The use of
propidium iodide, whose uptake indicates cell death or damage, was investigated to assess the viability of heat-inactivated and chemically inactivated Giardia muris
cysts. This was done by comparing
propidium iodide staining with excystation. We first determined that
propidium iodide could be used with an immunofluorescence detection procedure by showing that the percentages of Giardia lamblia
cysts stained with this
dye before and after subjecting them to a fluorescence detection method were similar. G. muris
cysts were then exposed to heat (56 degrees C), 0.5 to 4 mg of
chlorine per liter (pH 7.0, 5 degrees C), 0.1 to 10 mg of a
quaternary ammonium compound per liter, or 2 mg of preformed and forming
monochloramine per liter (pH 7.2, 18 to 20 degrees C). A good positive correlation between percent
propidium iodide-stained
cysts and lack of excystation was demonstrated for G. muris
cysts exposed either to heat or to the
quaternary ammonium compound. However, no significant correlation between absence of excystation and
propidium iodide staining was found for
cysts exposed to
chlorine or monochloramines. These results demonstrate that the
propidium iodide staining procedure is not satisfactory for determining the viability of G. muris
cysts exposed to these two commonly used
drinking water disinfectants.