| Abstract
| - Interaction force profiles between single Cryptosporidium parvum oocysts and positively charged, silane-coatedsilica particles were measured in aqueous solutions using an atomic force microscope. The oocysts were immobilizedfor the measurements by entrapment in Millipore polycarbonate membranes with 3 μm pore size. Experiments wereperformed in both NaCl and CaCl2 solutions at ionic strengths ranging from 1 to 100 mM. For both electrolytes, thedecay length of the repulsive force profile was found to be nearly independent of the ionic strength and always muchlarger than the theoretical Debye length of the system. In addition, the magnitude of the force was found to beessentially the same for both electrolytes, suggesting that the long-range repulsive forces are primarily steric in nature.These results support the theory that the interaction force between oocysts and surfaces is controlled by an outer,weakly charged or uncharged carbohydrate layer. Measurements were also performed with oocysts that had beendeactivated using either chemical (formalin) or heat treatment. The force profiles obtained with formalin-treatedoocysts appear to be essentially the same as for the untreated oocysts, whereas the profiles measured with the heat-treated oocysts show a much stronger dependence on solution ionic strength. With either the heat-treated or formalin-treated oocysts, adhesion was observed much more frequently than with untreated oocysts, which is consistent withthe increased deposition rate observed with treated oocysts by Kuznar and Elimelech (Kuznar, Z. A.; Elimelech, M.Langmuir2005, 21, 710−716). These results also suggest that treated oocysts, especially ones that have been inactivatedby heating, may not be good surrogates for viable oocysts in laboratory studies.
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