Gut bacterial metabolism of corticoids and other host-produced steroids

Abstract

The human gut microbiome is important to human health, and disruptions in the human gut microbiota community are linked to a variety of human disease states. However, until recently, few studies have examined how the gut microbiota impacts human health and disease. While most reports describe correlations between the gut microbiome and different disease states, mechanistic studies are necessary to fully understand how the microbiota impacts the host. Small molecules produced by the gut microbiome are known to affect the human host, but the repertoire of molecular output by the gut microbiome has yet to be fully elucidated. Steroid hormones are important signaling molecules involved in immune function, sex characteristic development, metabolism, and neurological signaling. Several studies have described associations between the gut microbiome, steroid hormones, and human health. However, few reports have investigated the mechanisms underlying these associations. Here, I describe studies in which I have leveraged recently developed chemical, microbiological, and genetic tools to investigate the causal links between the gut microbiome, steroid hormones, and human health. Chapter 2 describes an exploratory metabolism screen of prevalent and abundant gut bacterial strains against different steroid hormones known to be present in human bile and feces. Several published and novel metabolic activities were found in this screen, and results from this screen lay a foundation for further investigations identifying the bacterial genes responsible for the production of steroid hormone metabolites. Chapter 3 discusses a metabolic activity that was not observed in the exploratory screen from Chapter 2 but has the potential for great importance to human health, particularly women’s health. Studies from the 1960s, 70s, and 80s reported that gut bacteria can 21-dehydroxylate corticoids in bile to produce a different class of steroid hormones, the progestins. However, the bacterial genes responsible for this metabolism were never determined, and the 21-dehydroxylating strain described in these early reports was never deposited in a culture collection. We used a variety of microbiological, genetic, and chemical tools to identify a putative gene cluster responsible for this metabolism. We have also shown that 21-dehydroxylation occurs in vivo and that this activity potentially affects host behavior. Chapter 4 describes cooperative metabolism between hydrogen-producing gut bacteria and 21-dehydroxylating bacteria described in Chapter 3. Although cooperative metabolism between different species and strains of gut bacteria has been described, to our knowledge, no studies have described how a microbially produced gas can impact the reductive metabolism of another gut microbe. To fully understand this phenomenon, we developed novel methods to quantify hydrogen gas in bacterial cultures, and E. coli mutants deficient in hydrogen production were used to genetically link hydrogen production to 21- dehydroxylation by E. lenta. Each of these chapters required the use of analytical chemistry to detect and quantify steroid hormone substrates and products; thus, Chapter 5 discusses the development of sensitive and specific mass spectrometry methods to analyze host-produced steroids and their metabolites from different steroid hormone classes. In particular, UHPLC-MS method development on a Thermo Fisher Orbitrap Exploris led to the quantification of steroids in pregnant and non-pregnant human feces, revealing that the levels of bacterially produced steroid metabolites are orders of magnitude higher in pregnant human feces than in non-pregnant healthy human feces. Together, these studies represent an advancement in our understanding of gut microbial metabolism of host-derived steroid hormones and lay the groundwork for future work elucidating the causal roles of these metabolites in host health and disease.