Peroxiredoxins, the enzymes that catalyze the reduction of hydrogen peroxide and organic hydroperoxides, are ubiquitous proteins that protect organisms from damage by reactive oxygen species. Helicobacter pylori contains three members of the peroxiredoxin family: AhpC (alkyl hydroperoxide reductase), Tpx (thiol-specific peroxidase), and bacterioferritin comigratory protein (BCP). In this study, we characterized H. pylori bcp mutant strains and wild-type BCP. Compared to the parent strain and the ahpC mutant strain, the bcp mutant showed moderate sensitivity to the superoxide-generating agent paraquat and to organic hydroperoxides. Upon exposure of 10(8) cells to air for 10 h, 10(6) wild-type cells survived but none of the 10(8) bcp mutant cells were recovered. Introduction of an intact bcp gene at an unrelated locus in the bcp strain restored the wild-type-like oxidative stress resistance phenotype. Purified BCP was shown to be a thiol peroxidase that depends on the reducing activity of thioredoxin and thioredoxin reductase. Among a series of peroxides tested, linoleic acid hydroperoxide was the preferred substrate of BCP. By examining the profiles of protein expression within H. pylori cells, we confirmed that AhpC is much more abundant than BCP. The overlapping functions and activities of BCP and AhpC probably explain why the bcp mutant displayed a relatively weak oxidative stress resistance phenotype. The bcp mutant strain could colonize mouse stomachs, although colonization by the wild-type strain was slightly better than that by the mutant strain at 1 week after host inoculation. However, at 3 weeks after inoculation, the colonization ability of the wild type was significantly greater than that of the bcp mutant; for example, H. pylori was recovered from 10 of 11 mouse stomachs inoculated with the wild-type strain but from only 4 of 12 mice that were inoculated with the bcp mutant strain. This indicates that H. pylori BCP plays a significant role in efficient host colonization.