Impacts of viability and purification on the specific gravity of Cryptosporidium oocysts

The specific gravity of unpurified and purified Cryptosporidium oocysts was measured using an isopycnic gradient centrifugation technique. Specific gravity varied depending on the viability of the oocysts, as defined by permeabilty to 4′,6-diamidino-2-phenylindole (DAPI) and propidium iodide (PI), the presence or absence of internal structures, and whether or not oocysts were purified. The modal range of densities for a population of 1.4-week-old unpurified oocysts, was 1070–1073 kg/m3. This range was higher than that determined for 14-week-old purified oocysts, 1067–1070 kg/m3. Eleven- and 12-week-old unpurified populations exhibited a bimodal distribution of densities, with densities most frequently in the 1005–1041 and in the 1077–1108 kg/m3 range. In these populations, a high percentage of the oocysts having densities greater than 1077 kg/m3 were viable, while oocysts in the 1005–1024 kg/m3 range were predominately nonintact, and oocysts in the 1024–1041 kg/m3 range were intact, but permeable to DAPI and PI (nonviable). This work demonstrates the importance of controlling factors that may impact the viability of oocysts when conducting studies that examine the transport of these microorganisms in the environment and through water treatment processes.