Aggregation by depletion attraction in cultures of bacteria producing exopolysaccharide

In bacteria, the production of exopolysaccharides—polysaccharides secreted by the cells into their growth medium—is integral to the formation of aggregates and biofilms. These exopolysaccharides often form part of a matrix that holds the cells together. Investigating the bacterium Sinorhizobium meliloti, we found that a mutant that overproduces the exopolysaccharide succinoglycan showed enhanced aggregation, resulting in phase separation of the cultures. However, the aggregates did not appear to be covered in polysaccharides. Succinoglycan purified from cultures was applied to different concentrations of cells, and observation of the phase behaviour showed that the limiting polymer concentration for aggregation and phase separation to occur decreased with increasing cell concentration, suggesting a ‘crowding mechanism’ was occurring. We suggest that, as found in colloidal dispersions, the presence of a non-adsorbing polymer in the form of the exopolysaccharide succinoglycan drives aggregation of S. meliloti by depletion attraction. This force leads to self-organization of the bacteria into small clusters of laterally aligned cells, and, furthermore, leads to aggregates clustering into biofilm-like structures on a surface.     

Mechanism of microbial aggregation during capillary electrophoresis

We studied the aggregation of a rod-shaped bacteria, Bifidobacterium infantis, during capillary electrophoresis (CE). A microscope with an intensified CCD camera was employed to monitor the migration and aggregation of bacteria, which are labeled with fluorescent dye Syto 9 and excited with a 488-nm argon ion laser. A collision-based aggregation mechanism is proposed, in which collisions between microbes result from different mobilities and migration directions in the electric field. Individual microbes are aligned differently with respect to the direction of the electric field and exhibit different drag coefficients. The long-range forces include van der Waals attraction and electrostatic repulsion as qualitatively described by DLVO theory. Collisions in CE produce sufficient energy to overcome electrostatic repulsion, thus improving the efficiency of aggregation. This is supported by the fact that higher electric fields always resulted in faster aggregation. Also, when sodium phosphate buffer was used, increasing the ionic strength resulted in faster aggregation. However, when Tris-boric acid-EDTA (TBE, pH 9.1) buffer was used, the aggregation speed decreased when the ionic strength increased. We attribute this to the change of the surface of the bacteria at high borate and EDTA concentration, such as the loss of polysaccharides or the presence of complexation. This reduces the hydrophobicity of the surface and, thus, the short-range attractive forces. The addition of 0.05% poly(ethylene oxide) (PEO) into high ionic strength TBE buffer increased the aggregation rate. This can be attributed to the bridging effect of PEO between microbes. Further increase in the concentration of polymer reduced the aggregation rate, especially when the electric field was low, due in part to the increase in viscosity. The decrease in migration velocity produced lower collision energies and lower aggregation efficiencies as well.     

Aggregation behaviour of some β-cyclodextrin-based side-chain azo amphiphilic polyurethanes

New β-cyclodextrin-based side-chain azo amphiphilic polyurethanes were prepared. Aggregation behaviour was studied by employing steady-state fluorescence and UV–Vis spectroscopy techniques and, determination of the hydrodynamic diameter of micellar aggregates in N,N-dimethyl formamide and N,N-dimethyl formamide/ethanol mixture solutions was done. An apparent polarity evidenced by increasing I ₁/I ₃ ratio with the increasing concentration of polyurethanes except the polyurethanes with stronger intra- and intermolecular hydrogen bonding of hard segments was obtained. Pyrene resides predominantly in the interfacial region in micellar aggregates that is sensitive to polarity changes due to solvent penetration. Apparent critical micelle concentration due to the presence of β-cyclodextrin was determined by steady-state fluorescence and UV–Vis spectroscopy techniques which ranged between 10^?2 and 10^?1 g/L. Binding constants from both spectroscopy techniques were determined. The values of hydrodynamic diameter for the amphiphilic poly(ether urethane)s show that their aggregation behaviour is different in function of chemical composition, type and content of hard segment and hydrophobe/hydrophile ratio.     

In situ study of aggregate sizes formed in chalcopyrite-quartz mixture using temperature-responsive polymers

An interesting property of temperature-responsive polymers, such as poly(N-isopropylacrylamide) (PNIPAM), is the ability to behave as flocculants above a lower critical solution temperature (LCST). This study examines the aggregation of a chalcopyrite-quartz mixture using a sulfide-selective temperature-responsive polymer, P(NIPAM-co-ethyl xanthate methacrylate (EXMA)) in a continuously-sheared suspension, relative to polyacrylamide (PAM). The investigation was carried out in situ using imaging and Focused Beam Reflectance Measurement techniques to obtain real-time chord length distributions. While particle aggregates were observed in the presence of PNIPAM only upon heating above the LCST, P(NIPAM-co-EXMA) induced particle aggregation below the LCST, due to the attraction between the xanthate moiety and the sulfide surfaces. The largest aggregates were observed with P(NIPAM-co-EXMA) (1.5 MDa), followed by PNIPAM, PAM, and P(NIPAM-co-EXMA) (115?kDa). Particle aggregates formed with PAM did not exhibit further breakage under increasing shear to 1100?s^?1, while large-scale fragmentation was observed with the PNIPAM-based flocculants. Unlike PNIPAM, addition of P(NIPAM-co-EXMA) to suspension above the LCST was able to yield particle aggregation, attributed to the formation of charge-stabilised micelles. The influence of the shear rate on the size of the aggregates formed with P(NIPAM-co-EXMA) is unaffected by the polymer addition and measurement temperature below or above the LCST.     

A DEM-based analysis of the influence of aggregate structure on suspension shear yield stress

This study theoretically examined the effect of aggregate structure on the suspension shear yield stress. The aggregation process of colloidal particles was simulated using the discrete element model (DEM) combined with the well-known DLVO theory. The predicted aggregate structural characteristics, namely the coordination number and inter-particle forces were then used in a modified version of the Flatt and Bowen mechanistic model [6] to calculate the corresponding suspension yield stress. The effect of key parameters such as solid volume fraction, suspension pH and ionic strength on the aggregate structure and hence the yield stress of the suspension was investigated.The results showed that the yield stress increased significantly under conditions that were favourable for formation of complex net-like aggregate structures, such as high solid volume fractions, pH values near the iso-electric point, and high ionic strengths. In such cases, the mean coordination number reached a maximum value which was considered to be dependent on the particle size and size distribution. The suspension yield stress exhibited a power law dependency on the solid volume fraction. The interconnected network structure developed at high solid volume fractions was found to be the major contributing factor to the observed high suspension yield stress. As the particle–particle repulsion became significant, a decrease in both the number of bonds and the mechanical bonding strength of the aggregate structure was observed. That was considered to be responsible for the reduction in the suspension yield stress. The suspension yield stress became independent of the suspension ionic strength when the ionic strength exceeded the critical coagulation concentration. Satisfactory agreements were obtained between simulation results and the published experimental data.     

Effect of the surface characteristics of Methanosarcina barkeri on immobilization to support materials

To realize a highly efficient anaerobic treatment, it is necessary to immobilize high concentrations of methanogens within a fermenter. In this study, we experimentally examine the effect of the surface characteristics of the acetate-utilizing methanogen Methanosarcina barkeri (JCM 10043) on immobilization to support materials. To this aim, we measured the electrostatic and hydrophobic properties of M. barkeri. The electrophoretic mobility of M. barkeri decreased with increasing ionic strength of the cell suspension and was fitted by the Ohshima equation using two parameters: spatial charge density in the polyelectrolyte region (ZeN = –1.15 × 10⁶ C/m³) and the softness parameter (1/λ = 3.35 × 10⁻⁹ m). M. barkeri showed an affinity to n-hexadecane, with adhesion of more than 60%. M. barkeri showed hydrophobicity relative to Escherichia coli. We also carried out a microbial adhesion test to support materials. M. barkeri showed better adhesion to the anion-exchange resin than to the hydrophobic resin. The microbial cells adhered uniformly to the anion-exchange resin, while coagulated cells adhered non-uniformly to the hydrophobic resin. M. barkeri showed poor adhesion to the cation-exchange resin. The anion-exchange resin is most effective in immobilizing M. barkeri within the fermenter.     

Effect of ionic strength solution on the stability of chitosan–DNA nanoparticles

Chitosan–DNA nanoparticles employed in gene therapy protocols consist of a neutralised, stoichiometric core and a shell of the excess of chitosan which stabilises the particles against further coagulation. At low ionic strength, these nanoparticles possess a high stability; however, as the ionic strength increases, it weakens the electrostatic repulsion which can play a decisive part in the formation of highly aggregated particles. In this study, new results about the effect of ionic strength on the colloidal stability of chitosan–DNA nanoparticles were obtained by studying the interaction between chitosans of increasing molecular weights (5, 10, 16, 29, 57 and 150?kDa) and calf thymus DNA. The physicochemical properties of polyplexes were investigated by means of dynamic light scattering, static fluorescence spectroscopy, optic microscopy, transmission electronic microscopy and gel electrophoresis. After subsequent addition of salt to the nanoparticles solution, secondary aggregation increased the size of the polyplexes. The nanoparticles stability decreased drastically at the ionic strengths 150 and 500?mM, which caused the corresponding decrease in the thickness of the stabilising shell. The morphologies of chitosan/DNA nanoparticles at those ionic strengths were a mixture of large spherical aggregates, toroids and rods. The results indicated that to obtain stable chitosan–DNA nanoparticles, besides molecular weight and N/P ratio, it is quite important to control the ionic strength of the solution.     

Directional reversals enable Myxococcus xanthus cells to produce collective one-dimensional streams during fruiting-body formation

The formation of a collectively moving group benefits individuals within a population in a variety of ways. The surface-dwelling bacterium Myxococcus xanthus forms dynamic collective groups both to feed on prey and to aggregate during times of starvation. The latter behaviour, termed fruiting-body formation, involves a complex, coordinated series of density changes that ultimately lead to three-dimensional aggregates comprising hundreds of thousands of cells and spores. How a loose, two-dimensional sheet of motile cells produces a fixed aggregate has remained a mystery as current models of aggregation are either inconsistent with experimental data or ultimately predict unstable structures that do not remain fixed in space. Here, we use high-resolution microscopy and computer vision software to spatio-temporally track the motion of thousands of individuals during the initial stages of fruiting-body formation. We find that cells undergo a phase transition from exploratory flocking, in which unstable cell groups move rapidly and coherently over long distances, to a reversal-mediated localization into one-dimensional growing streams that are inherently stable in space. These observations identify a new phase of active collective behaviour and answer a long-standing open question in Myxococcus development by describing how motile cell groups can remain statistically fixed in a spatial location.     

Features of the Microrheological Behavior of Blood in Children

The blood of healthy children aged 2–14 was investigated. No differences between the aggregation parameters depending on the sex were revealed. In the children under the age of five, the amplitude of aggregation and the time of formation of linear aggregates were reliably lower and the strength of the largest of them and their total strength were decreased in comparison with the other age groups. An inverse correlation between the strength of the aggregates and the number of reticulocytes and leukocytes and HbA₂ in the blood has been revealed.     

Effects of coarser fine aggregate on tensile properties of ultra high performance concrete

This experimental research investigates the mechanical properties and shrinkage of ultra high performance concrete (UHPC) incorporating coarser fine aggregates with maximum particle size of 5 mm. To adequately design UHPC mixtures using various sizes of solid constituents, particle packing theory was adopted. UHPC mixtures containing either dolomite or basalt, and four fiber volume fractions up to two volume percent were investigated. Uniaxial tension test was performed to evaluate the first cracking tensile strength, ultimate tensile strength, tensile strain capacity and cracking pattern. The UHPC mixtures with dolomite and steel fibers with more than one volume percent achieved more than 150 MPa of compressive strength at the age of 56 days, and showed strain hardening behavior and limited decrease in tensile strength compared to typical UHPC without coarser fine aggregates. The experimental results highlight the potential of dolomite used as coarser fine aggregate in UHPC.     

Effect of high-density electric current pulses on precipitation and mechanical properties of a Cu–Zn alloy

ABSTRACT

By applying high-density electric current pulses (ECP) on the Cu–Zn alloy, the high-temperature precipitated phase is explored and the related mechanical properties are investigated. Results show that the nucleation rate of the high-temperature β precipitation can be greatly accelerated due to the ECP treatment. Especially, an obvious orientation relationship can be detected between the matrix α phase and the product β precipitation by increasing the electric current density. In addition, for the samples with a dense distribution of refined β precipitation, a slight decrease in the tensile strength and a great increase in the elongation-to-failure can be observed.

This paper is part of a themed issue on Materials in External Fields.     

Macroscopic and microstructural properties of engineered cementitious composites incorporating recycled concrete fines

Recycled concrete fines (RCF) are fine aggregates and particles from the demolition waste of old concrete. Unlike recycled coarse aggregates, RCF is seldom used to replace sands in concrete due to its high surface area and attached old mortar on the surface of RCF. This study investigated potential use of RCF as microsilica sand substitute in the production of engineered cementitious composites (ECC), a unique high performance fiber-reinforced cementitious composites featuring extreme tensile strain capacity of several percent. The results showed that it is viable to use RCF as microsilica sand substitute in the production of ECC and the resulting RCF-ECCs possess decent compressive strength and strain capacity. Microstructure investigation on the component level revealed that RCF size and content modify matrix toughness and fiber/matrix interface properties. The influence of RCF size and content on ECC properties was clearly revealed and explained by the resulting fiber bridging σ(δ) curves of RCF-ECCs calculated from the micromechanical model. Micromechanics-based design principle can therefore be used for ingredients selection and component tailoring of RCF-ECCs.     

Trace metal measurements in low ionic strength synthetic solutions by diffusive gradients in thin films

In view of conflicting reports regarding the performance of DGT in low ionic strength solutions (I < 1 mM), further investigations have been carried out. Minimal washing of the diffusive gel and deployment in 1.0 and 10 mM NaNO3 solutions containing Cu and Cd gave the theoretical response of 1 for [C](DGT)/[C](SOLN), where [C](DGT) is the concentration of metal measured by DGT and [C](SOLN) is the concentration of metal measured directly in the solution by an appropriate analytical method. Erroneously high values for [C](DGT)/[C](SOLN) were obtained when these same gels were deployed at I = 0.1 mM, presumably due to a net negative charge on the gel, attributable to the presence of initiation products of polymerization. However, washing the diffusive gels completely, where the storage solution pH equaled that of deionized water, gave values of approximately 0.5 for [C](DGT)/[C](SOLN) from deployments at I = 0.1 mM, consistent with the lower measured value of the diffusion coefficients at this ionic strength. These results can be explained by the presence of a net positive charge on the gel when it is exhaustively washed, which reduces the effective diffusion coefficient of metal ions by changing their concentration at the gel-solution interface (Donnan partitioning). Diffusive gel equilibration experiments showed the presence of low capacity sites capable of binding metals irrespective of ionic strength. This binding within the diffusive gel does not affect most DGT measurements, as short (4 h) deployments at concentrations of 10 ppb gave theoretical results. Incomplete washing of the resin-gel caused a 5-15% measurement error and a decrease in precision, even at ionic strengths of 10 mM. A high level of accuracy and precision (typically <5%) was maintained during all aspects of this work, even at ionic strengths of 0.1 mM, in contrast to previous results. This is attributable to three factors: (1) exhaustive washing and conditioning protocols, (2) improvements to the DGT sampling device, and (3) low and reproducible blanks due to ultraclean handling procedures. Provided effective diffusion coefficients measured at the same ionic strength are used, the established DGT theory is obeyed irrespective of ionic strength.     

Preparation of belt-like aggregates of a perylene derivative

Perylene 3,4,9,10-tetracarboxylic tetraethyl ester (PTCTEE) was synthesized via an esterification of perylene 3,4,9,10-tetracarboxylic dianhydride (PTCDA) with ethanol and iodoethane. After the chloroform solution of PTCTEE was mixed with n-hexane the luminescence intensity decreases, indicating the aggregation of the PTCTEE molecules. The characterizations of AFM and TEM showed that PTCTEE could form belt-like aggregates in this mixed solvents. The process of the formation and growth of the belt-like aggregates was tracked using AFM and the relation between the length of the belt-like aggregates and time could be well described using an exponential function. This aggregate growth is a one-dimensional crystallization of the PTCTEE molecules mainly due to the strong π–π stacking ability of perylene cores. The proper interaction between the side chains and solvent also plays an important role in creating one-dimensional aggregates of PTCTEE.     

Conductivity and strength behaviour of aluminawhisker-zirconia composites

Yttria stabilized zirconia (8 mol%) composites were fabricated by tape casting with either alumina powder or alumina whiskers, and pressureless sintered. Sintering behaviour, ionic conductivity and mechanical strength were analysed. For all compositions analysed, increasing alumina content reduced the sintered density. For whisker-reinforced zirconia, the rigid whiskers prevented matrix densification along their axis. The ionic conductivity was measured by the complex impedance method from 500–1000 °C and the activation energy for ionic conduction calculated over that range. The ionic conductivity of the alumina-zirconia composites decreased with increasing alumina content as expected by the rule of mixtures. However, the ionic conductivity of the whisker-zirconia composites decreased more than expected possibly due to contamination from the whiskers. The strength of the whisker-zirconia composites was also found to be affected by the porosity. At 5 vol%, the average strength was measured at 39.9 kgf mm^–2, which decreased to 24 kgf mm^–2 at 20 vol%.     

The aggregation behaviour of chitosan bioelectret in aqueous solution using a fluorescence probe

A kind of intramolecular charge-transfer compound, 3-hydroxy-6-methyl-4-N,N-dimethylamino flavone derivatives (PF), was used to detect the self-association of chitosan bioelectret in aqueous solution. Results showed that the fluorescence intensity of PF at 430 nm increased with increasing chitosan concentration in 0.1 M acetic acid, and concomitantly, the emission at 515 nm decreased. However, no emission was observed at 515 nm after the polarization of chitosan solution and only an increase in the peak of PF at 430 nm could be found. When 0.1 M hydrochloric acid was used to dissolve chitosan and all other conditions were the same as above, the PF peak was weak and did not disappear after the chitosan solution was polarized. In addition, the effect of ionic strength on chitosan aggregation with a concentration of 5 mg ml-1 before and after polarization was examined. The results also showed that the fluorescence intensity of the probe increased remarkably at 430 nm with increasing ionic strength after polarization of the chitosan solution. All the results indicated that polarization can promote the aggregation behaviour of chitosan in aqueous solution. © 1998 Chapman & Hall     

Influence of electrolyte and poloxamer 188 on the aggregation kinetics of solid lipid nanoparticles (SLNs)

Solid lipid nanoparticles (SLNs) have attracted increasing attention as colloidal drug carriers due to theirs advantages including low toxicity, drug targeting and modified release. However, undesired particle aggregation in aqueous dispersions would limit the applicability of SLNs for drug delivery. The purpose of the present article is to investigate the aggregation behavior of the SLNs and quantitatively evaluate how the concentration of NaCl and F68 affect the stability of the SLNs. The early stage aggregation kinetics of the SLNs was investigated over a wide range of NaCl concentrations by employing dynamic light scattering (DLS). In the presence of the NaCl, aggregation kinetics of the SLNs exhibited reaction-limited (slow) and diffusion-limited (fast) regimes. These results indicated that the aggregation behavior of these new nanoparticles can be well explained by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The critical coagulation concentration (CCC) of SLNs containing 0.0%, 0.1%, 0.5%, 2.0%, and 4.0% of Poloxamer 188 (F68) was 416, 328, 519, 607, and 602 mM, respectively, suggesting that the F68 influences the aggregation behavior of the SLNs. F68 made the SLNs more sensitive to the electrolyte when its concentration is low (0.1%), the bush of the polymer F68 has a bridging effect that accelerated the aggregation process of the SLNs. However, at the high concentration, F68 can provide the steric repulsion to the nanoparticles, which effectively stabilized the SLNs dispersions.     

Growth of fractal patterns in ZnO films doped with Fe

ZnO films both undoped and doped with Fe were deposited on Si substrates using rf-magnetron sputtering. The results showed that fractal features were clearly exhibited in the ZnO film doped with Fe. It is proposed that the fractal aggregates were the result of cluster diffusion-limited aggregation (CDLA) of magnetic particles on the surface of the film. The fractal dimension of a main branch (D=1.47) was smaller than that expected by the CDLA model (D=1.72). In this paper the growth mechanism of the observed fractal aggregates is discussed in terms of the magnetism of FeO, nanoparticle aggregation and surface tension changes.     

Synthesis of nanostructured TiO2 particles in room temperature ionic liquid and its photocatalytic performance

Nanostructured TiO₂ particles were synthesized by sol-gel method with room temperature ionic liquid (RTIL) 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) as a reaction medium. The structure and morphology of TiO₂ nanoparticles were characterized with X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The as-prepared TiO₂ nanoparticles present anatase crystal phase even without being calcined at high temperature, and show better photocatalytic performance in the degradation of methyl orange. The photocatalytic efficiency increases evidently along with increasing the concentration of nanostructure TiO₂, and the degradation percent can reach 100% at the optimal catalyst concentration (2.0 g/L).     

Self-generated oxygen gradients control collective aggregation of photosynthetic microbes

For billions of years, photosynthetic microbes have evolved under the variable exposure to sunlight in diverse ecosystems and microhabitats all over our planet. Their abilities to dynamically respond to alterations of the luminous intensity, including phototaxis, surface association and diurnal cell cycles, are pivotal for their survival. If these strategies fail in the absence of light, the microbes can still sustain essential metabolic functionalities and motility by switching their energy production from photosynthesis to oxygen respiration. For suspensions of motile C. reinhardtii cells above a critical density, we demonstrate that this switch reversibly controls collective microbial aggregation. Aerobic respiration dominates over photosynthesis in conditions of low light, which causes the microbial motility to sensitively depend on the local availability of oxygen. For dense microbial populations in self-generated oxygen gradients, microfluidic experiments and continuum theory based on a reaction–diffusion mechanism show that oxygen-regulated motility enables the collective emergence of highly localized regions of high and low cell densities.