Flow cytometry with fluorescence activated cell sorting (FACS) was used to purify Cryptosporidium (oo)cysts and Giardia cysts from water. With this purification step Cryptosporidium and Giardia were found in a higher percentage of the samples and significantly higher Giardia concentrations were detected in these positive samples. Because FACS removed most of the debris from the concentrated water sample, the microscopic preparations could be examined more rapidly for the presence of (oo)cysts and the morphological characteristics of the (oo)cysts could be interpreted more unambiguously than with the conventional immunofluorescence microscopy method. The use of FACS made it possible to apply PI-staining on environmental samples to determine the fraction of dead (oo)cysts. Sample processing did not appear to influence the PI-staining characteristics of the Cryptosporidium (oo)cysts, but did increase the percentage of PI-positive Giardia cysts. This suggests that this protocol can be used for determining the percentage of dead Cryptosporidium oocysts in environmental samples. Analysis of environmental samples suggests that reservoir storage increases the percentage of PI-positive (dead) oocysts.

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