Gut microbiota metabolites regulate host-pathogen interactions by altering pathogen virulence and host AhR signaling
Author(s):
Jason Xiao; Robert Keskey; Callie Winters; Alexander Zaborin; Olga Zaborina; John Alverdy
Background:
The determinants that confer survival following an LD50 dose of a given infectious inoculum remain unknown. In a previous model in which mice received intraperitoneal LD50 dose of Serratia marcescens, we demonstrated that survival depends on gut microbial production of tryptophan metabolites (i.e indole isoforms) with the capacity to activate a key receptor in macrophages, the aryl hydrocarbon receptor (AhR). Yet, whether gut-derived microbiota metabolites alter pathogen virulence and/or interfere with pathogens for AhR signaling has not been explored.
Hypothesis:
We hypothesized that gut microbiota metabolites alter pathogen virulence and compete with pathogens for AhR signaling in host cells.
Methods:
Serratia marcescens and Pseudomonas aeruginosa were tested in vitro for their response to gut microbial metabolites, including indoles and short-chain fatty acids (SCFAs). After exposure to metabolites, bacterial growth, biofilm formation, and virulence (using a small animal model of Galleria mellonella) were analyzed. The pathogen’s ability to inhibit AhR was studied using a murine AhR reporter cell line in which AhR activation by indoles was previously established.
Results:
Microbiota-derived metabolites impacted the growth and virulence of S. marcescens and P. aeruginosa in a concentration-dependent and metabolite-specific manner. SCFAs decreased S. marcescens growth by 27.0% (butyrate), 30.4% (acetate), and by 49.4% (propionate). Indoles reduced the growth and biofilm production of S. marcescens and P. aeruginosa strains by >15%. Specifically, 1mM indole-3-carboxyaldehyde (I3C) and tryptophol strongly inhibited S. marcescens growth (16% and 35%, respectively) and biofilm production (20% and 18%, respectively). Co-administering tryptophol, indole-3-lactic acid, or indole-3-acetic acid (I3A) reduced S. marcescens virulence as judged by delayed mortality in G. mellonella (n=10/group, p=0.0484). Similarly, co-administering SCFAs reduced S. marcescens virulence (n=10/group, p=0.0465). AhR signaling in response to indoles was suppressed by secretomes from S. marcescens (87.0%) and P. aeruginosa (65.8%), indicating a potential mechanism by which pathogens directly subvert host immunity.
Conclusions:
Gut microbiota-derived metabolites of tryptophan alter pathogen virulence and compete for AhR signaling in host cells. The ability of gut microbiota to produce these metabolites may explain, in part, why some hosts survive while others do not when exposed to a given infectious inoculum.