Rama Bansil

Boston University | BU · Department of Physics

Helicobacter spp., including the well-known human gastric pathogen H. pylori, can cause gastric diseases in humans and other mammals. They are Gram-negative bacteria that colonize the gastric epithelium and use their multiple flagella to move across the protective gastric mucus layer. The flagella of different Helicobacter spp. vary in their locati...

To colonize on the gastric epithelium Helicobacter pylori bacteria have to swim across a gradient of pH from 2-7 in the mucus layer. Previous studies of H. pylori motility have shown that at pH below 4 do not swim in porcine gastric mucin (PGM) gels. To separately assess the influence of gelation of PGM and that of pH on motors and pH sensitive rec...

The swimming strategies of unipolar flagellated bacteria are well known but little is known about how bipolar bacteria swim. Here we examine the motility of Helicobacter suis, a bipolar gastric-ulcer-causing bacterium that infects pigs and humans. Phase-contrast microscopy of unlabeled bacteria reveals flagella bundles in two conformations, extende...

We present particle tracking microrheology results on human mucins, isolated from normal surface and gland mucosa and one tumor sample, and examine the motility of Helicobacter pylori in these mucins. At 1.5% concentration human mucin solutions are purely viscous, with viscosity η (gland mucin) > η (surface mucin) > η (tumor mucin). In the presence...

In this review we discuss mucus, the viscoelastic secretion from goblet or mucous producing cells that lines the epithelial surfaces of all organs exposed to the external world. Mucus is a complex aqueous fluid that owes its viscoelastic, lubricating and hydration properties to the glycoprotein mucin combined with electrolytes, lipids and other sma...

It has frequently been hypothesized that the helical body shapes of flagellated bacteria may yield some advantage in swimming ability. In particular, the helical-shaped pathogen Helicobacter pylori is often claimed to swim like a corkscrew through its harsh gastric habitat, but there has been no direct confirmation or quantification of such claims....

The helical shape of the human stomach pathogen Helicobacter pylori has been suggested to provide mechanical advantage for penetrating the viscous stomach mucus layer. Using single-cell tracking and quantitative morphology analysis we document marked variation in cell body helical parameters and flagellum number among H. pylori strains leading to d...

The gastric ulcer and cancer causing bacteria, Helicobacter pylori have uniquely adapted to swim across the viscoelastic mucus gel that lines the stomach epithelial surface and colonize in the harsh acidic environment of the stomach. In this paper we first briefly review results of bacteria tracking and oscillatory shear rheology studies to suggest...

An optical blood coagulation monitor and method. The monitor has a blood sample holder, a laser with its output light directed through the blood sample, a two-dimensional detector that is able to detect light at the laser light wavelength and that has a detector output, optics for imaging onto the detector laser light that is forward scattered by t...

The bacterium Helicobacter pylori (H. pylori), has evolved to survive in the highly acidic environment of the stomach and colonize on the epithelial surface of the gastric mucosa. Its pathogenic effects are well known to cause gastritis, peptic ulcers, and gastric cancer. In order to infect the stomach and establish colonies on the mucus epithelial...

Recently, Farjas and Roura (FR) have proposed a universal scaling law to describe nonisothermal crystallization kinetics based on a modification of the conventionally used Avrami model. In this letter, we apply the approach of Farjas and Roura to analyze the kinetics of an order–order phase transition in a diblock copolymer solution. We present an...

We present time-resolved small-angle X-ray scattering (SAXS) measurements of the kinetics of a barotropic disorder–order transition in polystyrene–polyisoprene (SI) diblock micelles in diethyl phthalate (DEP), a styrene selective solvent. Because of the fast equilibration in a pressure jump we were able to observe the initial induction stage during...

Helicobacter pylori colonizes the stomach of ~50% of all humans. Helical cell shape is thought to enhance H. pylori’s flagellar-based motility and to enable bacteria to move through the highly viscous mucus layer via a corkscrew mechanism, allowing efficient colonization of the stomach. In support of this theory, deletion of cell shape-determining...

Mucin glycoprotein consist of tandem repeating glycosylated regions flanked by non-repetitive protein domains with little glycosylation. These non-repetitive domains are involved in the pH dependent gelation of gastric mucin, which is essential to protecting the stomach from autodigestion. We have examined the folding of the non-repetitive sequence...

Previous studies with gelatin have observed four distinct stages during the physical gelation process [Normand et al. Macromolecules, 2000, 33, 1063]. In this presentation we report measurements of microrheology in an effort to examine the time evolution of the gel on short length scales and time scales. By tracking latex particles in gelatin solut...

Synchrotron based time-resolved small angle x-ray scattering (SAXS) was used to study the kinetics of the formation of a gyroid phase in solutions of a poly (styrene -isoprene) (SI) diblock copolymer in dimethyl phthalate, a selective solvent for the polystyrene block. Temperature ramp measurements over the range of 70-130C show the transition from...

In this talk I will describe the molecular mechanisms involved in the remarkable ability of the mucus lining of the stomach for protecting the stomach from being digested by the acidic gastric juices that it secretes. These physical properties can be attributed to the presence of a high molecular weight glycoprotein found in mucus, called mucin. Rh...

Synchrotron based time-resolved small angle x-ray scattering (SAXS) was used to study the kinetics of the order-disorder transition (ODT) in a 30% (w/v) solution of a diblock copolymer of poly(styrene -- isoprene) (SI 18-12) in diethyl phthalate, a selective solvent for the PS block using pressure jump methods. Time resolved pressure jump SAXS expe...

Synchrotron based time-resolved small angle x-ray scattering (SAXS) was used to study the kinetics of the formation of a gyroid phase in solutions of a poly (styrene -isoprene) diblock copolymer in dimethyl phthalate, a selective solvent for the polystyrene block. From temperature ramp measurements on an 80% (w/v) sample, a hexagonally-packed cylin...

We have investigated the question of heterogeneity in gels using Brillouin scattering, depolarized Rayleigh scattering and dynamic light scattering measurements on methylmethacrylate (MMA) gels crosslinked with varying amount of ethylene-dimethacrylate (EDMA). We find that the k dependence of the phase velocity changes on increasing the cross-link...

The kinetics of the transformation from the hexagonal packed cylinder (HEX) phase to the face-centered-cubic (FCC) phase was simulated using Brownian Dynamics for an ABA triblock copolymer in a selective solvent for the A block. The kinetics was obtained by instantaneously changing either the temperature of the system or the well-depth of the Lenna...

Synchrotron based time-resolved small angle x-ray scattering (SAXS), was used to study the kinetics of the formation of a gyroid phase in solutions of a poly (styrene - isoprene) diblock copolymer in dimethyl phthalate, a selective solvent for the polystyrene block. From temperature ramp measurements on a 75% (w/v) sample, a hexagonally-packed-cyli...

We investigated the dynamics of a block copolymer SEBS (triblock of styrene (S) and ethylene-butylene, EB) in selective solvents using X-ray Intensity Fluctuation Spectroscopy (XIFS). We examined the temperature dependence of dynamics in cylindrical, spherical and lamellar phases as well as in coexisting phases through the transitions. We were able...

The ulcer-causing gastric pathogen Helicobacter pylori is the only bacterium known to colonize the harsh acidic environment of the human stomach. H. pylori survives in acidic conditions by producing urease, which catalyzes hydrolysis of urea to yield ammonia thus elevating the pH of its environment. However, the manner in which H. pylori is able to...

Synchrotron based time-resolved small angle x-ray scattering (SAXS) was used to study the kinetics of the order-disorder transition (ODT) in a 30% (w/v) solution of a diblock copolymer of poly(styrene -- isoprene) (SI 18-12) in diethylphthalate (DEP), a selective solvent for the PS block using pressure jump methods. The results show that the ODT te...

Confocal light absorption and scattering spectroscopic (CLASS) microscopy is a novel optical technique for observing submicron intracellular structures in living cells. It allows monitoring nondestructively cell function and cell dynamics in vivo and in real time. CLASS microscopy, having accuracy well beyond the diffraction limit, does not require...

We report Brownian Dynamics simulations on 400 bead-spring chains of triblock copolymer, A10B10A10, in a selective solvent for the A block using a FENE potential together with Lennard-Jones (LJ) for B-B attraction and a Weeks-Chandler-Anderson potential for A-B and A-A repulsions. On varying volume fraction and temperature T (in units of ε/kB, wher...

Time-resolved small-angle X-ray scattering (SAXS) was used to examine the kinetics of the transition from hexagonal (hex) cylinders to body-centered cubic (bcc) spheres at various temperatures in poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) in mineral oil, a selective solvent for the middle ethylene-co-butylene (EB) block. Temperature-ramp...

We examined the kinetics of the transformation from the lamellar (LAM) to the hexagonally packed cylinder (HEX) phase for the triblock copolymer, polystyrene-b-poly (ethylene-co-butylene)-b-polystyrene (SEBS) in dibutyl phthalate (DBP), a selective solvent for polystyrene (PS), using time-resolved small-an le X-ray scattering (SAXS). We observe the...

Time-resolved small angle x-ray scattering (SAXS) was used to examine the kinetics of the transition from HEX cylinders to BCC spheres at various temperatures in poly(styrene-b- ethylene-co-butylene-b-styrene) (SEBS) in mineral oil, a selective solvent for the middle EB block. Temperature-ramp SAXS and rheology measurements show the HEX to BCC orde...

Gastric mucin, a high molecular weight glycoprotein, is responsible for providing the gel-forming properties and protective function of the gastric mucus layer. Bulk rheology measurements in the linear viscoelastic regime show that gastric mucin undergoes a pH-dependent sol-gel transition from a viscoelastic solution at neutral pH to a soft viscoel...

We have developed a novel optical method for observing submicrometer intracellular structures in living cells, which is called confocal light absorption and scattering spectroscopic (CLASS) microscopy. It combines confocal microscopy, a well-established high-resolution microscopic technique, with light-scattering spectroscopy. CLASS microscopy requ...

We discuss the results of Monte Carlo simulations of a recent kinetic percolation model applicable to free radical initiated copolymerization of bifunctional and tetrafunctional units in the presence of a solvent. We find that the critical exponent for gelation and the critical amplitudes are unchanged by the inclusion of solvent and mobility. We a...

Mucin glycoproteins consist of tandem repeating glycosylated regions flanked by non-repetitive protein domains with little glycosylation. These non-repetitive domains are involved in polymerization of mucin via disulfide bonds and play an important role in the pH dependent gelation of gastric mucin, which is essential to protecting the stomach from...

Synchrotron based small angle x-ray scattering (SAXS) was used to study the pressure dependence of the order-disorder transition (ODT) in a 30% (w/v) solution of a diblock copolymer of poly(styrene -- isoprene) (SI 18-12, Polymer Source) in diethylphthalate (DEP), a selective solvent for the PS block. This diblock copolymer solution undergoes an or...

Time-resolved small-angle x-ray scattering measurements reveal that a long-lived intermediate bcc state forms when a poly(styrene-b-isoprene) diblock copolymer solution in an isoprene selective solvent is rapidly cooled from the disordered micellar fluid at high temperature to an equilibrium fcc state. The kinetics of the epitaxial growth of the [1...

An understanding of the basic structure, viscoelastic properties and interactions of mucin glycoproteins is of considerable interest to food science because of the important protective role that these macromolecules play in gastric physiology. The polymeric/colloidal behavior of mucins is complicated due to their large size (2–50 MDa) and complex s...

Why does the stomach not digest itself? The food we put in our stomach is efficiently broken down in this harsh acidic environment and yet somehow the acid does not damage the stomach itself. The answer is that the mucus gel layer lining the inside of the stomach serves as a protective barrier. For the past several years we have been engaged in a c...

We have used small-angle light-scattering (SALS), microscopy, and measurements to study structural changes produced in unbuffered agarose gels as ions migrate under applied electric fields (3-20 V/cm). Anisotropic, bowtielike, light-scattering patterns were observed, whose development occurred more quickly at higher fields. The horizontal lobes wer...

The kinetics of the transition of HEX cylinders to BCC spheres was studied by coupling anisotropic fluctuations on cylinders, similar to the pearling instability, according to which the amplitude of a transverse wave along the length of the cylinder grows causing the cylinder to break up into spheroidal droplets. We find that the sphere BCC phase a...

Synchrotron based time-resolved small angle x-ray scattering (SAXS), was used to study the kinetics of ordering transition (OOT) between cylindrical micelles in HEX phase and lamellar (LAM) phase in a 0.4 (w/v) solution of a triblock of polystyrene (PS) and poly(ethylene-co-butylene) (PEB), SEBS (PS-PEB-PS) copolymer in Dibutyl Phthalate (DBP), a s...

Mammalian gastric mucin, at high concentration, is known to form a gel at low pH, behavior essential to the protection of the stomach from auto-digestion. Atomic force microscopy (AFM) measurements of dilute solutions of porcine gastric mucin in an aqueous environment in the pH range 6-2 provide a direct visualization of extended fiberlike molecule...

Gastric mucin is a glycoprotein known to undergo a pH-dependent sol-gel transition that is crucial to the protective function of the gastric mucus layer in mammalian stomachs. We present microscope-based dynamic light scattering data on porcine gastric mucin at pH 6 (solution) and pH 2 (gel) with and without the presence of tracer particles. The da...

We discuss the mechanism by which gastric mucin forms a gel at low pH, which serves to protect the stomach from being damaged by the acidic gastric juice that it secretes. Frequency dependence of viscoelastic moduli of pig gastric mucin gels obtained by microscopic dynamic light scattering is presented. Atomic Force Microscopy provides direct visua...

We have examined solutions of a polystyrene-polybutadiene pentablock copolymer in n-heptane, a strongly selective solvent for polybutadiene. Small angle neutron scattering from 7 to 15% samples reveals domains about 10 nm in radius formed by the association of ∼200 polystyrene blocks. Dynamic light scattering measurements on 8 and 9% samples showed...

We utilized fluorescence recovery after photobleaching (FRAP) and fluorescence correlation spectroscopy (FCS) to examine the role of gallbladder mucin (GBM) in promoting the aggregation and/or fusion of cholesterol enriched vesicles. By fluorescent labeling either the vesicle or the mucin, we could examine the change in vesicle size as well as chan...

We used time resolved small angle x-ray scattering to study the kinetics of HEX-BCC transition in a 0.45(w/v) solution of a polystyrene-block-poly (ethylene-co-butylene)- block-polystyrene triblock copolymer in mineral oil, a selective solvent for the middle block. Temperature jumps were made for both the HEX-BCC (110-145 C) and the reverse transit...

We performed Brownian Dynamics simulations of multiblock copolymers of A and B polymers in a solvent selective for the A block at a volume fraction of 20%. Tri-, penta- and heptabocks were simulated. Fourier transformation reveals micellar clusters arranged in a BCC lattice, in agreement with scattering experiments. The clusters were analyzed using...

The kinetics of the disorder−order transition (ODT) in a triblock of polystyrene (PS) and poly(ethylene-co-butylene) (PEB), in mineral oil, a selective solvent for the middle PEB block, was followed by time-resolved SAXS using both temperature ramp and rapid temperature quench techniques. Data analysis, using the Percus−Yevick interacting hard-sphe...

We have used 2-D small angle light scattering and microscopy to study the structural changes an applied electric field produces in agarose gels. Anisotropic light scattering patterns were observed and the time evolution of these patterns was followed. The development and decay of the scattering patterns occurs more quickly at higher fields. The hor...

We have used time resolved small angle x-ray scattering (SAXS), to study the kinetics of order-order transition (OOT) in a 0.45(w/v) solution of Kraton G1650, a polystyrene-block-poly (ethylene-co-butylene)- block-polystyrene (SEBS) triblock copolymer in mineral oil, a selective for the middle EB block. Temperature ramp, temperature jump and equili...

We have examined solutions of a polystyrene–polybutadiene pentablock copolymer in 1,4-dioxane, a slightly selective solvent for polystyrene and a θ solvent for polybutadiene, with static light scattering (SLS), dynamic light scattering (DLS), and small-angle neutron scattering (SANS). The SANS data have been analyzed with the Percus–Yevick model to...

We have used small-angle x-ray scattering (SAXS), and small-angle neutron scattering (SANS) to study the micelle structure of a polystyrene-block-poly(ethene-co-butene)-block-polystyrene triblock copolymer in dilute - semidilute solutions in solvents selective for either the outer styrene block (dioxane) or for the middle block (heptane or tetradec...

We present data from small angle neutron scattering (SANS) on the polyelectrolyte side chain liquid crystalline phases of porcine stomach mucin. There is a polydomain nematic phase at concentrations above the point of brush overlap. Also, the proteoglycans were seen to orient under the influence of a magnetic field at concentrations below which the...

The effect of varying the crosslink content on molecular probe dynamics is studied in semidilute solutions of branched poly(methyl methacrylate) below the gel point and in gels in the vicinity of the gel point using dynamic light scattering (DLS). Toluene was used as solvent to simplify the analysis because it is isorefractive with the PMMA matrice...

We have used dynamic light scattering (DLS) and small angle neutron scattering (SANS) to study the association behavior of a pentablock copolymer of polystyrene (PS) and polybutadiene (PB), PS-PB-PS-PB-PS in semidilute solutions in solvents selective either for the PS block (dioxane) or for the PB block (heptane).DLS data show two modes: the fast o...

We report the first results from a SANS study of purified porcine gastric mucin solutions in D2O. The ability of this glycoprotein to protect the stomach epithelium from acid damage, may be due to a pH dependent conformational transition which leads to gelation at low pH Cao et. al. (Biophysical. J. 76, 1250, 1999). SANS measurements were made over...

We have used time resolved small angle x-ray scattering (SAXS), to study the kinetics of ordering transitions in a 20% solution of Kraton G1650, a polystyrene(PS)- block-poly (ethylene-co-butylene)- block-polystyrene (SEBS) triblock copolymer in mineral oil, a selective for the middle PEB block. Temperature ramp, temperature jump and equilibrium st...

We report on the first results from an AFM study of porcine gastric mucin employing the tapping mode technique in aqueous solution. This glycoprotein is responsible for protecting the stomach epithelium from acid damage. Mucin was imaged on a mica substrate at pH7, and at pH2. At the higher pH we detected individual molecules in disordered configur...

The temperature dependence of dynamic properties of solutions of a triblock copolymer with polystyrene outer blocks and a middle block of fully hydrogenated polybutadiene mass fraction of PS 0.28) was studied in a selective solvent for the middle block, n-heptane. In dilute solutions, a gradual decomposition of the flower-like micelles was observed...

We have used static and dynamic light scattering and pulsed field gradient NMR to study the effect of varying concentration on the dynamics of the triblock copolymer, polystyrene block poly(ethylene, butylene) block polystyrene (PS-PEB-PS), dissolved in n-heptane, a selective solvent for the middle block. The correlation function for a dilute solut...

We have used small angle x-ray scattering (SAXS) and neutron scattering (SANS) to study the micelle structure of a polystyrene(PS)-block-poly(hydrogenated butadiene)(PHB)-block-polystyrene triblock copolymer in dilute - semidilute solutions in solvents selective for either the outer PS block (dioxane) or for the middle PHB block (heptane or decane)...

We present synchrotron-based time-resolved small-angle x-ray scattering (SAXS) measurements of spinodal decomposition in a covalently cross-linked N-isopropylacrylamide gel. The range of wave numbers examined is well beyond the position of the maximum in the structure factor S(q,t). The equilibrium structure factor is described by the sum of a Lore...

We present dynamic light scattering (DLS) and hydrophobic dye-binding data in an effort to elucidate a molecular mechanism for the ability of gastric mucin to form a gel at low pH, which is crucial to the barrier function of gastric mucus. DLS measurements of dilute mucin solutions were not indicative of intermolecular association, yet there was a...

The dependence of the dynamics of probe particles in polyacrylamide solutions and gels as a function of crosslink content, scattering angle and size of the probe is studied using photon correlation spectroscopy. Several fitting methods were applied to the auto-correlation functions obtained from samples ranging through the gelation threshold. We ob...

The authors report here a calculation of the spatial correlation function of the fully polymerised four-functional units in the kinetic gelation model in three dimensions. The results indicate that the fully polymerised four-functional units form small correlated regions in space. This is in sharp contrast to normal percolation where the spatial di...

The authors have analysed the cluster size distribution for three-dimensional kinetic gelation. The simulations are done on cubic lattices of size up to 453 and they include 30% solvent molecules which are capable of diffusing. They observe that the cluster distribution as a function of cluster size is non-monotonic. The peak of this distribution s...

Phase separation in solid poly(styrene-stat-methyl methacrylate) copolymers (styrene volume fractions 0.1–0.9) was investigated by polarized Rayleigh–Brillouin and Rayleigh light scattering methods. Both these methods showed evidence of microphase separation caused by chemical polydispersity of the copolymers.

The kinetics and thermodynamics of phase separation by temperature increase into the two-phase region (HIPS process) have been studied in a polyethersulfone membrane-forming solution. The binodal temperature (T_cx) was determined from the measurement of turbidity at different heating rates by extrapolating to zero heating rate. The spinodal tempera...

Dynamic light scattering was used to follow the tracer diffusion of phospholipid/cholesterol vesicles in aqueous polyacrylamide solutions and compared with the diffusive behavior of polystyrene (PS) latex spheres of comparable diameters. Over the range of the matrix concentration examined (Cp = 0.1-10 mg/ml), the diffusivities of the PS spheres and...

The association of styrene−methyl methacrylate statistical copolymers in the selective solvent acetone (a thermodynamically good solvent for poly(methyl methacrylate and bad for polystyrene) was examined by static and dynamic light scattering methods. In dilute solutions, the dynamics was dominated by a single fast mode, whereas in semidilute solut...

Dynamic light scattering studies show that concentration and pH play important roles in determining pig gastric mucin's (PGM) ability to aggregate and gel. At low concentrations, PGM macromolecules exist in solution predominantly in the form of monomers. At high concentrations, PGM macromolecules aggregate to form supra-macromolecular clusters. Whe...

We present experimental results of the rich variety of two-dimensional precipitation patterns that are formed in a gel when varying some geometrical parameters. Because the patterns arise from a reaction-diffusion process of the typeA+B→C and the reagents as well as the gel are maintained always the same, the various shapes of the precipitate depen...

We have performed experiments and computer simulations of the formation of two types of precipitate profiles which appeared when electrolyte solutions of Na2HPO4 and CaCl2 are allowed to diffuse into an agarose gel from both ends of the gel. The formation of a spatially confined precipitate occurred when the electrolyte flux J was the same at both...

We have used SAXS to study the kinetics of the ordering phase transition in polystyrene-poly (hydrogenated butadiene)-polystyrene (Kraton) triblock copolymer solution in the selective solvent heptane under a variety of quench conditions. Heptane is a poor solvent for the outer PS block. This solution undergoes a microphase separation below 90 C; or...