The surface chemical composition and physicochemical properties (hydrophobicity and zeta potential) of two lactic acid bacteria, Lactococcus lactis subsp. lactis bv. diacetilactis and Lactobacillus helveticus, have been investigated using cells harvested in exponential or stationary growth phase. The surface composition determined by X-ray photoelectron spectroscopy (XPS) was converted into a molecular composition in terms of proteins, polysaccharides, and hydrocarbonlike compounds. The concentration of the last was always below 15% (wt/wt), which is related to the hydrophilic character revealed by water contact angles of less than 30°. The surfaces of L. lactis cells had a polysaccharide concentration about twice that of proteins. The S-layer of L. helveticus was either interrupted or crossed by polysaccharide-rich compounds; the concentration of the latter was higher in the stationary growth phase than in the exponential growth phase. Further progress was made in the interpretation of XPS data in terms of chemical functions by showing that the oxygen component at 531.2 eV contains a contribution of phosphate in addition to the main contribution of the peptide link. The isoelectric points were around 2 and 3, and the electrophoretic mobilities above pH 5 (ionic strength, 1 mM) were about ؊3.0 ؋ 10 ؊8 and ؊0.6 ؋ 10 ؊8 m 2 s ؊1 V ؊1 for L. lactis and L. helveticus, respectively. The electrokinetic properties of the latter reveal the influence of carboxyl groups, while the difference between the two strains is related to a difference between N/P surface concentration ratios, reflecting the relative exposure of proteins and phosphate groups at the surface.In many instances, the behavior of lactic acid bacteria is dependent on interfacial processes and thus on cell surface physicochemical properties and chemical composition. A better knowledge of these aspects would allow a deeper understanding of the autolysis of lactic acid bacteria (29, 36) and the production of texturing exopolysaccharides (4). It would help in controlling the sedimentation of starters for commercial production (7) and in understanding the roles of specific and nonspecific interactions in phage attachment (7,46,56).In dairy product manufacturing, adhesion of lactic bacteria to a material may be the first step leading to biofilm formation, which can be either deleterious (contamination, taste alteration, and biofouling on heat exchangers) (17, 25) or beneficial (continuous inoculation in yogurt or cheese making) (5). The adhesion behavior of microbial cells has been shown to depend on the balance of electrostatic and van der Waals interactions and on the hydrophobic character of the surfaces involved (38,42,53,54), pointing to the possible influence of the respective zeta potentials and surface hydrophobicities. Moreover, the production of extracellular substances either at the cell surface or in the surrounding medium has been shown to influence adhesion (14, 55).The surface hydrophobicity and composition of lactic acid bacteria have been studie...

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Atomic force microscopy (AFM) has been used to probe, under physiological conditions, the surface ultrastructure and molecular interactions of spores of the filamentous fungus Phanerochaete chrysosporium. High-resolution images revealed that the surface of dormant spores was uniformly covered with rodlets having a periodicity of 10 ± 1 nm, which is in agreement with earlier freeze-etching measurements. In contrast, germinating spores had a very smooth surface partially covered with rough granular structures. Force-distance curve measurements demonstrated that the changes in spore surface ultrastructure during germination are correlated with profound modifications of molecular interactions: while dormant spores showed no adhesion with the AFM probe, germinating spores exhibited strong adhesion forces, of 9 ± 2 nN magnitude. These forces are attributed to polysaccharide binding and suggested to be responsible for spore aggregation. This study represents the first direct characterization of the surface ultrastructure and molecular interactions of living fungal spores at the nanometer scale and offers new prospects for mapping microbial cell surface properties under native conditions.

In this paper, direct measurement by atomic force microscopy (AFM) of the cell surface softness of a fibrillated oral streptococcal strain Streptococcus salivarius HB and of a nonfibrillated strain S. salivarius HBC12 is presented, and the data interpretation is validated by comparison with results from independent techniques. Upon approach of the fibrillated strain in water, the AFM tip experienced a long-range repulsion force, starting at approximately 100 nm, attributed to the compression of the soft layer of fibrils present at the cell surface. In 0.1 M KCl, repulsion was only experienced when the tip was closer than approximately 10 nm, reflecting a stiffer cell surface due to collapse of the fibrillar mass. Force-distance curves indicated that the nonfibrillated strain, probed both in water and in 0.1 M KCl, was much stiffer than the fibrillated strain in water, and a repulsion force was experienced by the tip at close approach only (20 nm in water and 10 nm in 0.1 M KCl). Differences in cell surface softness were further supported by differences in cell surface morphology, the fibrillated strain imaged in water being the only specimen that showed characteristic topographical features attributable to fibrils. These results are in excellent agreement with previous indirect measurements of cell surface softness by dynamic light scattering and particulate microelectrophoresis and demonstrate the potential of AFM to directly probe the softness of microbial cell surfaces.

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Atomic force microscopy (AFM) was used to characterize, under water, the surface morphology and molecular interactions of two types of microbial cells: fungal spores (Phanerochaete chrysosporium) and bacteria (Lactococcus lactis). High-resolution deflection images showed that the spore surface was uniformly covered with patterns of rodlets that were several hundred nanometres in length and had a periodicity of ~10 nm. Such surface organization was not detected on the bacterial surface, which showed a sponge-like structure. Force-distance curves revealed very different molecular interactions for the two microorganisms: upon approach, no significant curvature was seen in the contact region for spores, contrary to bacteria, pointing to a difference in cell softness; and upon retraction, no adhesion forces were detected on spores but multiple unbinding events and elongation forces attributed to macromolecular bridging were observed on bacteria. These results show that AFM is a powerful tool for probing the surface properties of native microbial cells on the nanometre scale.

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