However, in contrast to chloramphenicol that enhanced the bactericidal effect of H2O2 (Figure 8, left half, cross-hatched bar), the addition of ampicillin reduced the bactericidal activity of H2O2 for unknown reasons click here (Figure 8, left half, compare horizontally hatched bar to diagonally-hatched bar). This indicates that the synergistic effect of chloramphenicol on the bactericidal activity of H2O2 is not due to its bacteriostatic effect and suggests that protein synthesis is important for E. coli to resist the killing by H2O2. Figure 8 Chloramphenicol enhanced the bactericidal activity of H 2 O 2 . The wild type E. coli (WT) and the ΔarcA mutant E. coli (ΔarcA) were incubated in M9

minimal medium containing 1.5 mM H2O2 for 6 hours at 37°C. The survival of bacteria was determined by plating. Bacterial concentration following each treatment (open bars, no treatment; diagonally-hatched Selleckchem LEE011 bars, H2O2; vertically-hatched bars, 25 μg ml-1 of chloramphenicol; cross-hatched bars, H2O2 and 25 μg ml-1 of chloramphenicol; dotted line-hatched bars, 50 μg

ml-1 ampicillin and horizontally-hatched bars, H2O2 and 50 μg ml-1 ampicillin) was plotted on the graph. The horizontal dashed line indicates the starting concentration of bacteria. Similar assays were carried out with the ΔarcA mutant E. coli and the results were consistent with those of the wild type E. coli. While incubation with H2O2 alone reduced the concentration of the ΔarcA mutant E. coli by over 5log10 after 6 hours of incubation (Figure 8, right half, diagonally-hatched bar), the addition of chloramphenicol to the assay eliminated all E. coli (Figure 8). The synergistic effect of the bactericidal activity of H2O2 and chloramphenicol on the ΔarcA mutant E. coli is not because it is more susceptible to chloramphenicol (Figure 8, vertically-hatched bars). Similarly to that observed with wild type E. coli, ampicillin reduced the bactericidal activity Glutamate dehydrogenase of H2O2, and the ΔarcA mutant E. coli survived better

in the presence of both ampicillin and H2O2 than H2O2 alone (1.7 × 105 CFU/ml vs. 1.0 × 102 CFU/ml) (Figure 8). Discussion Although the ArcAB Akt tumor system has been extensively investigated for its role as the global control system of E. coli in anaerobic growth, its role, if any, in aerobic growth is much less understood. We have previously reported that ArcA is necessary for the pathogenic bacterium Salmonella enterica to resist reactive oxygen and nitrogen species under aerobic conditions [38]. In this report, we used E. coli as our model to further explore the role of both ArcA and ArcB in ROS resistance, and to investigate the mechanism of ROS resistance mediated by the ArcAB two-component system. Here we demonstrate that deletion mutants of ArcA and ArcB were more susceptible to H2O2, suggesting that both ArcA and ArcB were necessary for E. coli to resist the stress caused by H2O2 (Figure 1), and that their functions were not limited to anaerobic growth of bacteria.