Direct force measurement between bio-colloidal Giardia lamblia cysts and colloidal silicate glass particles

Force-separation measurements between Giardia lamblia cysts and an inorganic oxide (silicate glass) have been obtained by using an atomic force microscope (AFM). The cysts are compressible on the scale of the loads applied during force measurement, with the surface compressibility expressed in terms of an interfacial spring constant (Kint). The force of interaction prior to this Hookean region, on approach, is long-range and repulsive. The long-range force has been compared to models of the electrical double layer as well as an electrosteric layer. The comparison has led to the conclusion that the cyst surface can be described as a polyelectrolyte brush at intermediate separations (5-115 nm from linear compliance) with an electrical double layer often observed at larger separations. The dependence of the interaction force on surface retraction suggests that tethering between the cyst and siliceous surface can occur. The variation of the interaction with pH and upon variation with ionic strength has also been assessed. The information gained from the measurement of the interaction between G. lamblia and this model sandlike surface informs water treatment processes. Similar studies have been performed by us for the Cryptosporidium parvum (C. parvum) oocyst system to which this work is compared.