A new method to determine Giardia cyst viability: correlation of fluorescein diacetate and propidium iodide staining with animal infectivity.

The viability of Giardia muris cysts was studied with the fluorogenic dyes fluorescein diacetate (FDA) and propidium iodide (PI). G. muris cysts were seen to fluoresce intensely green with FDA at an excitation wavelength of 450 to 490 nm. Cysts stained with PI fluoresced bright orange at an excitation wavelength of 450 to 490 nm and bright red at 545 to 546 nm. Examination of isolated G. muris cyst preparations stained with FDA-PI revealed that greater than 85% of the cysts stained green with FDA and less than 15% stained orange-red with PI. Using the mouse model for giardiasis, we inoculated FDA- or PI-stained cysts into neonatal mice. Feces were examined at days 3, 5, 8, and 11 postinoculation for the presence of cysts. Using 1,000 FDA-stained cysts as the inoculum, we detected cysts at days 5, 8, and 11 postinoculation in 19 of 19 mice, whereas a 50-fold greater dose of cysts produced infection in 27 of 27 mice at day 3 as well as at days 5, 8, and 11 postinoculation. Inoculation of mice with either 5,000 or 50,000 PI-stained G. muris cysts did not produce infection in any of the animals. Necropsy of mice infected with FDA-stained cysts showed trophozoites within the intestines. No trophozoites were detected within animals inoculated with PI-stained cysts. These results demonstrate that FDA-positive cysts are viable, as determined by infectivity, while PI-positive cysts are nonviable and incapable of producing G. muris infections in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)