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.

The association of Cryptosporidium oocysts with biofilm communities can influence the propagation of this pathogen through both environmental systems and water treatment systems. We observed the capture and retention of C. parvum oocysts in Pseudomonas aeruginosa biofilms using laboratory flow cells. Biofilms were developed in two different growth media using two different strains of P. aeruginosa, a wild-type strain (PAO1) and a strain that overproduces the exopolysaccharide alginate (PDO300). Confocal laser-scanning microscopy was used in conjunction with image analysis to assess the structure of the biofilms prior to introducing oocysts into the flow cells. More oocysts were captured by the biofilm-coated surfaces than the abiotic glass surface in both media. There was no significant difference in capture across the two strains of P. aeruginosa biofilm, but the fraction of oocysts captured was positively related to biofilm roughness and surface-area-to-volume ratio. Once captured, oocysts were retained in the biofilm for more than 24 h and were not released after a 40-fold increase in the system flow rate. We believe the capture and retention of oocysts by biofilm communities can impact the environmental transmission of C. parvum, and this interaction should be taken into consideration when predicting the migration of pathogens in the environment.The human pathogen Cryptosporidium parvum is responsible for numerous waterborne disease outbreaks in the United States (15,20,34,41,43). Outside its host, C. parvum exists as a nonreproductive oocyst, ϳ5 m in diameter, that is resistant to typical environmental stresses (6,29,39,40). C. parvum oocysts originate from the waste of infected hosts and are discharged in large quantities from municipal wastewater treatment facilities, animal agriculture, and wildlife populations (2,18,26,47). Because oocysts are persistent in the environment, the transmission of viable oocysts from sources to public water supplies can result in human infection even over long transport distances. Therefore, protection of public health requires a clear understanding of the factors that control the migration of Cryptosporidium in the environment.The transport of C. parvum oocysts can be influenced by interactions with surface-attached microbial communities, generally termed biofilms. Biofilms are ubiquitous in aquatic environments, where they form on rocks, plants, and sediments, and are also prevalent in wastewater treatment systems. Biofilm microorganisms are encased in a heterogeneous matrix of extracellular polymeric substances (EPS) composed of polysaccharides, proteins, lipids, and nucleic acids (10, 16). Both the morphology and chemical characteristics of biofilms are expected to promote the deposition and retention of C. parvum oocysts. Previous studies in both laboratory and environmental systems have shown that colloidal particles such as latex beads, bacteria, and virions can be readily transferred to biofilm communities from the surrounding bulk fluid (4,13,17,32,33,37,44,45). As...

Abstract:Stream-subsurface exchange strongly influences the transport of contaminants, fine particles, and other ecologically relevant substances in streams. We used a recirculating laboratory flume (220 cm long and 20 cm wide) to study the effects of particle size, overlying velocity, and biofilm formation on stream-subsurface exchange of particles. Sodium chloride was used as a non-reactive dissolved tracer and 1-and 5-µm fluorescent microspheres were used as particulate tracers. Surface-subsurface exchange was observed with a clean sand bed and a bed colonized by an autotrophic-heterotrophic biofilm under two different overlying velocities, 0Ð9 and 5 cm s 1 . Hydrodynamic interactions between the overlying flow and sand bed resulted in a reduction of solute and particle concentrations in the water column, and a corresponding accumulation of particles in both the sediments and in the biofilm. Increasing overlying velocity and particle size resulted in faster removal from the overlying water due to enhanced mass transfer to the bed. The presence of the biofilm did not affect solute exchange under any flow condition tested. The presence of the biofilm significantly increased the deposition of particles under an overlying velocity of 5 cm s 1 , and produced a small but statistically insignificant increase at 0Ð9 cm 1 . The particles preferentially deposited within the biofilm matrix relative to the underlying sand. These results demonstrate that hydrodynamic transport conditions, particle size, and biofilm formation play a key role in the transport of suspended particles, such as inorganic sediments, particulate organic matter, and pathogenic microorganisms in freshwater ecosystems, and should be taken into consideration when predicting the fate and transport of particles and contaminants in the environment.

The transfer of Cryptosporidium oocysts from the surface water to the sediment beds of streams and rivers influences their migration in surface waters. We used controlled laboratory flume experiments to investigate the deposition of suspended Cryptosporidium parvum oocysts in streambeds. The experimental results demonstrate that hydrodynamic interactions between an overlying flow and a sediment bed cause oocysts to accumulate in the sediments and reduce their concentrations in the surface water. The association of C. parvum with other suspended sediments increased both the oocysts' effective settling velocity and the rate at which oocysts were transferred to the sediment bed. A model for the stream-subsurface exchange of colloidal particles, including physical transport and physicochemical interactions with sediment grains, accurately represented the deposition of both free C. parvum oocysts and oocysts that were attached to suspended sediments. We believe that these pathogen-sediment interactions play an important role in regulating the concentrations of Cryptosporidium in streams and rivers and should be taken into consideration when predicting the fate of pathogens in the environment.

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