Abstract
Oxidative stress resistance is one of the key properties that enable pathogenic bacteria to survive the toxic reactive oxygen species released by the host. In a previous study characterizing oxidative stress resistance mutants of Helicobacter pylori, a novel potential antioxidant protein (MdaB) was identified by the observation that the expression of this protein was significantly upregulated to compensate for the loss of other major antioxidant components. In this study, we characterized an H. pylori mdaB mutant and the MdaB protein. While the wild-type strain can tolerate 10% oxygen for growth, the growth of the mdaB mutant was significantly inhibited by this oxygen condition. The mdaB mutant is also more sensitive to H(2)O(2), organic hydroperoxides, and the superoxide-generating agent paraquat. Although the wild-type strain can survive more than 10 h of air exposure, exposure of the mutant strain to air for 8 h resulted in recovery of no viable cells. The oxidative stress sensitivity of the mdaB mutant resulted in a deficiency in the ability to colonize mouse stomachs. H. pylori was recovered from 10 of 11 mouse stomachs inoculated with the wild-type strain, with about 5,000 to 45,000 CFU/g of stomach. However, only 3 of 12 mice that were inoculated with the mdaB mutant strain were found to harbor any H. pylori, and these 3 contained less than 2,000 CFU/g of stomach. A His-tagged MdaB protein was purified and characterized. It was shown to be a flavoprotein that catalyzes two-electron transfer from NAD(P)H to quinones. It reduces both ubiquinones and menaquinones with similar efficiencies and preferably uses NADPH as an electron donor. We propose that the physiological function of the H. pylori MdaB protein is that of an NADPH quinone reductase that plays an important role in managing oxidative stress and contributes to successful colonization of the host.