Surface of lactic acid bacteria: relationships between chemical composition and physicochemical properties.

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 degrees. 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 x 10(-8) and -0.6 x 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.

Direct probing by atomic force microscopy of the cell surface softness of a fibrillated and nonfibrillated oral streptococcal strain.

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.

Modification of the aggregation behaviour of the environmental Ralstonia eutropha-like strain AE815 is reflected by both surface hydrophobicity and amplified fragment length polymorphism (AFLP) patterns.

After inoculation of the plasmid-free non-aggregative Ralstonia eutropha-like strain AE815 in activated sludge, followed by reisolation on a selective medium, a mutant strain A3 was obtained, which was characterized by an autoaggregative behaviour. Strain A3 had also acquired an IncP1 plasmid, pLME1, co-aggregated with yeast cells when co-cultured, and stained better with Congo red than did the AE815 strain. Contact angle measurements showed that the mutant strain was considerably more hydrophobic than the parent strain AE815, and scanning electron microscopy (SEM) revealed the production of an extracellular substance. A similar hydrophobic mutant (AE176R) could be isolated from the AE815-isogenic R. eutropha-like strain AE176. With the DNA fingerprinting technique repetitive extragenic palindromic-polymerase chain reaction (REP-PCR), no differences between these four strains, AE815, A3, AE176 and AE176R, could be revealed. However, using the amplified fragment length polymorphism (AFLP) DNA fingerprinting technique with three different primer combinations, small but clear reproducible differences between the banding patterns of the autoaggregative mutants and their non-autoaggregative parent strains were observed for each primer set. These studies demonstrate that, upon introduction of a strain in an activated sludge microbial community, minor genetic changes readily occur, which can nevertheless have major consequences for the phenotype of the strain and its aggregation behaviour.

Direct probing of the surface ultrastructure and molecular interactions of dormant and germinating spores of Phanerochaete chrysosporium.

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.