Asma Eskhan

The effects of glucose starvation on Pseudomonas putida biofilm formation and adhesion on model sand surfaces were explored. Using short sand columns, we investigated P. putida biofilm survival and growth during 80‐day period under glucose‐fed and glucose‐starved conditions. Extracellular polymeric substances (EPS) were extracted and assayed for protein and carbohydrate contents, whereas biofilm adhesion to a model surface was quantified by atomic force microscopy (AFM). Upon glucose starvation, P. putida produced more proteins and that was associated with stronger adhesion to sand surfaces compared to glucose‐fed biofilms. These results affirm the capability of P. putida to support structurally robust, adhesive biofilms under extended nutrient deprivation. The findings have implications on the geotechnical and environmental applications of starving P. putida biofilms (e.g., for production of a tough biobarrier under low‐nutrient conditions for soil stabilization and groundwater remediation).

Bacterial strains can be divided into pathogenic and nonpathogenic strains. Distinguishing between the characteristics of these two types will help us understand the mechanisms that bacteria use to cause infections. Thus, the differences in the adhesion to a model hydrophilic silicon nitride surface and in the conformational properties between pathogenic and nonpathogenic Listeria species were probed using atomic force microscopy (AFM). The AFM force-distance approach curves were fitted to two steric models, the steric model and the extended-steric model, which assume the presence of one or two brushes on the bacterial cell surface, respectively. Our results indicated that no significant differences were noticed in the mean adhesion forces measured for pathogenic and nonpathogenic strains using the silicon nitride model surface. However, a larger number of adhesion peaks was found in the AFM retraction curves of the pathogenic strains. Similarly, when the conformational properties, represented by the mean thickness and the mean grafting density of the biopolymer brush, were determined using the steric model, no significant differences were observed between the pathogenic and nonpathogenic strains. However, when the conformational properties, represented by the mean thickness and the mean grafting density of the two brush layers, were quantified using the extended-steric model, it was found that the pathogenic strains had a lower mean grafting density for the first long brush and a higher mean grafting density for the second short brush. Thus, our findings demonstrate that the extended-steric model provides a more detailed view of the conformational properties of Gram-positive Listeria strains and allows for the detection of existing differences.

The treatment of oily wastewater poses a significant environmental challenge, creating a demand for advanced separation technologies. Membrane technologies, especially ultrafiltration (UF), offer a promising solution. A novel composite polyvinylidene fluoride (PVDF) and polypyrrole (PPy) membrane was created via an in situ polymerization method, which enhances the membrane’s functionality by combining the chemical stability of PVDF with the outstanding properties of PPy, through a simple two-step process that decreases manufacturing costs. The PPy content in the PVDF matrix varies from 0 to 1.5 wt%. The membranes were analyzed for their structure, morphology, hydrophilicity, porosity, mechanical strength, flux, oil rejection, and antifouling performance. Fourier-transform infrared spectroscopy (FTIR) confirmed the successful integration of PPy, which increased hydrophilicity; the contact angle dropped from 68° for pure PVDF to 55.6° at a 1.5% PPy concentration. Scanning electron microscopy (SEM) images showed an evident increase in surface porosity and macrovoid formation; calculated porosity increased from 59.5% to 79.9%, and the hydraulic pore size increased from 2.8 nm to 28.5 nm with 1.5% PPy. Although porosity improved, mechanical strength decreased due to the formation of voids. The enhancement in hydrophilicity and porosity resulted in improved flux recovery (FR), with the PP-1 membrane achieving 93% FR and 93% fouling resistance (Rt), indicating an optimal balance for practical use. These modified membranes successfully reduce fouling, making them easier to clean in oil–water separation applications. PP-1 showed only a reduction in flux but maintained an oil rejection rate over 99%, demonstrating high stability. This combination of PVDF’s durability and PPy’s functionality makes a cost-effective, high-performance membrane that transforms oil/water separation processes for sustainable water security.

The scarcity of fresh water resources is one of the biggest challenges encountered in the world. To address this challenge, water treatment and reuse may provide an alternative solution to increase water availability and alleviate the burden on governments. Adsorption is an effective and economical water treatment method that can be applied for the removal of contaminants from water. Naturally available and low-cost materials such as olivine have shown promising results as adsorbents for removing diverse contaminants in water purification. Olivine is one of the most abundant minerals on Earth, comprising more than 50 % of the Earth’s upper mantle. It is a low-cost material that has shown a promising potential as an adsorbent material. This short review summarizes the potential applications of olivine as an adsorbent for water purification applications in the removal of a variety of contaminants from water. The adsorption mechanisms proposed for olivine as an adsorbent in water treatment applications are summarized. Furthermore, other uses of olivine in the field of environmental remediation are also listed. The future prospects for the use of olivine in the field of water treatment are finally outlined. This topic is relevant and timely, given the increasing demand for sustainable materials in water purification applications.

Forces acting between an atomic force microscopy silicon nitride cantilever and the bacterial surface biopolymers of Escherichia coli or Pseudomonas putida were spatially probed in water. The interactions were fitted to a model of steric repulsion to estimate the bacterial surface biopolymer brush length and grafting density. The forces were further fitted to a Hertz model of contact mechanics modified by Sneddon et al. to quantify Young’s modulus of elasticity for the cells. Contour plots of the quantified properties described above (i.e., the bacterial surface biopolymer brush length and grafting density, and Young’s modulus of elasticity for the cells) based on the location coordinates on the bacterial surfaces were generated. Our contour plots indicated the bacterial cells organize their biopolymers uniquely to help them survive in the environment. Specifically, our results showed that the perimeter of a bacterial cell is characterized by a more flexible as well as longer biopolymer brush compared to those estimated at the center top of the cell. These results suggest that bacteria are likely to use their longer brushes on the edges to facilitate their adhesion by bridging surfaces. Also, they maintain their structural reinforcement by developing higher densities of grafted biopolymers and hence higher elasticities at their centers. Moreover, a stronger linear relationship was observed between the brush thicknesses and the grafting densities for the collapsed brush at the center of the cells when compared to the perimeter of the cells.

Atomic force microscopy was utilized to estimate the adhesion strengths to silicon nitride as well as the cellular elasticities of pathogenic Listeria monocytogenes EGDe cells cultured in media adjusted to five different pH conditions of growth (5, 6, 7, 8, and 9) under water with 0.0027 fixed ionic strength. Particularly, the role of adhesion on the bacterial elastic properties was investigated. The nonadhesive Hertz model of contact mechanics was used to extract Young's moduli of elasticity of bacterial cells from the approach force-indentation data. Additionally, the adhesive models of contact mechanics: Johnson–Kendall–Roberts (JKR) and Derjaguin–Muller–Toporov (DMT) were used to estimate Young's moduli of elasticity of bacterial cells from the retraction force-indentation data. Our results indicated that adhesion to silicon nitride was the highest for cells cultured at a pH of 7. Similarly, bacterial cells cultured at pH 7 were characterized by the highest Young's moduli of elasticities compared to the lower or higher pH conditions of growth. Young's moduli of elasticities estimated from the Hertz model were stiffer than those estimated using JKR or DMT models. As the adhesion between bacterial cells and indenters increased, the difference between the Hertz model and JKR or DMT models estimates of Young's moduli of elasticity increased as well. Contradicting the current norm of using the Hertz model to quantify bacterial elasticity in the literature, our results highlight the extreme importance of utilizing contact mechanics models with adhesion components in them such as the JKR and DMT models to estimate bacterial elasticity.

The roles of the bacterial surface biopolymers of pathogenic Listeria monocytogenes EGDe grown under variable pH conditions in governing their adhesion to a model surface of silicon nitride were investigated using atomic force microscopy under water. Our results indicated that the adhesion forces were the highest for cells cultured in media adjusted to pH 7 followed by 1.39, 1.49, 1.57, and 2.18-fold reductions at pH 6, 8, 9, and 5, respectively. Adhesion energies followed the same trends with 1.35, 1.67, 2.20, and 2.79-fold reductions in energies at pH 6, 8, 9, and 5, respectively, compared to the energy measured at pH 7. Furthermore, the structural properties of the bacterial surface biopolymer brush represented by the biopolymer brush thickness (L_o) and the molecular density (Γ) were determined by fitting a steric model of repulsion to the approach force–distance data. The L_o values followed the same trends as adhesion forces and energies, with thickness being highest at pH 7 followed by 1.82, 2.99, 3.11, and 4.66-fold reductions at pH 6, 8, 9, and 5, respectively. Γ was the highest at pH 5 and was followed by 1.26, 1.27, 1.70, and 2.82-fold reductions at pH 8, 9, 6, and 7, respectively. Our results indicated that bacterial adhesion forces and energies increased linearly with the product of L_o and Γ representing the number of biopolymers per unit length of the bacterial surface. To predict the adhesion forces and energies measured, a force-averaging model of the soft-particle analysis of the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory was used. In addition to the standard parameters accounted for in the soft-particle analysis of the DLVO theory such as surface potential, hydrophobicity, and size, this averaging model incorporates in it structural bacterial parameters such as L_o and Γ as well as a surface coverage factor (ϕ) that represents the fraction of the bacterial surface covered by biopolymers. When the soft-particle analysis of DLVO was considered, repulsive hydrogen bond strengths were predicted at close distances of approach (<0.3 nm). In comparison, the force-averaging model predicted that attractive hydrogen bonds dominate the bacterial adhesion strengths quantified. The highest adhesion quantified for cells grown at pH 7 was related to longer and more spaced biopolymers, higher contents of cellular carbohydrates, and more hydrophilic biopolymers, each of which contributes to higher possibilities for hydrogen bonding formation. These results are significant in designing new strategies that aim at controlling bacterial adhesion to surfaces.

Halloysite nanotubes (HNTs) were assembled into mesoporous/macroporous microparticles (c-g-HNTs MPs) using Pickering template-assisted approach. To unravel the stabilization mechanism in Pickering emulsion form, several emulsions and microparticles were prepared at various conditions and visualized using confocal laser scanning microscopy. The prepared c-g-HNTs MPs were used to treat emulsified oil solutions resulting in a maximum removal efficiency of 94.47%. The kinetics data of oil adsorption onto c-g-HNTs MPs was best fitted by the pseudo second order kinetic model (R2 = 0.9983). The maximum monolayer adsorption capacity of oil onto c-g-HNTs MPs as predicted by the multilayer BET model was found to be 788 mg/g. Compared with pristine HNTs, c-g-HNTs MPs exhibited higher self-settleability rates in aqueous solutions as well as in emulsified oil solutions demonstrating their candidacy for practical water treatment applications. The c-g-HNTs MPs were repeatedly used for five adsorption-desorption cycles with minimal losses noticed in their performance.