Filtration–elution of Cryptosporidium oocysts assisted by electrostatic interactions

Influence of ionic strength and soil characteristics on the behavior of Cryptosporidium oocysts in saturated porous media

The physico-chemical behavior of Cryptosporidium oocysts was investigated during their transfer through an alluvial formation from Les Cayes (Haiti) via batch tests. Five approximately 3 kg soil samples were collected and combined prior to batch tests from the alluvial formations. The experiments were carried out at soil pH by equilibrating different ranges of pure oocysts concentrations and soil samples with 3mM CaCl2 and 1mM NaBr as electrolyte. We used the Debye-Hückel equation describing ion activity in a solution for a given ionic strength. The equilibrium adsorption mechanism is used to enumerate the oocysts in the soil. The results suggest that the oocysts behavior in porous media depends on soil characteristics such as soil pH, the nature of the mineral and organic constituents of the soil and the ionic strength and activities in solution. These results show that a total transfer in batch containing NaBr solutions against a partial one in batch containing CaCl2 solutions depends on the oocysts media concentration. To confirm the oocysts number retained in soil, confocal microscopy was successfully used and the images demonstrate that the majority of oocysts were retained at the range of concentrations tested. The findings from this study demonstrated that the retention of C. Parvum in soils may be influenced by chemical conditions and soils characteristics, which are important for groundwater risk assessment.

Modifying the surface charge of pathogen-sized microspheres for studying pathogen transport in groundwater

Consuming pathogen-contaminated groundwater has caused many waterborne disease worldwide. Microspheres are often used as pathogen surrogates because they can be made similar to pathogens in terms of their sizes, buoyant densities, and shapes. Laboratory studies have, however, shown that the surface charges of microspheres are very different from those of pathogens of comparable sizes, and that their attenuation and transport behaviors differ significantly to those of the pathogens mimicked. Thus, for microspheres to be better surrogates, their surface charges need to be modified. We have demonstrated that the surface charge of a microorganism can be closely mimicked by microspheres covalently coated with a protein that has a similar pHPZC to the microorganism. Using MS2 bacteriophage to test our concept, 20 nm carboxylated microspheres were covalently coated with casein. Zeta potentials as a function of pH were determined for purified MS2, casein, and uncoated and coated microspheres. The uncoated microspheres were significantly more negatively charged than MS2. The coated microspheres displayed zeta potentials and a pHPZC value similar to MS2. The modified surface charge on the microspheres was stable for at least 4 mo. Using the concept developed from this study, surrogates for many specific pathogens of concern can be developed, and the results can be corrected with pathogen die-off determined independently in the laboratory. Protein-coated microspheres could provide a new and alternative approach to investigate pathogen transport in groundwater. Future research is required to validate the surrogates' resemblances to pathogens in terms of their attenuation and transport behaviors in groundwater.

Investigation of the interaction force between Cryptosporidium parvum oocysts and solid surfaces

Interaction force profiles between single Cryptosporidium parvum oocysts and positively charged, silane-coated silica particles were measured in aqueous solutions using an atomic force microscope. The oocysts were immobilized for the measurements by entrapment in Millipore polycarbonate membranes with 3 microm pore size. Experiments were performed in both NaCl and CaCl2 solutions at ionic strengths ranging from 1 to 100 mM. For both electrolytes, the decay length of the repulsive force profile was found to be nearly independent of the ionic strength and always much larger than the theoretical Debye length of the system. In addition, the magnitude of the force was found to be essentially 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 been deactivated using either chemical (formalin) or heat treatment. The force profiles obtained with formalin-treated oocysts 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 with the increased deposition rate observed with treated oocysts by Kuznar and Elimelech (Kuznar, Z. A.; Elimelech, M. Langmuir 2005, 21, 710-716). These results also suggest that treated oocysts, especially ones that have been inactivated by heating, may not be good surrogates for viable oocysts in laboratory studies.

Interaction force profiles between Cryptosporidium parvum oocysts and silica surfaces

The interaction force profile between single Cryptosporidium parvum oocysts and silica particles was measured in aqueous solutions using an atomic force microscope. The oocysts were immobilized during the measurements by entrapment in Millipore polycarbonate membranes with a 3 microm pore size. Experiments were performed in both NaCl and CaCl2 solutions at ionic strengths ranging from 1 to 100 mM. For both electrolytes the decay length of the repulsive force profile, obtained via the slope of a plot of the logarithm of interaction force versus separation, was found to be essentially independent of the ionic strength and always much larger than the theoretical Debye length of the system. In addition, the magnitude of the force was found to be essentially the same for both electrolytes, suggesting that the long-range repulsive forces are primarily steric in nature. Fitting the force to an expression for the steric repulsive force between two grafted brush layers yields a layer thickness of approximately 115 nm. These results support the idea that the oocysts are covered by a relatively thick layer of uncharged (or weakly charged) carbohydrates, possibly mixed with a thinner layer of charged protein.

Association of Cryptosporidium parvum with suspended particles: impact on oocyst sedimentation

The association of Cryptosporidium parvum oocysts with suspended particles can alter the oocysts' effective physical properties and influence their transport in aquatic systems. To assess this behavior, C. parvum oocysts were mixed with various suspended sediments under a variety of water chemical conditions, and the resulting settling of the oocysts was observed. Direct microscopic observations showed that oocysts attached to suspended sediments. Settling column and batch experiments demonstrated that oocysts are removed from suspension at a much higher rate when associated with sediments. The rate of oocyst sedimentation depended primarily on the type of sediment with which the oocysts were mixed. Changes in background water conditions had a relatively small impact on the extent of oocyst-particle association and the resulting oocyst deposition. We believe that the ubiquitous association of C. parvum oocysts with suspended particles enhances the sedimentation of oocysts in natural waters and that this interaction should generally be considered when predicting the migration of pathogens in the environment.

Role of surface proteins in the deposition kinetics of Cryptosporidium parvum oocysts

A radial stagnation point flow system was used to investigate the influence of Cryptosporidium parvum surface properties on oocyst deposition kinetics onto solid surfaces. To determine the role of oocyst surface proteins in adhesion, the deposition kinetics of viable oocysts were compared with the deposition kinetics of oocysts treated (inactivated) with either heat or formalin. Results showed a significantly higher deposition rate with formalin and heat-treated oocysts compared to viable oocysts under identical solution ionic strengths. Low deposition rates and corresponding attachment efficiencies were observed with viable oocysts over the entire range of solution conditions investigated, even at high ionic strengths where DLVO theory predicts the absence of an electrostatic energy barrier. An "electrosteric" repulsion between the viable Cryptosporidium oocyst and the quartz substrate, attributed to proteins on the oocyst surface, is surmised to cause this low deposition rate. Inactivation of the oocysts with either formalin or heat resulted in increased attachment efficiencies over the entire range of ionic strengths examined. It is hypothesized that formalin and heat treatments alter the structure of surface proteins and thus reduce steric repulsion. Formalin treatment was also found to impart an increased hydrophobicity to the oocyst surface and thus greater enhancement in oocyst deposition kinetics compared to heat treatment.

Adhesion kinetics of viable Cryptosporidium parvum oocysts to quartz surfaces

The transport and deposition (adhesion) kinetics of viable Cryptosporidium parvum oocysts onto ultrapure quartz surfaces in a radial stagnation point flow system were investigated. Utilizing an optical microscope and an image-capturing device enabled real time observation of oocyst deposition behavior onto the quartz surface in solutions containing either monovalent (KCl) or divalent (CaCl2) salts. Results showed a significantly lower oocyst deposition rate in the presence of a monovalent salt compared to a divalent salt. With a monovalent salt, oocyst deposition rates and corresponding attachment efficiencies were relatively low, even at high KCl concentrations where Derjaguin-Landau-Verwey-Overbeek (DLVO) theory predicts the absence of an electrostatic energy barrier. On the other hand, in the presence of a divalent salt, oocyst deposition rates increased continuously as the salt concentration was increased over the entire range of ionic strengths investigated. The unusually low deposition rate in a monovalent salt solution is attributed to "electrosteric" repulsion between the Cryptosporidium oocyst and the quartz surface, most likely due to proteins on the oocyst surface that extend into the solution. It is further proposed that specific binding of calcium ions to the oocyst surface functional groups results in charge neutralization and conformational changes of surface proteins that significantly reduce electrosteric repulsion.

Contribution of the comonomers to the bulk and surface properties of methacrylate copolymers

Relationships between formulation, bulk properties, and surface properties are investigated on series of copolymers prepared with hydroxyethylmethacrylate (HEMA), methylmethacrylate (MMA), and ethylmethacrylate (EMA) monomers, and on the homopolymers PMMA and PHEMA. The bulk water content, swelling ratio, and static (sessile drop and captive bubble) and dynamic (Wilhelmy plate technique) contact angles and the electrokinetic potential (streaming potential) are measured. The bulk water content and swelling ratio of HEMA copolymers are proportional to the amount of HEMA and are linearly correlated to the contact angle hysteresis. Periodic instabilities in the wetting cycles, similar to Haines jumps, are observed with HEMA copolymers and support a bidirectional relaxation of the hydrophilic groups respectively towards external water and capillary water. The origin of the electrokinetic potential of these nonionizable polymers is attributed to specific adsorption of [Formula: see text] ions. Its dependence on surface hydrophobicity and statistical length of the side-chains is interpreted in terms of the properties of water molecules near the interface.

Influence of surface characteristics on the stability of Cryptosporidium parvum oocysts

Microelectrophoresis is a common technique for probing the surface chemistry of the Cryptosporidium parvum oocyst. Results of previous studies of the electrophoretic mobility of C. parvum oocysts in which microelectrophoresis was used are incongruent. In this work we demonstrated that capillary electrophoresis may also be used to probe the surface characteristics of C. parvum oocysts, and we related the surface chemistry of C. parvum oocysts to their stability in water. Capillary electrophoresis results indicated that oocysts which were washed in a phosphate buffer solution had neutrally charged surfaces. Inactivation of oocysts with formalin did not influence their electrophoretic mobility, while oocyst populations that were washed in distilled water consisted of cells with both neutral and negative surface charges. These results indicate that washing oocysts in low-ionic-strength distilled water can impart a negative charge to a fraction of the oocysts in the sample. Rapid coagulation experiments indicated that oocysts did not aggregate in a 0.5 M NaCl solution; oocyst stability in the salt solution may have been the result of Lewis acid-base forces, steric stabilization, or some other factor. The presence of sucrose and Percoll could not be readily identified on the surface of C. parvum oocysts by attenuated total reflectance-Fourier transform infrared spectroscopy, suggesting that these purification reagents may not be responsible for the stability of the uncharged oocysts. These findings imply that precipitate enmeshment may be the optimal mechanism of coagulation for removal of oocysts in water treatment systems. The results of this work may help elucidate the causes of variation in oocyst surface characteristics, may ultimately lead to improved removal efficiencies in full-scale water treatment systems, and may improve fate and transport predictions for oocysts in natural systems.