| Abstract
| - The interaction force profile between single Cryptosporidiumparvum oocysts and silica particles was measured inaqueous solutions using an atomic force microscope. Theoocysts were immobilized during the measurements byentrapment in Millipore polycarbonate membranes with a3 μm pore size. Experiments were performed in bothNaCl and CaCl2 solutions at ionic strengths ranging from1 to 100 mM. For both electrolytes the decay length of therepulsive force profile, obtained via the slope of a plotof the logarithm of interaction force versus separation, wasfound to be essentially independent of the ionic strengthand always much larger than the theoretical Debye lengthof the system. In addition, the magnitude of the forcewas found to be essentially the same for both electrolytes,suggesting that the long-range repulsive forces areprimarily steric in nature. Fitting the force to an expressionfor the steric repulsive force between two grafted brushlayers yields a layer thickness of approximately 115 nm. Theseresults support the idea that the oocysts are covered bya relatively thick layer of uncharged (or weakly charged)carbohydrates, possibly mixed with a thinner layer of chargedprotein.
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