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

The surface chemical composition and physicochemical properties (hydrophobi city and zeta potential) of two lactic acid bacteria, Lactococcus lactis su bsp, lactis by. diacetilactis and Lactobacillus helveticus, have been inves tigated using cells harvested in exponential or stationary growth phase. Th e surface composition determined by X-ray photoelectron spectroscopy (XPS) was converted into a molecular composition in terms of proteins, polysaccha rides, and hydrocarbonlike compounds. The concentration of the last was alw ays below 15% (wt/wt), which is related to the hydrophilic character reveal ed by water contact angles of less than 30 degrees. The surfaces of L. lact is cells had a polysaccharide concentration about twice that of proteins. T he S-layer of L. helveticus was either interrupted or crossed by polysaccha ride-rich compounds; the concentration of the latter was higher in the stat ionary growth phase than in the exponential growth phase, Further progress was made in the interpretation of XPS data in terms of chemical functions b y 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 isoe lectric points were around 2 and 3, and the electrophoretic mobilities abov e pn 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 electrokine tic 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 prote ins and phosphate groups at the surface.