Components of the Gut Microbiome that Regulate Bone Fragility

Although modifications constituents of the gut microbiome have been shown to influence bone density and tissue strength, the specific microbial components responsible for this phenotype are not yet known. Here we used narrow spectrum antibiotics administered in drinking water to selectively remove components of the gut microbiota to identify microbial communities responsible for changes in bone. Male C57Bl/6 mice were divided into seven groups (n=7-10/group) and were administered the following treatments from 4-16 weeks of age: 1) ampicillin; 2) neomycin; 3) vancomycin; or 4) metronidazole; 5) a cocktail of all four antibiotics together with sweetener (to ensure adequate intake); 6) a sweetener only control group and 7) an untreated control group. The cocktail of all four antibiotics severely depleted the microbial population in the gut while individual antibiotics removed only distinct portions of the gut microbiota. Femora were collected at 16 weeks of age and bone geometry, whole bone strength, tissue strength, serum markers of bone turnover and fecal microbiota were examined. Animals receiving any of the antibiotics showed reduced femoral diaphyseal cross-sectional area and moment of inertia compared to untreated animals (p < 0.001). After accounting for differences in geometry, bone tissue strength was reduced in animals receiving neomycin alone (p=0.002) and was increased in the sweetener control group (p < 0.001). Reduced bone tissue strength in the neomycin group was associated with reduced mineral:matrix ratio determined using FTIR. The composition of the gut microbiome, determined using 16S rRNA sequencing, showed variations among groups. Functional components of the gut microbiome inferred from taxonomic results associate the capacity to synthesize vitamin K with impaired bone tissue strength. Our findings support the idea that microbiome-induced reductions in bone tissue strength in mice occur as a result of neomycin dosing, either due to a loss of neomycin-susceptible organisms or increased abundance of organisms resistant to neomycin.