5 27 5 ± 10 5 fslB 3 75 ± 1 51 8 17 ± 4 03 fslC 3 22 ± 1 61 6 33

5 27.5 ± 10.5 fslB 3.75 ± 1.51 8.17 ± 4.03 fslC 3.22 ± 1.61 6.33 ± 3.83 fslD 1.33 ± 0.45 2.07 ± 0.87 fslE 0.27 ± 0.10 0.30 ± 0.13 feoB 0.37 ± 0.19 0.46 ± 0.27 iglC 428 ± 161 11.1 ± 5.41 mglA 19.2 ± 12.5 B.D.L.b a The expression of the genes was analyzed by quantitative real-time PCR. Results are expressed as RCN means ± SEM of results three to five independent samples b Below Detection Limit The CAS plate assay is well-established for measurement of siderophore production in F. tularensis and we now

used it to assess the siderophore production in ΔmglA [13, 20, 28]. We did not observe any significant difference between the mutant and LVS. However, it should be noted that minor differences with regard to the siderophore production may not be detected in the assay. Together, the gene regulation of iron-starved bacteria and the CAS assay demonstrates that when subjected to severe iron-deficiency, ΔmglA regulates the fsl operon and similarly to LVS and has the Tipifarnib order capacity to MAPK inhibitor produce siderophores. Thus, it appears to have no inherent defects with regard to iron uptake. Hydrogen peroxide susceptibility of LVS and ΔmglA In view of the Alisertib molecular weight elevated catalase activity and aberrant iron uptake displayed by ΔmglA, we hypothesized that this would affect its susceptibility to H2O2. This was also the case since more than 2.0 log10 of LVS was killed during a 2 h incubation period when exposed to 0.1 mM H2O2, whereas the viability of ΔmglA decreased only

1.0 log10 by this treatment (P < 0.01) (Figure 4). Figure 4 Survival of LVS (white bars) or Δ mglA (black bars) after 2 h exposure to H 2 O 2 Prior to the Orotic acid H

2 O 2 challenge the bacteria had been cultivated for 2 h in CDM in the indicated milieu. The bars represent the average from four experiments with triplicate samples of each. The error bars indicate the SEM It was tested if growth in the microaerobic milieu, which diminished the catalase activity in ΔmglA and enhanced the iron uptake in LVS, affected the susceptibility of the strains to H2O2. Both LVS and ΔmglA were completely eradicated by a 2 h exposure to 0.1 mM H2O2 (Figure 4). In conclusion, our results show that the ΔmglA mutant compared to LVS displayed increased resistance to H2O2 under aerobic conditions whereas both showed markedly increased susceptibility to H2O2 under microaerobic conditions. Discussion It is well established that MglA plays an important role for the intracellular growth and virulence of F. tularensis, most likely through its regulation of genes of the igl operon and other genes of the Francisella Pathogenicity Island. There are also reports that MglA regulates the oxidative stress response in F. tularensis [8, 10] and that the F. novicida mglA mutant exhibits decreased survival during stationary-phase growth under nutrient-limiting conditions [10]. We observed that the LVS ΔmglA mutant did not grow to high densities in a nutrient-rich medium and generated only small colonies on solid agar plates.

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