPS, facilitating the release of bound water. Subsequently, PAM enhanced the flocculation of fine sludge particles resulting from the advanced oxidation processes (AOPs). Furthermore, analysis based on the Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory demonstrated shifts in interaction energies, highlighting the breakdown of energy barriers within the sludge and a transition in surface characteristics from hydrophilic (3.79 mJ m-2) to hydrophobic (-61.86 mJ m-2). This shift promoted the spontaneous aggregation of sludge particles. The innovative use of the Flory-Huggins theory provided insights into the sludge filtration mechanism from a chemical potential perspective, linking these changes to SFR. The introduction of Fe2+/PMS-PAM conditioning disrupted the uniformity of the EPS-formed gel layer, significantly reducing the chemical potential difference between the permeate and the water in the gel layer, leading to a lower SFR and enhanced dewatering performance. This thermodynamic approach significantly enhances our understanding of sludge dewatering and conditioning. These findings represent a paradigm shift, offering innovative strategies for sludge treatment and expanding our comprehension of dewatering and conditioning techniques.

Surface properties of mycoparasitic Pythium species and their interaction with model materials

Pythium oligandrum, a soil-born oomycete, is an effective biological control agent exhibiting antagonistic and parasitic activity against pathogenic fungi. This study is the first attempt to characterize its surface properties and to apply models of physicochemical interactions (thermodynamic, DLVO and XDLVO) to quantify its adhesion properties to a model material, represented by magnetic beads (MB). The predictions of interaction models were based on experimental data (contact angles, zeta potentials, size). Adhesion intensities (AI) were determined experimentally taking advantage of MB with different surface properties. The role of weak physicochemical interactions was estimated by comparing experimental AI with model predictions. The results revealed that the surface properties of the three Pythium spp. studied were very similar and fell within the range for hydrophilic microorganisms (ΔGTOT > 0) with a predominantly negative surface charge. The most reliable description of AI was obtained using the DLVO model, including Lifshitz-van der Waals and electrostatic interactions. The highest AI between Pythium spp. and all three MB was observed at pH 3, which was supported by the DLVO prediction. The greater agreement between the sphere-sphere geometric version of the DLVO model and experiment suggests that the surface protrusions of the oospores increase the efficiency of adhesion. The surface properties of the pathogenic fungi, characterized in this work, fell within the range defined by MB and therefore it can be expected that their physicochemical interactions with Pythium spp. will also be favourable.

Corrosion-induced changes in surface properties and roughness of orthodontic wires

INTRODUCTION

This study aimed to investigate the surface free energy and surface roughness (SR) of metallic alloys under the influence of acid solutions.

METHODS

The experiment involved the use of 270 rectangular wire samples measuring 0.019 × 0.025-in. These samples were sourced from 3 different commercial brands: Dentsply GAC, American Orthodontics, and Orthoclassic. This in vitro study categorized the samples into 3 groups based on the solutions employed: deionized water, citric acid, and phosphoric acid. Each group consisted of 90 samples, with 30 samples representing each type of alloy-stainless steel, nickel-titanium, and titanium molybdenum alloy (TMA). The wire segments were immersed in their respective solutions for 72 hours at a controlled temperature of 37°C, with continuous orbital agitation at 130 rpm. After the immersion period, the study analyzed both surface free energy and SR. The mean values obtained were subjected to an analysis of variance at a significance level of 5%.

RESULTS

All alloys displayed hydrophobic behavior, as indicated by interaction free energy values <0. In acidic environments (phosphoric acid and citric acid), significant differences were observed among different brands and alloys, affecting surface energy and interaction free energy. Variations in SR among metallic alloys included steel with the lowest SR variations, followed by nickel-titanium and TMA. Notably, the TMA alloy stood out with significantly higher surface energy compared with that of the other alloys (P <0.001).

CONCLUSIONS

In this study, all examined alloys demonstrated a hydrophobic nature, suggesting a limited attraction to water. Notably, TMA exhibited the least hydrophobic behavior among the alloys studied. However, when exposed to citric acid, TMA displayed the most substantial alterations in its surface properties. These results underscored the significance of accounting for the distinctive properties of each alloy and their responses to diverse challenges, such as exposure to acidic solutions, during the selection of orthodontic wires for orthodontics treatment.

Cation Chain Length of Nonhalogenated Ionic Liquids Matters in Enhancing SERS of Cytochrome c on Zr-Al-Co-O Nanotube Arrays

Surface-enhanced Raman scattering (SERS) is a remarkably powerful analytical technique enabling trace-level detection of biological molecules. The interaction of a probe molecule with the SERS substrate shows important distinctions in the SERS spectra, providing inherent fingerprint information on the probe molecule. Herein, nonhalogenated phosphonium-based ionic liquids (ILs) containing cations with varying chain lengths were used as trace additives to amplify the interaction between the cytochrome c (Cyt c) and Zr-Al-Co-O (ZACO) nanotube arrays, strengthening the SERS signals. An increased enhancement factor (EF) by 2.5-41.2 times compared with the system without ILs was achieved. The improvement of the SERS sensitivity with the introduction of these ILs is strongly dependent on the cation chain length, in which the increasing magnitude of EF is more pronounced in the system with a longer alkyl chain length on the cation. Comparing the interaction forces measured by Cyt c-grafted atomic force microscopy (AFM) probes on ZACO substrates with those predicted by the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, the van der Waals forces became increasingly dominant as the chain length of the cations increased, associated with stronger Cyt c-ZACO XDLVO interaction forces. The major contributing component, van der Waals force, stems from the longer cation chains of the IL, which act as a bridge to connect Cyt c and the ZACO substrate, promoting the anchoring of the Cyt c molecules onto the substrate, thereby benefiting SERS enhancement.

Durable Nano-Flower Structured Foam Coupled with Electrically-Driven in Situ Aeration Enable High-Flux Oil/Water Emulsion Separation with Dynamic Antifouling Ability

The conventional membranes used for separating oil/water emulsions are typically limited by the properties of the membrane materials and the impact of membrane fouling, making continuous long-term usage unachievable. In this study, a filtering electrode with synchronous self-cleaning functionality is devised, exhibiting notable antifouling ability and an extended operational lifespan, suitable for the continuous separation of oil/water emulsions. Compared with the original Ti foam, the in situ growth of NiTi-LDH (Layered double hydroxide) nano-flowers endows the modified Ti foam (NiTi-LDH/TF) with exceptional superhydrophilicity and underwater superoleophobicity. Driven by gravity, a rejection rate of over 99% is achieved for various emulsions containing oil content ranging from 1% to 50%, as well as oil/seawater emulsions. The flux recovery rate exceeds 90% after one hundred cycles and a 4-h filtration period. The enhanced separation performance is realized through the "gas bridge" effect during in situ aeration and electrochemical anodic oxidation. The internal aeration within the membrane pores contributes to the removal of oil foulants. This study underscores the potential of coupling foam metal filtration materials with electrochemical technology, providing a paradigm for the exploration of novel oil/water separation membranes.

Multitasking functions of bacterial extracellular DNA in biofilms

Bacterial biofilms are intricate ecosystems of microbial communities that adhere to various surfaces and are enveloped by an extracellular matrix composed of polymeric substances. Within the context of bacterial biofilms, extracellular DNA (eDNA) originates from cell lysis or is actively secreted, where it exerts a significant influence on the formation, stability, and resistance of biofilms to environmental stressors. The exploration of eDNA within bacterial biofilms holds paramount importance in research, with far-reaching implications for both human health and the environment. An enhanced understanding of the functions of eDNA in biofilm formation and antibiotic resistance could inspire the development of strategies to combat biofilm-related infections and improve the management of antibiotic resistance. This comprehensive review encapsulates the latest discoveries concerning eDNA, encompassing its origins, functions within bacterial biofilms, and significance in bacterial pathogenesis.

Synthesis, Characterization, and Evaluation of the Antimicrobial Effects and Cytotoxicity of a Novel Nanocomposite Based on Polyamide 6 and Trimetaphosphate Nanoparticles Decorated with Silver Nanoparticles

This study aimed to develop a polymeric matrix of polyamide-6 (P6) impregnated with trimetaphosphate (TMP) nanoparticles and silver nanoparticles (AgNPs), and to evaluate its antimicrobial activity, surface free energy, TMP and Ag+ release, and cytotoxicity for use as a support in dental tissue. The data were subjected to statistical analysis (p < 0.05). P6 can be incorporated into TMP without altering its properties. In the first three hours, Ag+ was released for all groups decorated with AgNPs, and for TMP, the release only occurred for the P6-TMP-5% and P6-TMP-10% groups. In the inhibition zones, the AgNPs showed activity against both microorganisms. The P6-TMP-2.5%-Ag and P6-TMP-5%-Ag groups with AgNPs showed a greater reduction in CFU for S. mutans. For C. albicans, all groups showed a reduction in CFU. The P6-TMP groups showed higher cell viability, regardless of time (p < 0.05). The developed P6 polymeric matrix impregnated with TMP and AgNPs demonstrated promising antimicrobial properties against the tested microorganisms, making it a potential material for applications in scaffolds in dental tissues.

Enzyme-enhanced acidogenic fermentation of waste activated sludge: Insights from sludge structure, interfaces, and functional microflora

Anaerobic fermentation is widely installed to recovery valuable resources and energy as CH4 from waste activated sludge (WAS), and its implementation in developing countries is largely restricted by the slow hydrolysis, poor efficiency, and complicate inert components therein. In this study, enzyme-enhanced fermentation was conducted to improve sludge solubilization from 283 to 7728 mg COD/L and to enhance volatile fatty acids (VFAs) yield by 58.6 % as compared to the conventional fermentation. The rapid release of organic carbon species, especially for tryptophan- and tyrosine-like compounds, to outer layer of extracellular polymeric substance (EPS) occurred to reduce the structural complexity and improve the sludge biodegradability towards VFAs production. Besides, upon enzymatic pretreatment the simultaneous exposure of hydrophilic and hydrophobic groups on sludge surfaces increased the interfacial hydrophilicity. By quantitative analysis via interfacial thermodynamics and XDLVO theory, it was confirmed that the stronger hydrophilic repulsion and energy barriers in particle interface enhanced interfacial mass transfer and reactions involved in acidogenic fermentation. Meanwhile, these effects stimulate the fermentation functional microflora and predominant microorganism, and the enrichment of the hydrolytic and acid-producing bacteria in metaphase and the proliferation of acetogenic bacteria, e.g., Rubrivivax (+9.4 %), in anaphase also benefits VFAs formation. This study is practically valuable to recovery valuable VFAs as carbon sources and platform chemicals from WAS and agriculture wastes.

Preparation of bioactive film based on chitosan and essential oils mixture for enhanced preservation of food products

Recently, the concept of biodegradable and bioactive packaging and surface coating has become a trend. In this work, the bioactive films of chitosan were elaborated following the casting method. Contrary to the films containing the Cinnamomum zeylanicum Blume, Thymus satureioides Cosson, and Syzygium aromaticum essential oils (EOs) mixtures, the control film was thin, colorless, and showed high moisture content, swelling degree, and elongation at break. Concerning the physicochemical parameters, the incorporation of the EOs mixtures minimized the hydrophobicity of the material (θw < 65°) and modified randomly its surface free energy components (γ-; γ+; γLW). The theoretical prediction of Aspergillus sp. and Rhizopus sp. adherence to the chitosan-based films was relatively correlated to the experimental results (r = -0.601). The latter showed that 6.80 % and 19.02 % of the control film surface was covered by Aspergillus sp. and Rhizopus sp. spores, respectively. In contrast, no fungal adherence was noticed in the case of the film incorporating the triple EOs mixture. These promising results revealed that chitosan film containing C. zeylanicum, T. satureioides, and S. aromaticum EOs mixtures could be utilized as a surface coating or bioactive packaging in the food industry.

The MATH test. A three-phase assay?

This study aimed to investigating the possible interference caused by glass test tubes on the quantification of bacterial adhesion to hydrocarbons by the MATH test. The adhesion of four bacteria to hexadecane and to glass test tubes was evaluated employing different suspending polar phases. The role of the ionic strength of the polar phase regarding adhesion to glassware was investigated. Within the conditions studied, Gram-positive bacteria adhered to both the test tube and the hydrocarbon regardless of the polar phase employed; meanwhile, Escherichia coli ATCC 25922 did not attach to either one. The capacity of the studied microorganisms to adhere to glassware was associated with their electron-donor properties. The ionic strength of the suspending media altered the patterns of adhesion to glass in a strain-specific manner by defining the magnitude of electrostatic repulsion observed between bacteria and the glass surface. This research demonstrated that glass test tubes may interact with suspended bacterial cells during the MATH test under specific conditions, which may lead to overestimating the percentage of adhesion to hydrocarbons and, thus, to erroneous values of cell surface hydrophobicity.

Quantifying interfacial interactions for improved membrane antifouling: A novel approach using triangulation and surface element integration method

To gain a thorough understanding of interfacial behaviors such as adhesion and flocculation controlling membrane fouling, it is necessary to simulate the actual membrane surface morphology and quantify interfacial interactions. In this work, a new method integrating the rough membrane morphology reconstruction technology (atomic force microscopy (AFM) combining with triangulation technique), the surface element integration (SEI) method, the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, the compound Simpson's approach, and the computer programming was proposed. This new method can exactly mimic the real membrane surface in terms of roughness and shape, breaking the limitation of previous fractal theory and Gaussian method where the simulated membrane surface is only statistically similar to the real rough surface, thus achieving a precise description of the interfacial interactions between sludge foulants and the real membrane surface. This method was then applied to assess the antifouling propensity of a polyvinylidene fluoride (PVDF) membrane modified with Ni-ZnO particles (NZPs). The simulated results showed that the interfacial interactions between sludge foulants in a membrane bioreactor (MBR) and the modified PVDF-NZPs membrane transformed from an attractive force to a repulsive force. The phenomenon confirmed the significant antifouling propensity of the PVDF-NZPs membrane, which is highly consistent with the experimental findings and the interfacial interactions described in previous literature, suggesting the high feasibility and reliability of the proposed method. Meanwhile, the original programming code of the quantification was also developed, which further facilitates the widespread use of this method and enhances the value of this work.

Extracellular Vesicles and Bacteriophages: New Directions in Environmental Biocolloid Research

There is a long-standing appreciation among environmental engineers and scientists regarding the importance of biologically derived colloidal particles and their environmental fate. This interest has been recently renewed in considering bacteriophages and extracellular vesicles, which are each poised to offer engineers unique insights into fundamental aspects of environmental microbiology and novel approaches for engineering applications, including advances in wastewater treatment and sustainable agricultural practices. Challenges persist due to our limited understanding of interactions between these nanoscale particles with unique surface properties and their local environments. This review considers these biological particles through the lens of colloid science with attention given to their environmental impact and surface properties. We discuss methods developed for the study of inert (nonbiological) particle-particle interactions and the potential to use these to advance our understanding of the environmental fate and transport of extracellular vesicles and bacteriophages.

Insights into the membrane biofouling behavior of planktonic anammox bacteria: Effect of solution pH and ionic strength

Understanding the effect of solution pH and ionic strength on membrane biofouling of anammox bacteria is essential for the widespread application of anammox MBRs. To provide an original elucidation, this study combined interfacial thermodynamics analysis and filtration experiments with an established planktonic anammox MBR to explore the biofouling behavior of anammox bacteria under varying solution pH and ionic strengths. Preliminary results showed that variation in solution pH and ionic strength has critical impacts on the thermodynamic properties of planktonic anammox bacteria and membrane surfaces. The further interfacial thermodynamics analysis and filtration experiments indicated that an increased pH and a decreased ionic strength could reduce membrane fouling by planktonic anammox bacteria. More specifically, a higher pH or lower ionic strength resulted in a stronger repulsive energy barrier due to the larger interaction distance covered by the dominant electrostatic double layer (EL) component compared to the Lewis acid-base (AB) and Lifshitz-van der Waals (LW) components, which corresponded to a reduction in the normalized flux (J/J₀) decline and the accumulation of cake resistance (R_c) during the filtration process. Furthermore, the aforementioned effect mechanism was verified by a correlation analysis of the thermodynamic properties and filtration behavior. These findings have generalized significance for understanding the biofouling or aggregation behavior of anammox bacteria.

Programmable Interactions of Cellulose Acetate with Octadecyltrichlorosilane-Functionalized SiO2 Nanoparticles

It is significant to understand the interfacial interactions involved between the cellulose acetate (CA) and dispersed nanomaterials, in which the enhanced interaction improves the mechanical behavior of CA. In this work, the amendments of CA with SiO₂ nanoparticles have been found to be endowed by grafting varying concentrations (0, 3, 5, and 6%) of octadecyltrichlorosilane (OTS). Aided by SiO₂ colloid probe atomic force microscopy (AFM with a probe diameter of 20 μm), the adhesion force between CA and SiO₂ is found to be programmable by tuning OTS concentrations functionalized onto SiO₂ surfaces. The adhesion forces of 5% OTS-functionalized SiO₂ with CA are the strongest, followed by the ones of 0, 3, and 6% OTS, resulting in a smoother and denser morphology on the film with 5% OTS. The AFM-measured approaching force-distance curves have been further compared to predictions by the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, in which the XDLVO force is summed as the Liftshitz-van der Waals force (F^LW), the electrostatic double-layer force (F^EL), and the acid-base interaction force (F^AB). F^LW and F^EL do not change significantly with OTS concentrations functionalized onto SiO₂. However, F^AB is sensitive to the functionalized OTS concentration onto SiO₂ and significantly contributes to the interaction force of the composite films with 5% OTS, promoting the formation of a smooth and dense surface feature with a considerable mechanical performance demonstrated by load-displacement curves from a nanoindenter. This is highly encouraging and suggests that nanomaterials can be incorporated into CA to effectively improve their mechanical compatibility by programming the interaction between the CA matrix and nanomaterials.

Colloid thermophoresis in the dilute electrolyte concentration regime: from theory to experiment

Colloid thermophoresis in aqueous media is vital for numerous applications in nanoscience and life sciences. To date, a general description of colloid thermophoresis in DI water has not been determined. Here, we describe a theoretical model within the framework of the Fokker-Planck formalism and the flickering cluster concept to describe the hydration entropy effect on the thermophoretic behaviour of colloids suspended in DI water and compare this to new experimental results. We built an experimental platform to allow for rapid and robust temperature control and investigate the thermophoretic behaviour of silica microspheres with different sizes at various background temperatures for comparison. In this work, the ionic shielding effect is accounted for by using the well-known Duhr-Dhont's model, and the hydration layer effect is determined using the developed theoretical model. For the latter, our model reveals that the sign of the Soret coefficient is governed by the interplay between the binding energy and the chemical potential of water molecules, which were found to be in the same order of magnitude. We show that our analysis accurately describes the experimental behaviour of colloidal particles that opens a new avenue for developing versatile trapping and separation techniques for various colloidal particles in aqueous systems according to their size and background temperature.

Critical role of extracellular DNA in the establishment and maintenance of anammox biofilms

Anaerobic ammonium oxidation (anammox) has been widely used for the sustainable removal of nitrogen from wastewater. Extracellular DNA (exDNA), as one of the main components of biofilms, not only determines the initial formation process, but also allows the three-dimensional structure to be maintained. Since the effects of exDNA on anammox biofilm formation are still poorly understood, this study elucidated the effects of exDNA on different stages of anammox biofilm establishment and maintenance under static conditions and its mechanism. The results revealed that exDNA mainly affected the maintenance stage of anammox biofilm formation. Compared with the absence of exDNA, nitrogen removal efficiency in the presence of exDNA was 6.17 % higher; the number of bacteria cells attached to the carrier was 2.23 times that in the absence of exDNA. The spatiotemporal distribution of bacteria was revealed by fluorescence in situ hybridization. After 30 days, the relative abundances of anammox in biofilms were 6.19 % and 0.4 % in the presence and absence of exDNA, respectively, indicating its positive role in anammox bacteria (AnAOB) adhesion and biofilm formation. The presence of exDNA in extracellular polymeric substances (EPS) promotes the synthesis of proteins and soluble microbial products. According to the extended Derjaguin-Landau-Verwey-Overbeek (X - DLVO) theory, the presence of exDNA also reduced the Lewis acid-base interaction energy and created favorable thermodynamic conditions for AnAOB adhesion. These findings advance our understanding of the role of exDNA in anammox-mediated biofilm formation and offer insights into the mechanism of exDNA in the establishment and maintenance stages.

Composite carrier enhanced bacterial adhesion and nitrogen removal in partial nitrification/anammox process

The rapid start-up and stable operation of one-stage (Partial nitrification/anammox) PN/A process for low-ammonium wastewater are difficult to be achieved, and many carriers are designed to solve this problem. Here, a composite carrier was developed, in which sepiolite and non-woven fabrics were assembled in polypropylene spherical shells. At the start-up phase, P_A reactor using the composite carriers reached a higher nitrogen removal rate of 134.50 ± 19.60 mg·N·L^-1d^-1, in contrast to that of 48.85 ± 19.64 mg·N·L^-1d^-1 in the P_B reactor without sepiolite carriers. When the final influent ammonium concentration of PN/A process is 100 mg/L, the total nitrogen removal efficiency can reach 72 ± 0.03 %. High biomass immobilization ability of composite carrier was evidenced by the greater adsorption trend between sludge and sepiolite than that between sludge and non-woven fabrics, where hydrophobic interaction and Van der Waals interaction played a major role. Extracellular protein (PN) content and the ratio of PN and extracellular polysaccharide of samples in P_A were significantly higher than those in P_B, verifying higher biofilm formation ability on the composite carrier. The composite carrier also increased the abundance of dominant bacteria in PN/A process, especially AOB, the relative abundance of which reached 46.11 %. And it increased the abundance of essential functional genes for nitrogen conversion as their perfect acid neutralizing effects. This study is of great significance in improving the start-up speed and stable operation of this process.

Elaboration and general evaluation of chitosan-based films containing terpene alcohols-rich essential oils

Recently, the scientific community is interested in the synthesis of biodegradable and bioactive packaging to replace oil-based ones. Therefore, the present study aims to elaborate an active and biodegradable material using chitosan (CS-film) combined with pelargonium, tea tree, marjoram, and thyme essential oils (EOs), and then evaluate their different properties and biological activities. The obtained data showed an augmentation in CS-film thickness and opacity following the addition of EOs ranging from 17 ± 3 to 42 ± 2 μm and from 1.53 ± 0.04 to 2.67 ± 0.09, respectively. Furthermore, a significant decrease in the water vapor transmission rate and moisture content parameters was recorded as regards the treated CS-films. On the other hand, the treatment with EOs engenders random modifications in the physicochemical and mechanical characteristics of the material. Concerning the biological activities, the treated CS-films scavenged around 60% of DPPH radical while the control CS-film exhibited a negligible antioxidant activity. Finally, the CS-films containing pelargonium and thyme EOs exhibited the strongest antibiofilm-forming activity against Escherichia coli, Enterococcus hirae, Staphylococcus aureus, and Pseudomonas aeruginosa with values of inhibition greater than 70%. These encouraging results verify the effectiveness of CS-films containing EOs such as pelargonium and thyme EOs as biodegradable and bioactive packaging.

High propensity of membrane fouling and the underlying mechanisms in a membrane bioreactor during occurrence of sludge bulking

While sludge bulking often occurring in activated sludge processes generally leads to serious membrane fouling in membrane bioreactors (MBR), the underlying causes are still unclear. In this study, fouling behaviors of a MBR operated at stages of normal and sludge bulking were compared, and the fouling mechanisms of the different behaviors were explored. It was found that, the MBR could be stably operated in normal stage without membrane cleaning for about 60 days, whereas, daily membrane cleaning had to be carried out when operated in sludge bulking stage. The bulking sludge possessed a rather high specific filtration resistance (SFR) of about 1.36×10¹⁴ m·kg^-1, which is over 5.33 times than that of the normal sludge. A series of characterizations demonstrated that the bulking sludge had rather lower dewaterability, smaller particle size, higher fractal dimension, higher viscosity, abundant filamentous bacteria and different functional groups of extracellular polymer sustains (EPS). It was suggested that microbial community transition was responsible for the occurrence of sludge bulking, further affecting membrane fouling. Based on these characterizations, it was reported that adhesion propensity (indicated by the thermodynamic interaction) of the bulking sludge to the membrane surface is about 3.6 times than that of the normal sludge. It was proposed that, extra force should be provided to offset a chemical potential gap caused by foulant layer structure transition during sludge bulking in order to sustain filtration of the bulking sludge, resulting in extremely high SFR. This study offered deep thermodynamic mechanisms of MBR fouling during occurrence of sludge bulking.

The effect of essential oils mixture on chitosan-based film surface energy and antiadhesion activity against foodborne bacteria

In the food sector, the formation of biofilms as a result of microbial adherence on food-grade surfaces causes a major problem resulting in significant economic losses. Thereby, this work aimed to elaborate a biodegradable film using chitosan (CS-film) and reinforce its antiadhesion activity by incorporating pelargonium, clove, thyme, and cinnamon essential oils (EOs). Firstly, the antibacterial activity of these EOs alone and combined against four foodborne bacteria were analyzed by the microdilution method. Synergism was observed in the case of EOs combination. Secondly, the physicochemical characteristics and antiadhesion behavior of the CS-films were assessed by the contact angle method and ESEM, respectively. Results revealed that the EOs mixture treatment impacted considerably the physicochemical characteristics of the CS-film and reduced its qualitative and quantitative hydrophobicity. Moreover, the treated CS-film showed a strong antiadhesion behavior against Enterococcus hirae, Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus with percentages of non-covered surface equal to 97.65 ± 1.43%, 98.76 ± 0.32%, 99.68 ± 0.28%, and 95.63 ± 1.32% respectively. From all these results, the CS-film treated with the mixture of EOs presents a great potential for application as surface coating and food packaging preventing microbial adhesion and thus, avoiding food contamination and spoilage.

Surface characterisation reveals substrate suitability for cyanobacterial phototaxis

Cyanobacteria respond to light stimulation, activating localised assembly of type IV pili for motility. The resulting phototactic response is highly dependent on the nature of the incoming light stimulus, and the final motility parameters depend on the surface properties. Conventionally, phototaxis studies are carried out on hydrogel surfaces, such as agarose, with surface properties that vary in time due to experimental conditions. This study considers five substrates, widely utilized in microfluidic technology, to identify the most suitable alternative for performing reliable and repeatable phototaxis assays. The surfaces are characterised via a contact angle goniometer to determine the surface energy, white light interferometry for roughness, zeta-potentials and AFM force distance curves for charge patterns, and XPS for surface composition. Cell motility assays showed 1.25 times increment on surfaces with a water contact angle of 80° compared to a reference glass surface. To prove that motility can be enhanced, polydimethylsiloxane (PDMS) surfaces were plasma treated to alter their surface wettability. The motility on the plasma-treated PDMS showed similar performance as for glass surfaces. In contrast, untreated PDMS surfaces displayed close to zero motility. We also describe the force interactions of cells with the test surfaces using DLVO (Derjaguin-Landau-Verwey-Overbeek) and XDLVO (extended DLVO) theories. The computed DLVO/XDLVO force-distance curves are compared with those obtained using atomic force microscopy. Our findings show that twitching motility on tested surfaces can be described mainly from adhesive forces and hydrophobicity/hydrophilicity surface properties. STATEMENT OF SIGNIFICANCE: The current article focuses on unravelling the potential Micro-Electro-Mechanical System (MEMS) compatible surfaces for studying phototactic twitching motility of cyanobacteria. This is the first exhaustive surface characterization study coupled with phototaxis experiments, to understand the forces contributing to twitching motility. The methods shown in this paper can be further extended to study other surfaces and also to other bacteria exhibiting twitching motility.

Marine microalgal biofilm development and its adhesion propensities on commercial membrane via XDLVO approach

An emerging biofilm immobilization method has enabled effortless biomass harvesting and promoted economic feasibility. The current limitation towards the adaptation of this technology is the inadequate understanding of the biofilm interaction towards microporous membrane. Cell adhesion is recognized as the most important step towards the immobilized cultivation of microalgae. Cell attachment kinetic was studied in a short-term batch culture of three marine diatoms, Amphora coffeaeformis, Cylindrotheca fusiformis and Navicula incerta over 96 h on submerged commercial polyvinylidene fluoride (PVDF) membrane under swirling motion of culture medium. Both the evolution of cell adhesion intensity and compositional changes of the extracellular polymeric substances (EPS) released were quantified throughout the cultivation period. To delve into the cell-substratum interactions, existing thermodynamics and colloidal extended Derjaguin, Landau, Vervey, and Overbeek (XDLVO) theory were employed. As a result, A. coffeaeformis and N. incerta recorded a higher cell colonization percentage than C. fusiformis being the lowest about 2.16±0.17% cell colonization due to their respective species-dependent EPS variation. Polysaccharide contents were at least two times higher than protein contents for both C. fusiformis and N. incerta except for A. coffeaeformis depicting a lower polysaccharide-to-protein ratio whereby the protein contents were maximized at 1.03 × 103 ± 64.14 pg m^-2 cell^-1 at 6th h. From the surface free energy point of view, both thermodynamics and XDLVO model elucidated that cells adhered reversibly in the secondary energy minimum and ranked C. fusiformis the lowest adhesion tendency among three. These findings establish fundamental knowledge about biofilm formation in porous substrate bioreactors.

Surface characterization of indirect restorative materials submitted to different etching protocols

OBJECTIVES

This in vitro study aimed to evaluate the effect of different times and concentrations of hydrofluoric acid etching on the surface of indirect restorative materials obtained from blocks used in CAD-CAM technology.

METHODS

Specimens (4 mm × 4 mm × 0.8 mm) were obtained for each indirect restorative material: zirconia-reinforced lithium monosilicate ceramic (Celtra Duo), nanoceramic resin (Lava Ultimate), and polymer-infiltrated ceramic network material (Vita Enamic). The materials were submitted to etching with 5% or 10% hydrofluoric acid for 20, 40, 60, or 90 s. A control group for each material was evaluated without any surface treatment, totaling nine experimental groups for each material (n = 10). The specimens were evaluated for surface roughness (R_a and R_z), confocal optical microscopy, the contact angle (θ), surface free energy (γs), total free interaction energy (∆G) using a goniometer, and microshear bond strength to resin cement. Specimen images were obtained using scanning electron microscopy, confocal optical microscopy, and atomic force microscopy. Data on the surface roughness, the contact angle, surface free energy, total free interaction energy, and bond strength were subjected to two-way ANOVA and Tukey´s test (α=0.05).

RESULTS

In general, Celtra Duo showed better results after etching with 10% hydrofluoric acid for 40 or 60 s. Lava Ultimate showed better performance after etching with 10% hydrofluoric acid for 20 or 40 s, whereas Vita Enamic showed better results after etching with 5% hydrofluoric acid for 90 s.

CONCLUSION

Each material showed different characteristics after etching with hydrofluoric acid. Knowledge of the proper protocol for each material is essential to ensure improvements in the adhesion process and durability of indirect restorations. In general, Celtra Duo presented mechanical properties superior to those of Lava Ultimate and Vita Enamic.

CLINICAL SIGNIFICANCE

Specific etching protocols must be recommended for each indirect material because longer exposure to hydrofluoric acid can jeopardize the surface, thus affecting the mechanical and bond strength properties.

Assessing organic fouling of ultrafiltration membranes using partition coefficients of dissolved organic matter in aqueous two-phase systems

Organic fouling caused by dissolved organic matter (DOM) is a critical challenge for membrane technologies. In this study, prediction models for the fouling of commercial polyether sulfone (PES) and regenerated cellulose (RC) ultrafiltration membranes by DOM were established based on the hydrophobicity of DOM. The organic fouling behavior of 40 natural water samples collected from Lake Taihu was investigated. The fouling propensity of water samples on ultrafiltration membranes was evaluated using the fouling index (FI). The hydrophobicity of DOM in water samples was quantified by its partition coefficient in an aqueous two-phase system (K_ATPS). The FI of water samples on RC membranes was lower than that on PES membranes due to stronger repulsive Lewis acid-base interactions, which reduced DOM-membrane interactions. A significant positive correlation was found between K_ATPS and FI, suggesting the important role of DOM hydrophobicity in the organic fouling of ultrafiltration membranes. FI prediction models using K_ATPS as the variable were established using a training group containing 20 water samples for PES and RC membranes, respectively. The resulting models were then validated using the additional 20 water samples, which suggested good prediction power (RMSE = 1.65). The pH effect on the organic fouling can be adequately predicted by the same model with K_ATPS values measured at given pH. The results suggest that K_ATPS can be used as a convenient index for assessing the initial organic fouling of ultrafiltration membranes by freshwater DOM.

Mechanistic insights into Ca-alginate gel-associated membrane fouling affected by ethylene diamine tetraacetic acid (EDTA)

While transparent exopolymer particles (TEP) is a major foulant, and ethylene diamine tetraacetic acid (EDTA) is a strong chelating agent frequently used for fouling mitigation in membrane-based water treatment processes, little has been known about TEP-associated membrane fouling affected by EDTA. This work was performed to investigate roles of EDTA addition in TEP (Ca-alginate gel was used as a TEP model) associated fouling. It was interestingly found that, TEP had rather high specific filtration resistance (SFR) of 2.49 × 10¹⁵ m^-1·kg^-1, and SFR of TEP solution firstly decreased and then increased rapidly with EDTA concentration increase (0-1 mM). A series of characterizations suggested that EDTA took roles in SFR of TEP solution by means of changing TEP microstructure. The rather high SFR of TEP layer can be attributed to the big chemical potential gap during filtration described by the extended Flory-Huggins lattice theory. Initial EDTA addition disintegrated TEP structure by EDTA chelating calcium in TEP, inducing reduced SFR. Continuous EDTA addition decreased solution pH, resulting into no effective chelating and accumulation of EDTA on membrane surface, increasing SFR. It was suggested that factors increasing homogeneity of TEP gel will increase SFR, and vice versa. This study revealed the thermodynamic mechanism of TEP fouling behaviors affected by EDTA, and also demonstrated the importance of EDTA dosage and pH adjustment for TEP-associated fouling control.

Effects of polysaccharides' molecular structure on membrane fouling and the related mechanisms

Fouling behaviors of polysaccharides vary with their structure, while the mechanisms underlying this phenomenon remain unexplored. This work was carried out to explore the thermodynamic fouling mechanisms of polysaccharides with different structure. Carrageenan and xanthan gum were selected as the model polysaccharides with structure of straight and branch chains, respectively. Batch filtration experiments showed that xanthan gum solution corresponded to a more rapid flux decline trend, and specific filtration resistance (SFR) of xanthan gum (2.32 × 10¹⁵ m^-1 kg^-1) was over 10 times than that of carrageenan (2.21 × 10¹⁴ m^-1 kg^-1). It was found that, xanthan gum possessed a more disordered structure and a rather higher viscosity (15.03 mPa·s V.S. 1.98 mPa·s for carrageenan). Calculation of extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory showed higher adhesion energy of xanthan gum (-42.82 my m^-2 V.S. -23.26 mJ m^-2 for carrageenan). Scanning electron microscopy (SEM) analyses showed that xanthan gum gel layer had a more homogenous structure and rigid polymer backbone, indicating better mixing with water to form a gel. As verified by heating experiments, such a structure tended to contain more bound water. According to this information, Flory-Huggins lattice theory was introduced to build a bridge between polymeric structure and SFR. It was revealed that branch structure corresponded to higher chemical potential change during gel layer formation, and higher ability to carry bound water, resulting in higher filtration resistance during filtration process. This work revealed the fundamental thermodynamic mechanism of membrane fouling caused by polysaccharides with different structure, deepening understanding of membrane fouling.

New insights into the anti-erosive property of a sugarcane-derived cystatin: different vehicle of application and potential mechanism of action

A new sugarcane-derived cystatin (CaneCPI-5) showed anti-erosive properties when included in solutions and strong binding force to enamel, but the performance of this protein when added to gel formulations and its effect on surface free energy (SFE) requires further studies.

Differences in exopolysaccharides of three microbial aggregates

Different microbial aggregates show substantial differences in morphology, and extracellular polymer substances have been confirmed to play a key role in the formation of aggregates. In this study, three different microbial aggregates and their exopolysaccharides were compared. The results show that the granular sludge was largest in size and the most compact in shape. Biofilms with a certain thickness had the next greatest density, and flocculent sludge, with the smallest particle size, was the loosest. The extended Derjaguin-Landau-Verwey-Overbeek analysis shows that hydrogen bonding, hydrophobic and electrostatic interactions affect the aggregation of microorganisms. A comparison of exopolysaccharides shows that granular sludge exopolysaccharides show the highest hydrophobicity (38.08%) and lowest surface charge (-20.5 mV), followed by biofilm exopolysaccharides (27.9% and -24.8 mV respectively). The results of Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy show that the contents of hydrophilic and hydrophobic functional groups and charged functional groups of exopolysaccharides affect the above properties of exopolysaccharides, thereby affecting microbial aggregation. In addition, the hydrogen bond content of exopolysaccharides in granular sludge (19.3%), biofilm (19.2%) and activated sludge (18.9%) decreased sequentially. This also affects the cross-linking of microbial exopolysaccharides to form hydrogels. Finally, the results of confocal laser scanning microscopy showed that, different from the other two aggregates, the extracellular α-polysaccharides of granular sludge are mainly distributed in the nucleus, which is more conducive to aggregation. The research results of this thesis provide a new understanding of the differences in the aggregation morphology of different aggregates from the perspective of exopolysaccharides.

Fundamental thermodynamic mechanisms of membrane fouling caused by transparent exopolymer particles (TEP) in water treatment

While transparent exopolymer particles (TEP) has high fouling potential, its underlying fouling mechanisms have not yet been well revealed. In current work, fouling characteristics of TEP under different Ca²⁺ concentrations (0 to 1.5 mM) were investigated. TEP quantification and filtration tests showed that TEP contents increased with Ca²⁺ concentration, while TEP's specific filtration resistance (SFR) under the influence of Ca²⁺ concentration presented a unimodal pattern. The peak of TEP's SFR reached at Ca²⁺ concentration of 1 mM when SA concentration was 0.3 g·L^-1. A series of characterizations suggested that microstructure transformation of TEP particles was the main contributor to the resistance variations of TEP solution. The optical microscope observation showed that above and below the critical Ca²⁺ concentration (1 mM when SA concentration is 0.3 g·L^-1 in this study), the formed TEP existed in the form of c-TEP (average particle size is 0.24 μm) and p-TEP (average particle size is 1.05 μm), respectively. Thermodynamic analysis showed that the adhesion ability of c-TEP (-249,989 and - 303,692 kT) was more than 19 times than that of p-TEP (-12,905 kT), which would accelerate foulant layer formation. In addition, below the critical value, the increased SFR with Ca²⁺ concentration could be explained by integrating Flory-Huggins lattice theory with the preferential intermolecular coordination. Above the critical value, the decreased SFR can be attributed to the formation of a "large-size crack structure" cake layer from the p-TEP. This study revealed fundamental mechanisms of membrane fouling caused by TEP, greatly deepening understanding of TEP fouling, and facilitating to development of effective fouling control strategies.

Review of Label-Free Monitoring of Bacteria: From Challenging Practical Applications to Basic Research Perspectives

Novel biosensors already provide a fast way to detect the adhesion of whole bacteria (or parts of them), biofilm formation, and the effect of antibiotics. Moreover, the detection sensitivities of recent sensor technologies are large enough to investigate molecular-scale biological processes. Usually, these measurements can be performed in real time without using labeling. Despite these excellent capabilities summarized in the present work, the application of novel, label-free sensor technologies in basic biological research is still rare; the literature is dominated by heuristic work, mostly monitoring the presence and amount of a given analyte. The aims of this review are (i) to give an overview of the present status of label-free biosensors in bacteria monitoring, and (ii) to summarize potential novel directions with biological relevancies to initiate future development. Optical, mechanical, and electrical sensing technologies are all discussed with their detailed capabilities in bacteria monitoring. In order to review potential future applications of the outlined techniques in bacteria research, we summarize the most important kinetic processes relevant to the adhesion and survival of bacterial cells. These processes are potential targets of kinetic investigations employing modern label-free technologies in order to reveal new fundamental aspects. Resistance to antibacterials and to other antimicrobial agents, the most important biological mechanisms in bacterial adhesion and strategies to control adhesion, as well as bacteria-mammalian host cell interactions are all discussed with key relevancies to the future development and applications of biosensors.

An integrated investigation on anaerobic membrane-based thickening of fecal sludge and the role of extracellular polymeric substances (EPS) in solid-liquid separation

Pre-thickening or primary treatment of fecal sludge (FS) is a major bottleneck in designing fecal sludge treatment plants. This research demonstrates a practical analysis to improve the thickening efficiency of FS using woven fiber microfiltration membrane. A laboratory-scale anaerobic membrane-based thickening tank (MBTT) was investigated. Firstly, the system was operated with unconditioned FS at a flux range of 1-3 L/m²h. Secondly, the system was operated at an optimized flux of 2 L/m²h with conditioners, chitosan, and charcoal dust, at their optimized dosages reported in previous studies. It was observed that the solids accumulation in MBTT was linear and was specific to net solids accumulation. Feed FS was thickened to around 15% of total solids (TS) in 9-14 days. The overall solids accumulation rate was higher with conditioned FS. The less EPS accumulation and higher dewaterability in conditioned FS reduced the brushing frequency of the membrane and consequently, the average filtration duration per cycle was increased. Strong correlations of dewatering time with floc size, EPS, and electrical conductivity, indicated that higher EPS in FS tends to form flocs which can increase the dewatering rate in unstabilized FS. In permeate, the average TS and COD_t removal observed were 72-78% and 87-91%, respectively.

Phosphorus deficiency leads to the loosening of activated sludge: The role of exopolysaccharides in aggregation

Whether phosphorus deficiency in influent will affect the aggregation and sedimentation of activated sludge needs to be further clarified. This paper systematically studied the structure, aggregation and settlement of activated sludge, and the composition, properties and chemical structure of extracellular polymers and microbial community structure of sludge under different influent phosphorus contents to determine the causes of sludge aggregation and structural deterioration. The results show that phosphorus deficiency in influent leads to a decrease in the aggregation capacity and a loose structure of activated sludge, and the reduction of hydrophobic interactions is the main factor of sludge aggregation and structural deterioration. The content, functional groups and protein secondary structure of extracellular protein were almost unchanged. An increase in the content and hydrophilicity of extracellular polysaccharide (PS) results in a decrease in sludge hydrophobicity. Under phosphorus deficiency, the changes in microbial species related to PS secretion were the reasons for the increase in PS content and hydrophilicity. The negative effects of PS content and hydrophilicity on sludge aggregation and structure are important findings of this work and are expected to be useful for better understanding the restoration of activated sludge performance in the treatment of phosphorus-deficient wastewater.

Insight into the role of exopolysaccharide in determining the structural stability of aerobic granular sludge

Extracellular polymeric substances (EPS) have a critical contribution to the stability of aerobic granular sludge (AGS), but the mechanism and details of EPS composition and function are far from clear. This work investigated the contribution of exopolysaccharide (PS) to maintaining the structural stability of AGS. The results revealed that PS hydrolysis induced by α-amylase, dextranase and cellulase significantly decreased the granular stability, whereas a substantial content reduction of extracellular protein (PN) was also observed. It was also found that hydrolysis of PS led to a decrease of sludge hydrophobicity, granular gel mechanical strength by 14.09 %, 38.67 %, respectively, and an increase of surface free energy by 49.59 %, which is not conducive to granular stability. Through fluorescent staining, existence of large amounts of PS and PN conjugates in EPS matrix was verified. It was proposed that these conjugates with PS as skeleton (PS-PN) dominate granular stability by affecting hydrophobicity interactions and hydrogen bonds system, which are two important parameters to gel properties, constituting a crucial finding of this work. This study offers an supplementation of EPS system theory and granular stability mechanism.

Effect of EPS and its forms of aerobic granular sludge on sludge aggregation performance during granulation process based on XDLVO theory

Different forms of extracellular polymeric substances (EPS) play different roles in the formation process of aerobic granular sludge (AGS). This work focused on the contribution of loosely bound EPS (LB-EPS), tightly bound EPS (TB-EPS) and EPS to the aggregation between sludge cells during the start-up of aerobic granular sludge in a sequencing batch reactor. By analyzing the changes of sludge surface characteristics before and after the extraction of each layer of EPS, the contribution of LB-EPS, TB-EPS and EPS to the adhesion and aggregation of sludge cells in the granulation was calculated by surface thermodynamics and the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. The experimental results showed that AGS reactor basically completed the granulation process and kept stable operation within 40 d. In the process of sludge granulation, the effect of LB-EPS on the aggregation of sludge cells shifted from attraction to repulsion. TB-EPS could improve the surface zeta potential and hydrophobicity and show an attractive effect in the granulation process, which was conducive to the adhesion between sludge cells and was the main contributor to the formation of granules. Additionally, EPS played an apparently positive role in sludge flocculation and could promoted cell aggregation in the whole granulation process.

In vitro Evaluation of Surface Free Energy of Dentin after Treatment with Sodium Trimetaphosphate Associated or Not with Fluoride, Exposed or Not to Calcium

It has been stated that sodium trimetaphosphate (TMP) promotes a more anionic dentin surface inducing greater calcium (Ca) and phosphate precipitation. The aim of the present study was to evaluate in vitro the surface free energy (γs) of dentin after treatment with TMP associated or not with fluoride (F), exposed or not to Ca, as well as the adsorption of TMP, F, and Ca by dentin. Bovine dentin blocks (n = 12 blocks/group) were treated with solutions containing TMP at 0, 1, 3, or 9 (w/v) followed or not by the application of Ca. These solutions were or were not associated to 1,100 ppm F. F, Ca, and TMP were determined in the solutions before and after the treatment to calculate the adsorption by dentin. To analyze the γs of dentin, the apolar (γsLW), and polar (γsAB), components were determined by contact angle measurement. Data were submitted to 2-way ANOVA followed by the Student-Newman-Keuls test (p < 0.05). TMP reduces γs of dentin and increases electron donor sites (γs-). Higher values of γs- led to higher adsorption of Ca (p < 0.001). The F/TMP association did not change γs or γsLW and reduced the values of γs-, but the adsorption of Ca was higher. There was correlation between the adsorption of TMP and γs- (Pearson's r = 0.801; p < 0.001) and F (Pearson's r = 0.871, p < 0.001). It is possible to conclude that TMP increased γs- and Ca adsorption, and reduced γs. The association with F increased the adsorption of TMP without rising γs-; however, there was higher adsorption of Ca.

Adhesion Behavior of Escherichia coli Strains on Glass: Role of Cell Surface Qualitative and Quantitative Hydrophobicity in Their Attachment Ability

Microbial adhesion to surfaces is thought to involve physicochemical interactions between the substrate and microbial cells. Understanding the physicochemical aspects involved in the adhesion phenomenon, as a critical step in biofilm formation, is essential to finding ways to prevent their formation and control biocontamination risks. The aim of this study was to investigate the relation between the adhesion behavior of 12 Escherichia coli strains isolated from food and their surface hydrophobicities using qualitative (θ _w ) and quantitative (ΔG _iwi ) approaches. The surface physicochemical properties of both bacterial cells and glass material were estimated through contact angle measurements. The adhesive behavior of E. coli strains on a glass surface was assessed. The results showed a good logarithmic relation between the percentage of the adhered cells and their surface hydrophobicity with the quantitative approach ΔG _iwi ; however, qualitative hydrophobicity (θ _w ) appeared to demonstrate no effect regarding adhesion behavior. This work lays the foundation for future studies and opens an important debate on the mechanisms underlying the adhesion behavior of E. coli strains by using the thermodynamic approach (ΔG _iwi ) as an important model of hydrophobicity that could explain and predict better bacterial adhesion ability.

Physiology of microalgal biofilm: a review on prediction of adhesion on substrates

In view of high energy cost and water consumption in microalgae cultivation, microalgal-biofilm-based cultivation system has been advocated as a solution toward a more sustainable and resource friendlier system for microalgal biomass production. Algal-derived extracellular polymeric substances (EPS) form cohesive network to interconnect the cells and substrates; however, their interactions within the biofilm are poorly understood. This scenario impedes the biofilm process development toward resource recovery. Herein, this review elucidates on various biofilm cultivation modes and contribution of EPS toward biofilm adhesion. Immobilized microalgae can be envisioned by the colloid interactions in terms of a balance of both dispersive and polar interactions among three interfaces (cells, mediums and substrates). Last portion of this review is dedicated to the future perspectives and challenges on the EPS; with regard to the biopolymers extraction, biopolymers’ functional description and cross-referencing between model biofilms and full-scale biofilm systems are evaluated. This review will serve as an informative reference for readers having interest in microalgal biofilm phenomenon by incorporating the three main players in attached cultivation systems: microalgae, EPS and supporting materials. The ability to mass produce these miniature cellular biochemical factories via immobilized biofilm technology will lay the groundwork for a more sustainable and feasible production.

Surface Free Energy, Interaction, and Adsorption of Calcium and Phosphate to Enamel Treated with Trimetaphosphate and Glycerophosphate

This study aimed to evaluate the surface (γs) and interaction (ΔGiwi) free energy and calcium (Ca2+) and phosphate (PO43-) adsorption to dental enamel treated with sodium trimetaphosphate (TMP) or calcium glycerophosphate (CaGP) that had or had not been exposed to CaPO4-containing solutions. Bovine enamel blocks (n = 192; 24 blocks/group) were treated (2 mL/block; 2 min) with TMP (0%, 1%, 3%, and 9%) and CaGP (0, 0.25, 0.5, and 1%) or exposed to a CaPO4-containing solution. The adsorption of these compounds by enamel was assessed before and after treatment. γs and ΔGiwi and their apolar (γsLW and ΔGiwiLW) and polar (γsAB and ΔGiwiAB) components and acid-base interactions (γs+/γs-) were determined by the contact angles. The data were subjected to ANOVA, followed by the Student-Newman-Keuls test (p < 0.05). The adsorption of TMP was dose dependent (p < 0.001), and it reduced γs and γsAB and increased ΔGiwiAB (ΔGiwi > 0) and γs- when compared with the group without TMP (p < 0.001). The immersion in CaPO4-containing solution increased γs and γsAB and reduced ΔGiwiAB (ΔGiwi > 0) and γs- (p < 0.001). There was a correlation between the adsorption of TMP and Ca2+ (r = 0.916; p < 0.001) and PO43- (r = 0.899; p < 0.001). The adsorption of CaGP on the enamel was dose dependent (p < 0.001), reducing γs, ΔGiwiAB (ΔGiwi < 0), γsLW, and γs- when compared to the group without CaGP (p < 0.001). When exposed to the CaPO4-containing solution, there was an increase in ΔGiwiAB (ΔGiwi > 0), γsLW, and γs- and a decrease in γsAB (p < 0.001) without adsorption of Ca2+ by enamel. It may be concluded that TMP and CaGP were adsorbed onto the enamel, producing hydrophilic and hydrophobic surfaces, respectively. TMP produces electron donor sites that induce Ca2+ adsorption, while CaGP releases Ca2+ into the medium.

A critical review on surface modifications mitigating dairy fouling

Thermal treatments performed in food processing industries generate fouling. This fouling deposit impairs heat transfer mechanism by creating a thermal resistance, thus leading to regular shutdown of the processes. Therefore, periodic and harsh cleaning-in-place (CIP) procedures are implemented. This CIP involves the use of chemicals and high amounts of water, thus increasing environmental burden. It has been estimated that 80% of production costs are owed to dairy fouling deposit. Since the 1970s, different types of surface modifications have been performed either to prevent fouling deposition (anti-fouling) or to facilitate removal (fouling-release). This review points out the impacts of surface modification on type A dairy fouling and on cleaning behaviors under batch and continuous flow conditions. Both types of anti-fouling and fouling-release coatings are reported as well as the different techniques used to modify stainless steel surface. Finally, methods for testing and characterising the effectiveness of coatings in mitigating dairy fouling are discussed.

Principal Component Analysis to Determine the Surface Properties That Influence the Self-Cleaning Action of Hydrophobic Plant Leaves

It is well established that many leaf surfaces display self-cleaning properties. However, an understanding of how the surface properties interact is still not achieved. Consequently, 12 different leaf types were selected for analysis due to their water repellency and self-cleaning properties. The most hydrophobic surfaces demonstrated splitting of the ν_s CH₂ and ν CH₂ bands, ordered platelet-like structures, crystalline waxes, high-surface-roughness values, high-total-surface-free energy and apolar components of surface energy, and low polar and Lewis base components of surface energy. The surfaces that exhibited the least roughness and high polar and Lewis base components of surface energy had intracuticular waxes, yet they still demonstrated the self-cleaning action. Principal component analysis demonstrated that the most hydrophobic species shared common surface chemistry traits with low intra-class variability, while the less hydrophobic leaves had highly variable surface-chemistry characteristics. Despite this, we have shown through partial least squares regression that the leaf water contact angle (i.e., hydrophobicity) can be predicted using attenuated total reflectance Fourier transform infrared spectroscopy surface chemistry data with excellent ability. This is the first time that such a statistical analysis has been performed on a complex biological system. This model could be utilized to investigate and predict the water contact angles of a range of biological surfaces. An understanding of the interplay of properties is extremely important to produce optimized biomimetic surfaces.

A novel optical biosensor for in situ and small-scale monitoring of bacterial transport in saturated columns

In situ monitoring techniques can provide new insight into bacterial transport after inoculating exogenous bacteria into contaminated soils for bioremediation. A real-time and non-destructive optical sensor (the optrode) was employed to monitor in situ transport of two fluorescently labelled bacteria - Green Fluorescent Protein (Gfp)-labelled, hydrophilic Pseudomonas putida and Tomato Fluorescent Protein (td)-labelled, hydrophobic Rhodococcus erythropolis, in a saturated sand column with and without rhamnolipid surfactant. In situ measurements were made at three sampling ports in the column with the optrode in two sets of column experiments. In Experiment 1, liquid samples were extracted for ex situ analyses (plate counts and fluorescence), while in Experiment 2 no liquid samples were extracted. Extracting liquid samples for ex situ analyses in Experiment 1 disturbed in situ measurements; in situ measured bacterial concentrations were lower, or a significant lag in breakthrough occurred relative to ex situ measurements. In Experiment 2, the optrode worked well in monitoring bacterial transport, which gave consistent transport parameters at each sampling port. Moreover, the optrode enabled the impact of bacterial hydrophobicity and rhamnolipid surfactant on bacterial transport to be observed. Specifically, hydrophilic P. putida was transported faster through the column than hydrophobic R. erythropolis; we infer from this result that fewer P. putida cells adsorb to sand particles than do R. erythropolis cells. The rhamnolipid surfactant enhanced the transport of both hydrophilic and hydrophobic bacteria. These two observations are consistent with Lifshitz-van der Waals forces and acid-base interactions between bacteria and sand.

Co-transport of negatively charged nanoparticles in saturated porous media: Impacts of hydrophobicity and surface O-functional groups

Graphene oxide (GO) and polystyrene nanoplastic (PSNP) are typical carbonaceous nanomaterials which likely co-exist in soil and sediment. Here, we describe the transport of GO, irradiation reduced GO (RGO) and PSNP in saturated quartz sand both in single and binary systems. In the single transport system, the materials exhibited mobility in the order of GO > RGO > PSNP, due to increased hydrophobicity and decreased negative surface charges. Nevertheless, the co-transport of (R)GO and PSNP in the binary transport system was much more intricate. In Na⁺ saturated porous media, PSNP preferred to interact with (R)GO relative to the highly negatively charged quartz sand, thus (R)GO carried PSNP to break through the sand column. However, in Ca²⁺ saturated porous media, the transport of both (R)GO and PSNP was depressed, attributed to the particle-collector and particle-particle bridging effects between Ca²⁺ and the metal-complexing moieties of the nanoparticles and sand grains. Moreover, GO influenced the co-transport of PSNP to a larger extent than RGO, especially at relatively high ionic strength, because of the more abundant surface O-functional groups on GO providing more complexion sites with Ca²⁺. These results demonstrated that the transport of negatively charged nanomaterials was greatly related to the hydrophobicity and surface O-functional groups.