Abstract. The adhesion of microorganisms to surfaces is influenced by long-range, short-range, and hydrodynamic forces. In the study of particle adhesion using well-defined, nonliving systems, long-range forces are adequately described by DLVO theory (due to Derjaguin and Landau, and Verwey and Overbeek), and hydrodynamic forces can be controlled. The quantitative description of short-range forces, however, remains a problem. In biological systems the application of DL VO theory as weIl as the quantitative description of short-range forces becomes difficult. Consequently, carefully designed experiments are required.The authors suggest that in order to advance the study of microbiaI adhesion, it is necessary for physical chemists and microbiologists to collaborate in a detailed study of adhesion in a weH characterized microbiological system.

The initial interaction between Streptococcus mutans and hard surfaces has been investigated using a rotating disc technique. The deposition to clean and BSA-coated glass of two strains of S.mutans, FA-1 (serotype b) and KPSK2 (serotype c), which exhibit different surface properties, was studied. Organisms were harvested from cultures grown in a chemostat at a dilution rate of 0.06 h-l and suspended in NaCl solutions of defined ionic strengths and pH values. The deposition of both strains showed a strong dependence on electrolyte concentration, particularly at low ionic strengths, which was inversely related to the zeta potentials of the organisms. Similarly, the ionic strength at which maximum deposition was first noted (critical coagulation concentration) for the two strains correlated with their relative potentials. Deposition was insensitive to changes in pH at an electrolyte concentration of 0.05 M. The maximum observed deposition did not approach values predicted by theory, suggesting that a further barrier to deposition, other than electrostatic repulsion, might exist. Under all experimental conditions, some of the deposited bacteria were observed to be oscillating, suggesting that they were held at a distance from the collector surface. The cells did not, however, appear to be deposited in a secondary minimum predicted by DLVO theory hence it may be that long-range polymer interactions are also involved in the deposition of these organisms.

A co-polymer with hydrophobic and hydrophilic segments was allowed to adsorb from aqueous solution onto glass previously made hydrophobic by derivatization with octadecyl dimethylchlorosilane. The polymer is thought to adsorb via its hydrophobic segments, leaving the hydrophilic segments free to extend into the water. After allowing cells to settle on the treated surface, the shear stress at the chamber wall required to remove red blood cells, Dictyostelium discoideum amoebae and Escherichia coli was determined in a calibrated laminar flow chamber. On octadecyl glass a shear stress of 2–3 Nm-2 evicts 50% of adherent red cells and E. coli. No D. discoideum amoebae could be removed at 5Nm-2. In striking contrast, the lowest experimentally obtainable shear stress of 0.03 Nm-2 removes 97.0-99.5% of cells of all three types from the polymer-treated surface, even after a cell residence time of 1 h without flow in the absence of free polymer. The minimum shear stress of 0.03Nm-2 corresponds to only approximately equal to 20 times the force of gravity on a red cell. The mechanism of action of the polymer and the implications of the results are discussed.

The heterogeneous nature of tea cream, with regard to solubility, is shown. A simple three-component system enables some of the observed cream characteristics to be explained. Suggestions are made concerning the compositions of these components and a model for the formation of cream particles, from a hot infusion, is proposed. Extraction of leaf tea with cold water gives an infusion incapable of creaming.