Laterally-resolved force microscopy of biological microspheres-oocysts of Cryptosporidium parvum

An atomic force microscope (AFM) has been used to measure the force of inte raction between individual biological microspheres (oocysts of Cryptosporid ium parvum) and an amorphous silica surface (hydrolyzed AFM tip). One of th e main barriers to oocyst contamination of drinking waters is provided by s and-bed filtration. The AFM tip has been functionalized to silica in order to investigate the interaction between oocysts and model sand (siliceous) p articles at the most fundamental level. The AFM force curves have been comp ared and contrasted with the zeta-potentials of silica particles and oocyst s obtained from electrophoretic mobility measurements. It has been conclude d that there is a steric interaction between the silica surface and the ooc yst material, in addition to electrical double-layer and van der Waals inte raction. The proteinaceous materials on the surface of the oocysts are cons idered to be responsible for the steric interaction. The magnitude of the s teric interaction is little changed by varying the pH and electrolyte conce ntration (i.e. changing the charge characteristics of surfaces has little e ffect on the magnitude of the interaction). Force curves suggest that once the silica and oocyst surfaces contact one another, protein-linked tetherin g can occur. Therefore, despite the large steric barrier, adhesion between oocysts and silica may occur. A measurement of the hardness of the Cryptosp oridium oocyst surface has shown that the surface has silica-like hardness, which may explain why the oocysts are so resistant to disinfection. Finall y, the effect on the surface force measurement of alignment between two cur ved surfaces has been reported. The findings have implications for all stud ies that force map surfaces that possess curvature.