Interactions between biosurfaces

Measuring forces with the AFM: polymeric surfaces in liquids

This review links together for the first time both the practicalities of force measurement and the work carried out to date on force detection between polymeric surfaces in liquids using the atomic force microscope (AFM). Also included is some of the recent work that has been carried out between surfactant surfaces and biologically coated surfaces with the AFM. The emphasis in this review is on the practical issues involved with force measurement between these types of surfaces, and the similarities and irregularities between the observed types of forces measured. Comparison is made between AFM and surface force apparatus (SFA) measurements, as there is a much longer history of work with the latter. Results indicate that forces between the surfaces reviewed here are a complicated mixture of steric-type repulsion, conformational behaviour on separation and long-range attraction, which is often ascribed to 'hydrophobic' forces. The origin of this latter force remains uncertain, despite its almost ubiquitous appearance in force measurements with these types of surfaces.

The measurement of Bacillus mycoides spore adhesion using atomic force microscopy, simple counting methods, and a spinning disk technique

An atomic force microscope has been used to study the adhesion of Bacillus mycoides spores to a hydrophilic glass surface and a hydrophobic-coated glass surface. AFM images of spores attached to the hydrophobic-coated mica surface allowed the measurement of spore dimensions in an aqueous environment without desiccation. The spore exosporium was observed to be flexible and to promote the adhesion of the spore by increasing the area of spore contact with the surface. Results from counting procedures using light microscopy matched the density of spores observed on the hydrophobic-coated glass surface with AFM. However, no spores were observed on the hydrophilic glass surface with AFM, a consequence of the weaker adhesion of the spores at this surface. AFM was also used to quantify directly the interactions of B. mycoides spores at the two surfaces in an aqueous environment. The measurements used "spore probes" constructed by immobilizing a single spore at the apex of a tipless AFM cantilever. The data showed that stretching and sequential bond breaking occurred as the spores were retracted from the hydrophilic glass surface. The greatest spore adhesion was measured at the hydrophobic-coated glass surface. An attractive force on the spores was measured as the spores approached the hydrophobic-coated surface. At the hydrophilic glass surface, only repulsive forces were measured during the approach of the spores. The AFM force measurements were in qualitative agreement with the results of a hydrodynamic shear adhesion assay that used a spinning disk technique. Quantitatively, AFM measurements of adhesive force were up to 4 x 10(3) times larger than the estimates made using the spinning disk data. This is a consequence of the different types of forces applied to the spore in the different adhesion assays. AFM has provided some unique insights into the interactions of spores with surfaces. No other instrument can make such direct measurements for single microbiological cells.

Atomic Force Microscopy Study of the Adhesion of Saccharomyces cerevisiae

An atomic force microscope (AFM) has been used to quantify directly the adhesion of metabolically active Saccharomyces cerevisiae cells at a hydrophilic mica surface, a mica surface with a hydrophobic coating, and a protein-coated mica surface in an aqueous environment. The measurements used "cell probes" constructed by immobilizing a single cell at the apex of a tipless AFM cantilever. Adhesion was quantified from force-distance data for the retraction of the cell from the surface. The data indicated stretching and sequential bond-breaking as the cell probe was retracted from all of the surfaces. Detailed studies were made for physiologically active cells, which were shown to have different adhesion properties to glutaraldehyde-treated cells. Greatest cell adhesion was measured at the hydrophobic surface. Prior adsorption of a bovine serum albumin protein layer at the hydrophilic surface did not significantly affect cell adhesion. Changes in yeast surface hydrophobicity and zeta-potential with yeast cell age were correlated with differences in adhesion. Cells from the stationary phase adhered most strongly to a mica surface. Time of surface contact was demonstrated to be important. Both the force needed to detach a cell from a hydrophilic mica surface and the length of the adhesive interaction increased after 5 min contact. The AFM cell probe technique gives unique insights into primary colonization events in biofilm formation. It will continue to aid both fundamental studies and the assessment of new procedures that are designed to lower cell adhesion at surfaces relevant to biotechnology, medicine, and dentistry Copyright 2001 Academic Press.

The surface properties of phospholipid liposome systems and their characterisation

The field of liposome (vesicle) research has expanded considerably over the last 30 years. In physical chemical terms liposomes have many of the characteristics of colloidal particles and their stability is determined in part by the classical surface forces. It is now possible to engineer a wide range of liposomes varying in size, phospholipid composition and surface characteristics. The surfaces of liposomes can be modified by the choice of bilayer lipid as well as by the incorporation and covalent linkage of proteins (e.g. antibodies and sugar binding proteins [lectins]), glycoproteins and synthetic polymers. Much of the impetus for liposome design has come from their potential value as drug delivery systems. The development of technologies for the production of such a range of liposome systems has presented interesting problems in the characterisation of their properties. The review addresses the progress that has been made in characterising the surfaces of different types of liposomes with specific reference to their electrophoretic properties and their interpretation and the physical interactions between liposomal bilayers.