Imaging of lactic acid bacteria with AFM--elasticity and adhesion maps and their relationship to biological and structural data

Importance of molecular details in predicting bacterial adhesion to hydrophobic surfaces

Electrostatic and hydrophobic forces are generally recognized as important in bacterial adhesion. Current continuum models for these forces often wrongly predict measurements of bacterial adhesion forces. The hypothesis tested here is that even qualitative guides to bacterial adhesion often require more than continuum information about hydrophobic forces; they require knowledge about molecular details of the bacteria and substrate surface. In this study, four different strains of bacteria were adsorbed to silica surfaces hydrophobized with alkylsilanes. The thickness of the lipopolysaccharide layers varied on the different bacteria, and the lengths of the alkylsilane molecules were varied from experiment to experiment. Bacterial adhesion was assessed using column experiments and atomic force microscopy (AFM) experiments. Results show that hydrophobized surfaces have higher bacterial sticking coefficients and stronger adhesion forces than bare silica surfaces, as expected. However, adhesion decreased as the solution Debye length became longer than the alkylsilane, perhaps since the silane molecules could not "reach" the bacterial surface. Similarly, those bacteria with a long o-antigen layer had decreased adhesion, perhaps since the silane molecules could not reach surface-bound proteins on the bacteria. This study reveals that macroscopic measurements such as contact angle are not able to fully describe bacterial adhesion; rather, additional details such as the molecular length are required to predict adhesion.

Tapping, pulling, probing: atomic force microscopy in drug discovery

The atomic force microscope is a versatile instrument that can image surfaces with nanoscale resolution, probe local mechanical properties, and measure a variety of interaction forces. It can be relatively easily operated in fluids, and thus experiments can be conducted under nearly physiological conditions, which is behind its numerous applications in biological problems. In the field of drug discovery, atomic force microscopy (AFM) presents a unique opportunity to study a variety of phenomena at the cellular and subcellular level, down to individual (macro)molecular complexes.:

The cell wall of lactic acid bacteria: surface constituents and macromolecular conformations

A variety of strains of the genus Lactobacillus was investigated with respect to the structure, softness, and interactions of their outer surface layers in order to construct structure-property relations of the Gram-positive bacterial cell wall. The role of the conformational properties of the constituents of the outer cell-wall layers and their spatial distribution on the cell wall is emphasized. Atomic force microscopy was used to resolve the surface structure, interactions, and softness of the bacterial cell wall at nanometer-length scales and upwards. The pH-dependence of the electrophoretic mobility and a novel interfacial adhesion assay were used to analyze the average physicochemical properties of the bacterial strains. The bacterial surface is smooth when a compact layer of globular proteins constitutes the outer surface, e.g., the S-layer of L. crispatus DSM20584. In contrast, for two other S-layer containing strains (L. helveticus ATCC12046 and L. helveticus ATCC15009), the S-layer is covered by polymeric surface constituents which adopt a much more extended conformation and which confer a certain roughness to the surface. Consequently, the S-layer is important for the overall surface properties of L. crispatus, but not for the surface properties of L. helveticus. Both surface proteins (L. crispatus DSM20584) and (lipo)teichoic acids (L. johnsonii ATCC332) confer hydrophobic properties to the bacterial surface whereas polysaccharides (L. johnsonii DSM20533 and L. johnsonii ATCC 33200) render the bacterial surface hydrophilic. Using the interfacial adhesion assay, it was demonstrated that hydrophobic groups within the cell wall adsorb limited quantities of hydrophobic compounds. The present work demonstrates that the impressive variation in surface properties displayed by even a limited number of genetically-related bacterial strains can be understood in terms of established colloidal concepts, provided that sufficiently detailed structural, chemical, and conformational information on the surface constituents is available.

Probiotics and the management of inflammatory bowel disease

The demonstration that immune and epithelial cells can discriminate between different microbial species has extended our understanding of the actions of probiotics beyond simple barrier and antimicrobial concepts. Several probiotic mechanisms of action, relative to inflammatory bowel disease, have been elucidated: (1) competitive exclusion, whereby probiotics compete with microbial pathogens for a limited number of receptors present on the surface epithelium; (2) immunomodulation and/or stimulation of an immune response of gut-associated lymphoid and epithelial cells; (3) antimicrobial activity and suppression of pathogen growth; (4) enhancement of barrier function; and (5) induction of T cell apoptosis in the mucosal immune compartment. The unraveling of these mechanisms of action has led to new support for the use of probiotics in the management of clinical inflammatory bowel disease. Though level 1 evidence now supports the therapeutic use of probiotics in the treatment of postoperative pouchitis, only levels 2 and 3 evidence is currently available in support of the use of probiotics in the treatment of ulcerative colitis and Crohn's disease. Nevertheless, one significant and consistent finding has emerged during the course of research in the past year: not all probiotic bacteria have similar therapeutic effects. Rigorously designed, controlled clinical trials are vital to investigate the unresolved issues related to efficacy, dose, duration of use, single or multi-strain formulation, and the concomitant use of probiotics, synbiotics, or antibiotics.