Repression of Salmonella entericaphoPExpression by Small Molecules from Physiological Bile

Abstract

Infection with Salmonella enterica serovar Typhi in humans causes the life-threatening disease typhoid fever. In the laboratory, typhoid fever can be modeled through the inoculation of susceptible mice with Salmonella enterica serovar Typhimurium. Using this murine model, we previously characterized the interactions between Salmonella Typhimurium and host cells in the gallbladder and showed that this pathogen can successfully invade gallbladder epithelial cells and proliferate. Additionally, we showed that Salmonella Typhimurium can use bile phospholipids to grow at high rates. These abilities are likely important for quick colonization of the gallbladder during typhoid fever and further pathogen dissemination through fecal shedding. To further characterize the interactions between Salmonella and the gallbladder environment, we compared the transcriptomes of Salmonella cultures grown in LB broth or physiological murine bile. Our data showed that many genes involved in bacterial central metabolism are affected by bile, with the citric acid cycle being repressed and alternative respiratory systems being activated. Additionally, our study revealed a new aspect of Salmonella interactions with bile through the identification of the global regulator phoP as a bile-responsive gene. Repression of phoP expression could also be achieved using physiological, but not commercial, bovine bile. The biological activity does not involve PhoPQ sensing of a bile component and is not caused by bile acids, the most abundant organic components of bile. Bioactivity-guided purification allowed the identification of a subset of small molecules from bile that can elicit full activity; however, a single compound with phoP inhibitory activity could not be isolated, suggesting that multiple molecules may act in synergy to achieve this effect. Due to the critical role of phoP in Salmonella virulence, further studies in this area will likely reveal aspects of the interaction between Salmonella and bile that are relevant to disease.

Bile Acids Function Synergistically To Repress Invasion Gene Expression in Salmonella by Destabilizing the Invasion Regulator HilD

“…2). Similar to previous reports (17,18,35), these results verified that the repression exhibited by bile was not unique to sipC expression but rather that it applied to invasion genes in general. Of note, in these assays deoxycholate was assayed at a higher pH than the other compounds (pH 8.0 instead of pH 6.7), which was necessary to maintain deoxycholate solubility (36,37).…”

Survival of the Fittest: How Bacterial Pathogens Utilize Bile To Enhance Infection

“…While physiological bovine bile also repressed phoP expression, commercial bovine bile and bile acids did not have the same effect. Therefore, different components of bile may be responsible for different responses in vitro and in vivo (128). Data from this recent analysis parallel the various findings of Vibrio research upon the utilization of different sources or components of bile, all of which highlight the importance of the use of physiological conditions to understand the complex interactions of each pathogen with bile.…”

“…The importance of this binding is not yet known, but it represents a curious observation for how Salmonella interacts with the environment during infection. Additional virulence factors repressed by bile in S. enterica serovar Typhimurium include the flagellar flhC, flgC, and fliC genes; the fimbrial fimI and fimZ genes; and other virulence genes (128,137). Unlike for Vibrio, motility appears to be repressed in the presence of bile, which could be linked to gallbladder colonization (see below).…”

“…First, the activation of the two-component regulatory system PhoPQ results in the remodeling of the lipid A structure of LPS (3,117,127). Although PhoPQ does not sense bile directly, it regulates several genes upon stimulation with bile, including central metabolism and respiration genes (124,128). Deletion of phoP decreases the viability of both S. enterica serovars Typhi and Typhimurium in bile, and furthermore, constitutively expressed PhoP increases bacterial fitness in media containing bile (124).…”

Repression of Salmonella Host Cell Invasion by Aromatic Small Molecules from the Human Fecal Metabolome

“…6 shows that this concentration is somewhere between 1 and 10 M. To further ascertain that 3,4-dimethylbenzoic acid is a repressor of hilA, we set out to confirm the results reported above using a different methodology. Although the PhilA::gfp reporter strain employed in our studies has been successfully used to determine the effects of environmental cues on hilA expression (19,20), it is based on a multicopy-plasmid system, and, as such, it may not reflect the actual levels of hilA mRNA and the real fold changes elicited by any given stimulus. Therefore, we repeated the same experiment, growing Salmonella in the absence or presence of 100 M 3,4-dimethylbenzoic acid to assess hilA expression; this time, however, we used the wild-type Salmonella strain SL1344 and measured hilA expression through real-time Figure 6C shows that, indeed, 3,4-dimethylbenzoic acid represses hilA expression, and to a further extent than what was observed in the GFP reporter experiments.…”