Abstract

Giardia intestinalis is the microaerophilic protozoon causing giardiasis, a common infectious intestinal disease. Giardia possesses an O₂-scavenging activity likely essential for survival in the host. We report that Giardia trophozoites express the O₂-detoxifying flavodiiron protein (FDP), detected by immunoblotting, and are able to reduce O₂ to H₂O rapidly (~3 μM O₂ × min × 10⁶ cells at 37 °C) and with high affinity (C₅₀ = 3.4 ± 0.7 μM O2). Following a short-term (minutes) exposure to H₂O₂ ≥ 100 μM, the O₂ consumption by the parasites is irreversibly impaired, and the FDP undergoes a degradation, prevented by the proteasome-inhibitor MG132. Instead, H₂O₂ 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.

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