Force interaction profiles between viable Cryptosporidium parvum oocysts and charged surfaces

Force interaction profiles between viable C. parvum oocyst surfaces and charged surfaces have been studied under a variety of solution conditions. For this work, a novel method of oocyst immobilization was developed, which eliminated the introduction of oocyst surface alterations during sample preparation, via entrapment of the oocyst in a Millipore polycarbonate membrane with pore size slightly smaller (3 mum) than that of the oocyst particle. It was found that the interaction force between an oocyst surface and a charged substrate is steric in nature for both a monovalent and a divalent salt solution. This is theorized to be due to a thin, charged polypeptide backbone on the oocyst surface which is shielded by a thick, uncharged carbohydrate layer. Subsequent data fitting and adhesion analyses have revealed this layer to be greater than 100 nm thick. Alterations of the oocyst surface by means of formalin treatment revealed no significant change in the force interaction with increasing ionic strength between the oocyst and a negatively-charged surface. However, alteration of the oocyst surface by means of heat treatment revealed a significant decrease in the steric repulsion between the oocyst and a negatively-charged surface for increasing ionic strength. Adhesion forces were also observed to increase for the treated oocysts as opposed to the untreated oocysts. This was especially true when calcium ions were present. The addition of humic acid in the presence of a 1 mM monovalent salt revealed no significant adsorption of the humic acid to the oocyst surface. However, significant adsorption of humic acid to the oocyst surface was observed in the presence of calcium ions. Force profiles obtained in the presence and absence of humic acid for increasing concentrations of calcium chloride, however, revealed no significant change in the force interaction between the oocyst and negatively-charged silica surface. This result was somewhat surprising and could be due to a lack of sensitivity in the AFM force measurements.