Giardia intestinalis escapes oxidative stress by colonizing the small intestine: A molecular hypothesis.

Giardia intestinalis is the microaerophilic protozoon causing giardiasis, a common infectious intestinal disease. Giardia possesses an O(2)-scavenging activity likely essential for survival in the host. We report that Giardia trophozoites express the O(2)-detoxifying flavodiiron protein (FDP), detected by immunoblotting, and are able to reduce O(2) to H(2)O rapidly (similar to 3 mu M O(2) x min x 10(6) cells at 37 degrees C) and with high affinity (C(50) = 3.4 +/- 0.7 mu M O(2)). Following a short-term (minutes) exposure to H(2)O(2) >= 100 mu M, the O(2) consumption by the parasites is irreversibly impaired, and the FDP undergoes a degradation, prevented by the proteasome-inhibitor MG132. Instead, H(2)O(2) does not cause degradation or inactivation of the isolated FDP. On the basis of the elevated susceptibility of Giardia to oxidative stress, we hypothesize that the parasite preferentially colonizes the small intestine since, compared with colon, it is characterized by a greater capacity for redox buffering and a lower propensity to oxidative stress.