A comparison of fluorescein diacetate and propidium iodide staining andin vitroexcystation for determiningGiardia intestinaliscyst viability

SummaryThe viability of 4 human isolates ofGiardia intestinaliscysts using either the fluorogenic vital dyes fluorescein diacetate (FDA) and propidium iodide (PI) orin vitroexcystation was assessed. Whereas viable cysts, as defined byin vitroexcystation were present in each of the 4 isolates, cysts from only 3 of the 4 isolates took up the vital dyes. FDA consistently over-estimated cyst viability whilst PI under-estimated non-viable cysts when compared within vitroexcystation. Followingin vitroexcystation, both FDA and PI stained a proportion of unexcysted cysts indicating that FDA stained cysts which were incapable of excystation, whereas PI did not stain all cysts which were incapable of excystation. One human cyst isolate, which underwentin vitroexcystation, could not be stained with either FDA or PI. In the absence of currently more specific fluorescent indicators of viability, PI alone could be used to determine the lower limit of nonviability in positive water-related samples, where small numbers of cysts are to be expected.