A study was made of binding of an antifoam preparation containing polydimethylsiloxane (PDS) to cells of Saccharomyces cerevisiae NCYC 366. These cells bound approximately 2.1 mug PDS per 2 x 107 organisms when suspended in 0.1 M KH2PO4 buffer (pH 4.5) containing Antifoam M-10 (to give 19 mug PDS per ml). A decrease in the concentration of any one of the two emulsifiers, polyoxyethylene sorbitan monostearate or glycerol monostearate caused an increase in the saturation concentration of PDS bound by the organisms, while a decrease in the concentration of the thickener, sodium carboxymethyl cellulose caused a decrease. The sites involved in binding PDS were indicated to be located in the cell-wall phosphomannan-protein of the organisms by studies made on PDS binding by whole cells, PDS release by saturated organisms, analyses of isolated walls and surface properties of organisms, before and after chemical and enzymic treatments. The surface charge of the organisms at different pH values had no effect on PDS binding. Binding of PDS had a masking effect on the electrophoretic mobility, binding of antibody and binding of concanavalin A by the organisms, but had no effect on the release of invertase. An examination was made of the ability of each of 4 strains of Saccharomyces cerevisiae to flocculate following the excision of the phosphodiester linkages of cell-wall phosphomannan. Treatment of isolated walls of each of the 4 strains with hydrofluoric acid (58 - 62%, v/v) removed most of the phosphorus without extensive losses of other components. Treatment of whole cells with the same reagent increased the sedimentation rates of both flocculent and non-flocculent cells. The presence of Ca2+ ions was found to be essential for the expression of flocculence either inherent or induced by hydrofluoric acid treatment. Esterification of surface carboxyl groups decreased the sedimentation rates of both untreated flocculent cells and hydrofluoric acid-treated organisms. Inclusion of mannose in the suspending medium deflocculated untreated flocculent cells but failed to deflocculate hydrofluoric acid-treated organisms. The phosphorus contents of the outer layers of the cell-wall as indicated by the electrophoretic mobility at pH 4.0 or the amount of calcium bound by isolated walls was not related to the flocculation characteristics of the organisms.
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