|Home | About | Journals | Submit | Contact Us | Français|
At present, Salmonella is considered to express two peroxiredoxin-type peroxidases, TsaA and AhpC. Here we describe an additional peroxiredoxin, Tpx, in Salmonella enterica and show that a single tpx mutant is susceptible to exogenous hydrogen peroxide (H2O2), that it has a reduced capacity to degrade H2O2 compared to the ahpCF and tsaA mutants, and that its growth is affected in activated macrophages. These results suggest that Tpx contributes significantly to the sophisticated defense system that the pathogen has evolved to survive oxidative stress.
Salmonella is an important human pathogen which causes a variety of diseases, including gastroenteritis, septicemia, and typhoid fever. In the host, salmonellae reside inside phagocytic cells and are exposed to various host defense mechanisms, including oxidative stress (13). The production of superoxide anion (O2−) is crucial, as individuals with chronic granulomatous disease, which is due to a defective phagocyte NADPH oxidase, are more susceptible to infections with Salmonella (10). Likewise, diminished NADPH oxidase activity leads to increased susceptibility to Salmonella in murine macrophages (20-22, 25). Superoxide anion (O2−) is weakly reactive and fails to pass through the bacterial cell wall. After conversion to H2O2 by either spontaneous or enzymatic dismutation by superoxide dismutases, it readily diffuses into the bacterial cell and forms reactive hydroxyl radicals (OH) that damage macromolecules such as DNA, proteins, and lipids (12, 17).
In principle, Salmonella possesses two classes of enzymes to degrade H2O2. Catalases degrade H2O2 to water and molecular oxygen independent of an additional reductant. Peroxiredoxin-type peroxidases (peroxiredoxins) reduce organic hydroperoxides to alcohols and hydrogen peroxide to water at the expense of NADH or NADPH. In a recent study by Hébrard et al., three members of the catalase family, KatG, KatE, and KatN, and two members of the peroxiredoxin family, AhpC and TsaA, were characterized in Salmonella (14). Previously it had been shown that single katE, katG, and katN Salmonella mutants did not show increased susceptibility to exogenous H2O2 (3, 24). In macrophages a katG katE katN triple mutant had no growth defect, whereas an ahpCF tsaA double mutant showed a reduced growth rate in macrophages (14). These observations point out the multiple routes that have evolved in Salmonella to protect the pathogen against oxidative stress and suggest that peroxiredoxins play a dominant role in the antioxidant defense during infection. In this study we characterized a third peroxiredoxin-type peroxidase, Tpx. Surprisingly, a simple tpx mutant of Salmonella enterica serovar Typhimurium (S. Typhimurium) was more susceptible to exogenous H2O2 than the wild type (WT). The mutant grew less well in activated macrophages and showed a reduced peroxidase activity toward H2O2.
The contribution of Tpx to the oxidative stress response of S. Typhimurium was assessed in vitro using exogenous H2O2. Deletion of tpx (STM1682) was performed by the method of Datsenko and Wanner (9). Overnight cultures of the wild type and the tpx mutant were adjusted to an optical density at 600 nm (OD600) of 0.2 and diluted 1:10 in LB medium, and 100 μl of the diluted suspension was challenged with 2 mM H2O2 at 37°C. Killing of S. Typhimurium was enumerated by plating for CFU on LB agar after the indicated time points (Fig. (Fig.1A).1A). After 6 h, the difference in survival between the wild type and the mutant became significant (P < 0.005 using a two-sided t test). While the wild type was reduced to ≈2.1 × 105 CFU/ml after 9 h, the tpx mutant was reduced to ≈6 × 103 CFU/ml. Expression of wild-type tpx on the pBAD30 plasmid (11) in the tpx mutant restored the wild-type phenotype upon challenge with H2O2 (Fig. (Fig.1A).1A). Taking into account that there are three catalases (KatE, KatG, and KatN) and two other peroxiredoxins (AhpC and TsaA) present in S. Typhimurium, the clear phenotype of the tpx mutant under exogenous H2O2 was surprising. Apparently the remaining scavenging capacity in the tpx mutant was not sufficient to protect S. Typhimurium from exogenous H2O2.
To address the role of Tpx during intracellular replication in gamma interferon (IFN-γ)-activated RAW 264.7 macrophages (10 ng/ml), cells were infected with S. Typhimurium as described previously (6). Intracellular proliferation was calculated by dividing the bacterial load of the macrophages at 6 h or 16 h postinfection by the number of intracellular bacteria at 2 h after infection. In activated macrophages, intracellular proliferation of the tpx mutant was decreased significantly after 16 h (Fig. (Fig.1B)1B) compared to that of the wild type (≈20.1-fold and ≈34.2-fold, respectively; P < 0.05). To test whether the attenuation of the tpx mutant in activated macrophages was due to an oxidative burst, RAW 264.7 cells were treated at 2 h postinfection with 10 μM diphenyleneiodonium (DPI), a known inhibitor of the phagocyte oxidase. In the presence of DPI, the tpx mutant showed the same proliferation rate as the WT and complemented strains (Fig. (Fig.1B).1B). Finally, the intracellular proliferation was investigated at 6 h after infection, as an oxidative burst in macrophages starts very early after infection. The tpx mutant showed a slightly decreased proliferation compared to the WT (≈2.9-fold and ≈3.4-fold, respectively; P > 0.05) which could be restored in the complemented strain (≈3.6-fold; P < 0.05).
Hébrard et al. reported that the catalases of S. Typhimurium are dispensable during infection of IFN-γ stimulated RAW cells, whereas a mutant lacking ahpCF/tsaA shows a reduced proliferation, similar to the result described here for the tpx mutant (14). These results emphasize the role of peroxiredoxins in intracellular growth of Salmonella in activated macrophages.
To test how much Tpx, AhpC, and TsaA contribute to the detoxification of H2O2 in S. Typhimurium, the peroxidase activities of tpx, ahpCF, and tsaA mutants were compared in cell extracts using H2O2 as a substrate. Deletion of ahpCF (STM0608, STM0609) and tsaA (STM0402) was performed using the method of Datsenko and Wanner (9). Overnight cultures of a tpx, ahpCF, or tsaA mutant were washed in 20 mM Tris-1 mM EDTA (pH 7.4), and extracts were prepared by mechanical disruption of cells. The degradation of H2O2 was measured using a xylenol orange assay quantifying H2O2 based on the oxidation of Fe2+ to Fe3+ in the presence of xylenol orange (18). A reaction mixture (100 μl) containing 1 mM H2O2 and 10 μg total protein of the cell extracts in 50 mM HEPES-1 mM EDTA (pH 7.4) was incubated for 15 min at room temperature. One milliliter of the xylenol assay reagent, consisting of 250 μM ammonium iron(II) sulfate, 125 μM xylenol orange, 25 mM H2SO4, and 100 mM sorbitol, was added and incubated for a further 15 min at room temperature. Absorbance was measured at 560 nm and compared to that of a known standard of H2O2. A concentration of 84.4 μM H2O2 was measured in extracts from the tpx mutant, compared to 18 μM, 38.9 μM, and 20.9 μM in extracts from the wild type and the tsaA and ahpCF mutants, respectively (Fig. (Fig.2),2), indicating that the tpx mutant possesses less reductive capacity toward H2O2 than the wild type, the ahpCF mutant, and the tsaA mutant.
The fitness of the mutant strain compared to the wild type was tested in a competition experiment in mice. Groups of five female BALB/cJ mice at the age of 6 to 8 weeks were infected intraperitoneally with 1 × 105 cells consisting of the wild-type and mutant strains at a 1:1 ratio. Mice were sacrificed, and the livers and spleens were removed and homogenized in phosphate-buffered saline (PBS) on day 1 and day 3 after infection. Bacteria were plated on LB agar and on LB agar containing chloramphenicol. Competitive indices (CIs) were calculated as described previously (2). The tpx mutant was not attenuated in the livers and spleens of infected mice compared to the wild type (CIs were 0.93 ± 0.13 and 1.27 ± 0.06 for the mutant versus the WT in livers and spleens, respectively).
In this study we identified a third peroxiredoxin, Tpx, in S. Typhimurium by testing three aspects of the oxidative defense system of Salmonella: in vitro H2O2 challenge, growth inside activated phagocytes, and virulence in mice. The attenuated phenotype of the tpx mutant in response to exogenous H2O2 and in activated macrophages seems to be surprising considering the redundant capacities to counteract oxidative stress in Salmonella. However, similar observations have been reported for other pathogens, such as Escherichia coli (4), Mycobacterium tuberculosis (15), Helicobacter pylori (8), and Enterococcus faecalis (19). A single deletion in tpx rendered all four pathogens more susceptible to exogenous H2O2. In vitro challenge with H2O2 in this study was done in whole-cell assays and in cell extracts. In both assays the tpx mutant was significantly affected either in survival or in its ability to degrade H2O2. We directly compared tpx, ahpCF, and tsaA mutants in cell extracts challenged with 1 mM H2O2. This comparison suggested that Tpx might be superior to AhpC and TsaA in its ability to protect against H2O2 even at millimolar concentrations. Biochemical studies indicate that Tpx has broad substrate specificity. E. coli Tpx has been shown to reduce H2O2 and organic peroxides (1, 5, 7). M. tuberculosis Tpx has a similar substrate range and, in addition, is known to reduce peroxynitrite (16, 23). Studies with purified Tpx and AhpC from E. coli revealed that Tpx is the most potent peroxidase. Inactivation of tpx in E. coli resulted in increased sensitivity to H2O2 and various organic hydroperoxides. The level of sensitivity was always higher than that of an ahpC mutant (3, 4). In E. faecalis the comparison of mutants lacking either an NADH peroxidase (Npr), an alkyl hydroperoxide reductase (AhpC), or a thiol peroxidase (Tpx) showed that Tpx mediates the most effective antioxidant activity for survival inside macrophages (19). In this study we did not compare growth of the ahpCF, tsaA, and tpx single mutants in macrophages. However, the presently available results suggest that all three peroxiredoxins play an essential role during growth in activated macrophages. In M. tuberculosis and in E. faecalis, a single tpx mutant was attenuated in mice. We could not detect reduced virulence of a single tpx mutant of Salmonella in mice. Hébrard et al. found that neither a double mutant with deletions in ahpCF and tsaA nor a triple mutant with deletions in katG, katE, and katN was attenuated in mice. Only a mutant with deletions in all five genes was highly attenuated in vivo (14). Thus, Salmonella has evolved multiple routes to survive oxidative stress in vivo. In summary, this study shows that Salmonella Tpx is efficient in scavenging H2O2 in vitro and facilitates intracellular growth in activated macrophages.
We thank S. Suerbaum for his support. We thank U. Maus and R. Maus for the RAW 264.7 cells and the group of M. W. Hornef for support in establishing the macrophage infections.
This work was supported by International Research Training Group 1273 funded by the German Research Foundation (DFG) to S.A.H., L.A.D., and S.F.R. and by the Niedersächsischer Verein zur Bekämpfung der Tuberkulose. The contributions of M.R. and S.F.R. were supported by the Swedish Medical Research Council.
Published ahead of print on 19 March 2010.