Adhesion heterogeneity of individual bacterial cells in an axenic culture studied by atomic force microscopy.

The evaluation of bacterial adhesive properties at a single-cell level is critical for under standing the role of phenotypic heterogeneity in bacterial attachment and community formation. Bacterial population exhibits a wide variety of adhesive properties at the single-cell level, suggesting that bacterial adhesion is a rather complex process and some bacteria are prone to phenotypic heterogeneity. This heterogeneity was more pronounced for Escherichia coli, where two subpopulations were detected. Subpopulations exhibiting higher adhesion forces may be better adapted to colonize a new surface, especially during sudden changes in environmental conditions. Escherichia coli was characterized by a higher adhesion force, a stronger ability to form biofilm and larger heterogeneity index calculated in comparison with Bacillus subtilis. Higher adhesion forces are associated with a more efficient attachment of bacteria observed in an adhesion assay and might provide a basis for successful colonization, survival and multiplications in changing environment. The atomic force microscopy provides a platform for investigation of the adhesion heterogeneity of individual cells within a population, which may be expected to underpin further elucidation of the adaptive significance of phenotypic heterogeneity in a natural environment.

Effect of ampicillin on adhesive properties of bacteria examined by atomic force microscopy.

Discovery of new antibacterial agents requires the development of novel techniques for bacteria surface characterization after treatment with antibiotics. In this study, we investigate the effect of ampicillin at MICs levels on adhesive properties of Gram-positive and Gram-negative bacteria, using atomic force microscopy (AFM). Our results revealed that the treatment leads to changes of bacterial surface properties, especially cell surface roughness. A nanomechanical alteration of the cells led to an increase of adhesive forces and rupture lengths. Changes in adhesive properties are determined not only by the modification of physicochemical cell properties but also by an increase in roughness, leading to an increase of the contact area with a cantilever tip. We discovered that the contribution of non-specific physicochemical interactions in the bacteria attachment to a substrate is not negligible and was significantly influenced by the presence of antibiotic. Ampicillin caused much greater change in the adhesion properties of Bacillus subtilis than Escherichia coli due to the mode of action of ß-lactam antibiotic. Adhesion measurements may by a new way to investigate subtle changes of the bacterial surface properties caused by antibiotic, especially those targeting the bacterial cell wall. In contrast to nanoindentation assays, they provide information on adhesive properties of the bacteria surface.

Mechanical transition in a highly stretched and torsionally constrained DNA.

We show results of our high force (up to 1.8 nN) atomic force microscopy force spectroscopy measurements of a double stranded DNA. We have found that the force spectra of torsionally constrained molecules display a small plateau occurring at a force of approximately 1 nN. This transition is absent in molecules with rotational freedom. Based on all-atom molecular dynamics simulations, we suggest that this plateau is a result of reducing the diameter of a double helix through extreme stretching. The simulation suggests that the molecule is forced into a form resembling an underwound P-DNA, with bases protruding outside of the backbones. These results broaden our understanding of the fundamental aspects of DNA nanomechanics.

Synthesis and Characterization of a Fluorescence Probe of the Phase Transition and Dynamic Properties of Membranes.

We describe the synthesis and characterization of a new fluorescence probe whose emission spectra, anisotropies, and wavelength-dependent decay times are highly sensitive to the phase state of phospholipid vesicles. This probe is 6-palmitoyl-2-[[2-(trimethylammonio)ethyl]methylamino] naphthalene chloride (Patman). The emission maximum of Patman shifts from 425 to 470 nm at the bilayer transition temperatures. The spectral properties of Patman reveal nanosecond time-dependent spectral shifts, which are the result of membrane relaxation around the excited state of Patman. The apparent fluorescence lifetimes of Patman are strongly dependent upon the emission wavelength, and the fluorescence phase and modulation data prove that the spectral shifts are due to an excited-state process, and not ground-state heterogeneity. As expected, the fluorescence anisotropies reflect the phase transitions of the bilayers. In addition, the anisotropies are dependent upon the emission wavelength because the duration of the excited state varies across the emission spectrum. The different apparent lifetimes across the emission spectrum allow the relaxed and unrelaxed emission spectra to be resolved by phase-sensitive detection of fluorescence. Also, the emission spectra of Patman show marked shifts to longer wavelengths as the excitation wavelength is increased. These red-edge excitation shifts are sensitive to the temperature and phase state of the bilayers.