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Tras La Pared - Blue Jeans.epub

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Tras La Pared - Blue Jeans.epub

The attenuation of a ΔsehAB mutant suggested that upon peroral inoculation Salmonella encounters conditions favouring the enhanced transcription of the sehAB locus. To confirm this, we inoculated mice with wild-type S. Typhimurium carrying a transcriptional reporter for sehAB (sehAB-gfp). Bacteria extracted from various organs were examined for the expression of GFP by flow cytometry and compared to reference strains grown in LB broth. Most Salmonella extracted from the mesenteric lymph nodes (MLN) of perorally inoculated mice showed GFP fluorescence levels (blue curve, Figure 6A) that were slightly higher than that of wild-type S. Typhimurium grown in LB broth (green curve, Figure 6A). This indicates that the majority of bacteria in the MLNS exhibit slightly higher expression of sehAB. Yet, we also observed in MLNs a second population with a mean GFP fluorescence similar to that observed for the ΔsehB strain grown in LB broth (red curve, Figure 6A). This population, hereafter referred to as GFPhigh bacteria, corresponded to Salmonella in which expression of sehAB is de-repressed. This GFPhigh population although not exceeding 5% of the total Salmonella population, was however significantly more prevalent in spleens and MLNs of mice inoculated via the peroral route (Figure 6B). The mean fluorescence of total bacteria was also significantly increased in populations extracted from perorally but not from intraperitoneally inoculated mice, when compared to bacteria grown in LB broth (Figure 6C). Finally, in perorally inoculated mice, the mean fluorescence of GFPhigh bacteria was significantly higher in Salmonella extracted form MLNs in comparison to their corresponding spleens (Figure 6C). Considering that having crossed the intestinal barrier the bacteria will reach the MLNs before the spleen [33], these results indicate that activation of the sehAB module is an early event in the process of infection through oral ingestion. To further emphasise this point we also analysed the consequences of the deletion of sehAB on the virulence attenuation of bacteria present in various organs. We found that the CI of wild-type versus ΔsehAB strains is much lower in MLNs (0.050.03) as compared to the spleen (0.510.19) and the liver (0.320.02) (Figure 6D). These results reveal that sehAB is especially important for the survival of bacteria in MLNs. 59ce067264


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