METABOLIC CHANGES IN GIARDIA-INTESTINALIS DURING DIFFERENTIATION

The oxygen uptake rate and metronidazole (MTZ) sensitivity in Giardia spp, cysts is greatly reduced from that in trophozoites. Thus, this pr oject was undertaken to assess when in the encystation process these p henomena occur. Oxygen uptake rates approximately doubled (from simila r to 4.9 to 8.3 mu M O-2 min(-1) 10(-6) cells) during the first 5 hr i nto encystation. This increase was followed by a marked decrease to 2. 3 mu M O-2 min(-1) 10(-6) by 12 hr. By 50 hr into encystation. oxygen uptake was 0.7 mu M O-2 min(-1) 10(-6) cells. Glucose stimulated oxyge n uptake by 89% in trophozoites but did not demonstrably stimulate oxy gen uptake in cells after 12 hr into encystment. Deoxy-D-glucose uptak e dropped by more than an order of magnitude in encysting cells compar ed to nonencysting cells. In contrast, aspartate uptake remained relat ively constant regardless of whether cells were encysting or not. This suggests that there is a change in the parasite's ability to transpor t glucose during cyst formation: a similar change in the parasite's ab ility to transport aspartate was not observed after 40 hr into encysta tion. MTZ inhibited oxygen uptake by 77% in trophozoites, but there wa s no detectable inhibition of oxygen uptake 8 hr after trophozoites we re transferred to encystation medium. We propose that this resistance to MTZ may be due to a change in metabolic flux away from the pyruvate ferredoxin oxidoreductase pathway. Oxygen uptake by noninduced cysts increased exponentially during the 30 min following the induction of e xcystation. Likewise, MTZ sensitivity returned within 15 min after the induction of excystation, and by 30 min into excystation full sensiti vity had returned.