Bacterial surface appendages strongly impact nanomechanical and electrokinetic properties of Escherichia coli cells subjected to osmotic stress

Electrophoretic mobility of sarcoplasmic reticulum vesicles - analytical model includes amino acid residues of A+P+N domain of Ca(2+)-ATPase and charged lipids

This work is an experimental and theoretical study of electrostatic and hydrodynamic properties of the surface of sarcoplasmic reticulum (SR) membrane using particle electrophoresis. The essential structural components of SR membrane include a lipid matrix and a dense layer of Ca(2+)-ATPases embedded in the matrix. The Ca(2+)-ATPase layer both drives and impedes vesicle mobility. To analyze the experimental mobility data, obtained at pH4.0, 4.7, 5.0, 6.0, 7.5, and 9.0 in 0.1M monovalent (1:1) electrolyte, an analytical solution for the vesicle mobility and electroosmotic flow velocity distribution was obtained by solving the Poisson-Boltzmann and the Navier-Stokes-Brinkman equations. The electrophoretic mobility model includes two sets of charges that represent: (a) charged lipids of the lipid matrix of the vesicle core, and (b) charged amino acid residues of APN domains of Ca(2+)-ATPases. APN domains are assumed to form a charged plane displaced from the surface of lipid matrix. The charged plane is embedded in a frictional layer that represents the surface layer of calcium pumps. Electrophoretic mobility is driven by the charged APN domain and by lipid matrix while the surface layer provides hydrodynamic friction. The charge of APN domain is determined by ionized amino acid residues obtained from the amino acid composition of SERCA1a Ca(2+)-ATPase. Agreement between the measured and the predicted mobility is evaluated by the weighted sum of mobility deviation squared. This model reproduces the experimental dependence of mobility on pH and predicts that APN domains are located in the upper half of the SR vesicle surface layer.

Thermo-regulated adhesion of the Streptococcus thermophilus Δrgg0182 strain

The physicochemical determinants governing the temperature-dependent adhesion of Streptococcus thermophilus to abiotic surfaces are identified under physiological condition for cells either lacking or not the Rgg0182 transcriptional regulator involved in their thermal adaptation. For that purpose, the wild type LMG18311 strain and Δrgg0182 mutant were imaged using highly resolved atomic force microscopy (AFM) at various cell growth temperatures (42 to 55 °C). The corresponding hydrophobic/hydrophilic balance of the cells was quantitatively addressed via the measurement by chemical force microcopy of their adhesion to a reference hydrophobic surface. Analysis of force-separation distance curves further allowed us to discriminate cell surfaces according to the presence or absence of biopolymers. These results were interpreted in relation to the measured adhesion of the Δrgg0182 mutant onto the hydrophobic wall of microwells in the temperature range from 46 to 52 °C. It is evidenced that the viscoelastic Δrgg0182 cell envelop behaves as a thermo-responsive film whose hydrophobicity increases with increasing temperature, thereby favoring cell attachment to hydrophobic surfaces. Regardless cell growth temperature, wild-type cells do not attach to hydrophobic surfaces and the presence of the Rgg0182 transcriptional regulator is associated with the synthesis of hydrophilic cell surface biopolymers. Throughout, the impact of electrostatics on bioadhesion is ruled out upon examination of electrohydrodynamic cell properties at 50 °C.

Adhesin contribution to nanomechanical properties of the virulent Bordetella pertussis envelope

Adherence to a biological surface allows bacteria to colonize and persist within the host and represents an essential first step in the pathogenesis of most bacterial diseases. Consequently, the physicochemical properties of the outer membrane in bacteria play a key role for attachment to surfaces and therefore for biofilm formation. Bordetella pertussis is a Gram-negative bacterium that colonizes the respiratory tract of humans, producing whooping cough or pertussis, a highly infectious disease. B. pertussis uses various adhesins exposed on its surface to promote cell-surface and cell-cell interactions. The most dominant adhesin function is displayed by filamentous hemagglutinin (FHA). B. pertussis Tohama I wild-type (Vir+) strain and two defective mutants, an avirulent (Vir-) and a FHA-deficient (FHA-) B. pertussis strains were studied by AFM under physiological conditions to evaluate how the presence or absence of adhesins affects the mechanical properties of the B. pertussis cell surface. Quantitative information on the nanomechanical properties of the bacterial envelope was obtained by AFM force-volume analysis. These studies suggested that the presence of virulence factors is correlated with an increase in the average membrane rigidity, which is largely influenced by the presence of FHA. Moreover, for this system we built a nanoscale stiffness map that reveals an inhomogeneous spatial distribution of Young modulus as well as the presence of rigid nanodomains on the cell surface.

Osmotic stress adaptations in rhizobacteria

Rhizobacteria have been reported to be beneficial to the plants in many different ways. Increasing salinity in the coastal agricultural zones has been shown to be a threat to the plant and microbial life in the area. Exposure of microorganisms to high-osmolality environments triggers rapid fluxes of cell water along the osmotic gradient out of the cell, thus causing a reduction in turgor and dehydration of the cytoplasm. The microorganisms have developed various adaptations to counteract the outflow of water. The first response to osmotic up shifts and the resulting efflux of cellular water is uptake of K⁺ and cells start to accumulate compatible solutes. Yet another mechanism is by altering the cell envelope composition resulting in changes in proteins, periplasmic glucans, and capsular, exo and lipopolysaccharides. Bacteria also initiate a program of gene expression in response to osmotic stress by high NaCl concentrations, which are manifested as a set of proteins produced in increased amounts in response to the stress. Genomics, transcriptomics and proteomics approaches have revealed the key components in molecular basis of bacteria salt adaptation. Understanding the mechanisms of osmo-adaptation in rhizobacteria would also be relevant from an ecological and an applicative point of view.

Automated force volume image processing for biological samples

Atomic force microscopy (AFM) has now become a powerful technique for investigating on a molecular level, surface forces, nanomechanical properties of deformable particles, biomolecular interactions, kinetics, and dynamic processes. This paper specifically focuses on the analysis of AFM force curves collected on biological systems, in particular, bacteria. The goal is to provide fully automated tools to achieve theoretical interpretation of force curves on the basis of adequate, available physical models. In this respect, we propose two algorithms, one for the processing of approach force curves and another for the quantitative analysis of retraction force curves. In the former, electrostatic interactions prior to contact between AFM probe and bacterium are accounted for and mechanical interactions operating after contact are described in terms of Hertz-Hooke formalism. Retraction force curves are analyzed on the basis of the Freely Jointed Chain model. For both algorithms, the quantitative reconstruction of force curves is based on the robust detection of critical points (jumps, changes of slope or changes of curvature) which mark the transitions between the various relevant interactions taking place between the AFM tip and the studied sample during approach and retraction. Once the key regions of separation distance and indentation are detected, the physical parameters describing the relevant interactions operating in these regions are extracted making use of regression procedure for fitting experiments to theory. The flexibility, accuracy and strength of the algorithms are illustrated with the processing of two force-volume images, which collect a large set of approach and retraction curves measured on a single biological surface. For each force-volume image, several maps are generated, representing the spatial distribution of the searched physical parameters as estimated for each pixel of the force-volume image.