Formate dehydrogenase, ubiquinone, and cytochrome bd-I are required for peptidoglycan recognition protein-induced oxidative stress and killing in Escherichia coli

Mammalian Peptidoglycan Recognition Proteins (PGRPs) kill bacteria through induction of synergistic oxidative, thiol, and metal stress. PGRPs induce oxidative stress in bacteria through a block in the respiratory chain, which results in decreased respiration and incomplete reduction of oxygen (O(2)) to hydrogen peroxide (H(2)O(2)). In this study we identify the site of PGRP-induced generation of H(2)O(2) in Escherichia coli. Tn-seq screening of E. coli Tn10 insertion library revealed that mutants in formate dehydrogenase (FDH) genes had the highest survival following PGRP treatment. Mutants lacking functional FDH-O had abolished PGRP-induced H(2)O(2) production and the highest resistance to PGRP-induced killing, and formate enhanced PGRP-induced killing and H(2)O(2) production in an FDH-dependent manner. Mutants in ubiquinone synthesis (but not menaquinone and demethylmenaquinone) and cytochrome bd-I (but not cytochromes bo(3) and bd-II) also had completely abolished PGRP-induced H(2)O(2) production and high resistance to PGRP-induced killing. Because electrons in the respiratory chain flow from dehydrogenases’ substrates through quinones and then cytochromes to O(2), these results imply that the site of PGRP-induced incomplete reduction of O(2) to H(2)O(2) is downstream from dehydrogenases and ubiquinone at the level of cytochrome bd-I, which results in oxidative stress. These results reveal several essential steps in PGRP-induced bacterial killing.