Effects of the bacterial conversion of lipids in the gut on mouse metabolism

The intestinal microbiota is a complex ecosystem that encodes millions of genes and produces many metabolites, the function of which in regulating host physiology is largely unknown. Shifts in fecal microbial profiles are often linked to metabolic diseases like obesity and type-2 diabetes, but the role of specific strains and associated mediators has scarcely been studied. The overall aim of this thesis was to study lipid metabolism by the gut microbiome, with a focus on secondary bile acid production and bacterial lipases. The work is divided into three chapters, each addressing a specific topic:In chapter I, I looked at the effects of bile acid dehydroxylation on host metabolism, which had only been studied using isolates from the human intestine. The effect of colonization by Extibacter muris, a murine isolate capable of 7α-dehydroxylation, on gut and liver responses was examined. Mice colonized from birth with the defined microbiota OligoMM12, with or without the addition of E. muris, were fed two diets varying in fat content. Successful colonization with E. muris (≤2.5% rel. abundance) was accompanied by the production of the secondary bile acids DCA and LCA, albeit at levels lower (up to 363 nmol/g cecal content) than those usually detected in conventional mice. Liver physiology was affected, i.e., protein pathways involved in amino acid, glucose, lipid, energy and drug metabolism were regulated. In chapter II, I focused on the class Coriobacteriia, some members of which can transform bile acids and have been linked to altered host lipid metabolism. The fecal microbiota of 346 KORA participants with detailed body composition data was analyzed using 16S rRNA amplicon sequencing. The occurrence of Eggerthellaceae was inversely associated with BMI, WHR and fat mass. Experimentally, four Coriobacteriia strains previously shown to affect lipid metabolism in gnotobiotic mice were used to colonize mice harboring the defined microbiota OligoMM12. Only Eggerthella lenta was able to engraft in this model (≤40% rel. abundance). Its presence did not significantly affect target host parameters, e.g., body weight and fat mass. As a first step towards future mechanistic studies, I then worked on developing engineered lactic acid bacteria strains expressing Coriobacteriia-derived putative lipases. In chapter III, I continued studying bacterial lipases by characterizing the biochemical properties of a novel lipolytic enzyme expressed by a mouse isolate of the species Clostridium symbiosum. To summarize, a new gnotobiotic mouse model for studying secondary bile acid production in vivo was established. No adverse effects on host metabolism were observed. Studying the role of Coriobacteriia on host metabolism is hampered by the complicated nature of these bacteria. Colonization in gnotobiotic mice was inconsistent. Their occurrence in human feces was inversely associated with disturbed host metabolic parameters. Studying the role of gut bacteria-encoded lipase with the help of engineered strains will open new avenues of research on the function of specific microbiota members in regulating host metabolism