Electron microscopy of lipids and membranes

Identification and evaluation of a novel peptide binding to the cell surface of Staphylococcus aureus

Identification of short peptides that serve as specific ligands to biological materials such as microbial cell surfaces has major implications in better understanding the molecular recognition of cell surfaces. In this study we screened a commercially available random phage-display library against Staphylococcus aureus cells and identified peptides specifically binding to the bacteria. A synthetic peptide (SA5-1) representing the consensus sequence (VPHNPGLISLQG) of the bacteria-binding peptide was evaluated for its binding potential against S. aureus. Dot-blot, immunoblot assay and ELISA results revealed the SA5-1 peptide to be highly specific to S. aureus. The SA5-1 peptide binding was optimal between pH 6.0 and 8.0. Nanogold Transmission Electron Microscopy demonstrated that the SA5-1 binds to the outer membrane surface of S. aureus. Diagnostic potential of the SA5-1 peptide was evaluated in human platelet samples spiked with S. aureus and specific detection of the bacteria by biotinylated-SA5-1 and streptavidin-conjugated fluorescent quantum dots. Fluorometry results indicated that the peptide was able to detect ∼100 organisms per ml in a spiked biological sample providing a proof-of-concept towards potential of this peptide as a S. aureus diagnostic tool that can be of use in different detection platforms.

Effect of polymyxin on the ultrastructure of the outer membrane of wild-type and polymyxin-resistant strain of Salmonella

The effect of polymyxin on two sets of Salmonella mutants was studied by thin-section and scanning electron microscopy. Polymyxin (in increasing concentrations, starting just below bactericidal effect) caused the appearance of the previously described rodlike projections on the cell surface of wild-type (smooth, polymyxin-sensitive) bacteria. These projections seemed to involve the outer membrane of the cell wall. In rough mutants, which are deficient in lipopolysaccharide, the projections were much smaller and flat. Higher concentrations of polymyxin were required to produce morphological effects in polyxmin-resistant mutants of both smooth and rough forms. Furthermore, in these mutants polymyxin caused vesicle-like bulging of the total outer membrane quite different in appearance from the rodlike projections of the wild type.

The ultrastructure of plasmodesmata

It is suggested that the central strand which traverses plasmodesmata is in open continuity with the endoplasmic reticulum of adjacent cells, and that this strand (desmotubule) represents a modulation of a normal ER membrane so that it comprises solely spherical protein subunits. This concept is used to illustrate how plasmodesmata could form a median nodule or anastomosing central strands. The implications of this model in relation to current theories of symplasmic transport are discussed, and the possibility for further experimental work is outlined.

Biophysical contributions to membrane structure

Applications of physical techniques to studies of membrane structure have increased greatly in the past few years and they have begun to provide more precise structural parameters for membranes and also some indication of the physical states of the molecular constituents in the membranes. Direct measurements of membrane features have been made by electron microscopy and by X-ray diffraction methods. Spectroscopic techniques especially infrared absorption, nuclear magnetic resonance absorption and optical rotatory dispersion and circular dichroism measurements have provided additional data relating to the physical states of the molecular components in the membranes.

A new interpretation of plasmodesmatal ultrastructure

It is shown that simple, unbranched, plasmodesmata between young xylem ray cells of willow have no direct intercellular continuity apart from the plasmalemma which limits the cytoplasm and lines the plasmodesmatal canal. Each plasmodesma is traversed by a 200 Å diameter tubule (the desmotubule) which has a wall with probably 11 subunits arranged around a central cavity through which runs a 40 Å diameter rod. This rod is connected to the inside of the tubule wall, by fine filaments. At the ends of each plasmodesma the plasmalemma and cell wall are closely appressed to the tubule, thus precluding direct continuity between the cytoplasm of adjacent cells. Through the central part of the plasmodesmata the tubule is separated from the plasmalemma by a 90-100 Å wide gap. Cytoplasmic microtubules in the same tissue have a diameter of approximately 250 Å and a wall probably composed of 13 subunits: both desmotubules and cytoplasmic microtubules therefore have a centre-to-centre subunit spacing of about 47 Å. It is suggested that the desmotubules are not microtubules but may be nuclear spindle fibres which become trapped in the wall during cell plate formation. The endoplasmic reticulum, while closely approaching the plasmodesmata, is not continuous across them. It is thought most unlikely that the endoplasmic reticulum traverses plasmodesmata, as the dimensions of the central tubule - found here as well as by other workers - are smaller than those which would be expected to allow a stable molecular configuration in a unit membrane. The plasmalemma, where it lines the plasmodesmatal canal, appears to have particulate subunits in the outer opaque layers and the presence of these subunits may be attributable to the need for stability in membranes arranged about so small a radius.