Components of the Gut Microbiome That Influence Bone Tissue-Level Strength.

Modifications to the constituents of the gut microbiome influence bone density and tissue-level strength, but the specific microbial components that influence tissue-level strength in bone are not known. Here, we selectively modify constituents of the gut microbiota using narrow-spectrum antibiotics to identify components of the microbiome associated with changes in bone mechanical and material properties. Male C57BL/6J mice (4 weeks) were divided into seven groups (n = 7-10/group) and had taxa within the gut microbiome removed through dosing with: (i) ampicillin; (ii) neomycin; (iii) vancomycin; (iv) metronidazole; (v) a cocktail of all four antibiotics together (with zero-calorie sweetener to ensure intake); (vi) zero-calorie sweetener only; or (vii) no additive (untreated) for 12 weeks. Individual antibiotics remove only some taxa from the gut, while the cocktail of all four removes almost all microbes. After accounting for differences in geometry, whole bone strength was reduced in animals with gut microbiome modified by neomycin (-28%, p = 0.002) and was increased in the group in which the gut microbiome was altered by sweetener alone (+39%, p < 0.001). Analysis of the fecal microbiota detected seven lower-ranked taxa differentially abundant in animals with impaired tissue-level strength and 14 differentially abundant taxa associated with increased tissue-level strength. Histological and serum markers of bone turnover and trabecular bone volume per tissue volume (BV/TV) did not differ among groups. These findings demonstrate that modifications to the taxonomic components of the gut microbiome have the potential to decrease or increase tissue-level strength of bone independent of bone quantity and without noticeable changes in bone turnover. © 2021 American Society for Bone and Mineral Research (ASBMR).

Obesity and load-induced posttraumatic osteoarthritis in the absence of fracture or surgical trauma.

Osteoarthritis is increasingly viewed as a heterogeneous disease with multiple phenotypic subgroups. Obesity enhances joint degeneration in mouse models of posttraumatic osteoarthritis (PTOA). Most models of PTOA involve damage to surrounding tissues caused by surgery/fracture; it is unclear if obesity enhances cartilage degeneration in the absence of surgery/fracture. We used a nonsurgical animal model of load-induced PTOA to determine the effect of obesity on cartilage degeneration 2 weeks after loading. Cartilage degeneration was caused by a single bout of cyclic tibial loading at either a high or moderate load magnitude in adult male mice with severe obesity (C57Bl6/J + high-fat diet), mild obesity (toll-like receptor 5 deficient mouse [TLR5KO]), or normal adiposity (C57Bl6/J mice + normal diet and TLR5KO mice in which obesity was prevented by manipulation of the gut microbiome). Two weeks after loading, cartilage degeneration occurred in limbs loaded at a high magnitude, as determined by OARSI scores (P < .001). However, the severity of cartilage damage did not differ among groups. Osteophyte width and synovitis of loaded limbs did not differ among groups. Furthermore, obesity did not enhance cartilage damage in limbs evaluated 6 weeks after loading. Constituents of the gut microbiota differed among groups. Our findings suggest that, in the absence of surgery/fracture, obesity may not influence cartilage loss after a single mechanical insult, suggesting that either damage to surrounding tissues or repeated mechanical insult is necessary for obesity to influence cartilage degeneration. These findings further illustrate heterogeneity in PTOA phenotypes and complex interactions between mechanical/metabolic factors in cartilage loss.

Bone-inspired microarchitectures achieve enhanced fatigue life.

Microarchitectured materials achieve superior mechanical properties through geometry rather than composition. Although ultralightweight microarchitectured materials can have high stiffness and strength, application to durable devices will require sufficient service life under cyclic loading. Naturally occurring materials provide useful models for high-performance materials. Here, we show that in cancellous bone, a naturally occurring lightweight microarchitectured material, resistance to fatigue failure is sensitive to a microarchitectural trait that has negligible effects on stiffness and strength-the proportion of material oriented transverse to applied loads. Using models generated with additive manufacturing, we show that small increases in the thickness of elements oriented transverse to loading can increase fatigue life by 10 to 100 times, far exceeding what is expected from the associated change in density. Transversely oriented struts enhance resistance to fatigue by acting as sacrificial elements. We show that this mechanism is also present in synthetic microlattice structures, where fatigue life can be altered by 5 to 9 times with only negligible changes in density and stiffness. The effects of microstructure on fatigue life in cancellous bone and lattice structures are described empirically by normalizing stress in traditional stress vs. life (S-N) curves by √ψ, where ψ is the proportion of material oriented transverse to load. The mechanical performance of cancellous bone and microarchitectured materials is enhanced by aligning structural elements with expected loading; our findings demonstrate that this strategy comes at the cost of reduced fatigue life, with consequences to the use of microarchitectured materials in durable devices and to human health in the context of osteoporosis.

Disruption of the Gut Microbiome Increases the Risk of Periprosthetic Joint Infection in Mice.

BACKGROUND: Periprosthetic joint infection (PJI) is one of the most devastating complications of total joint arthroplasty. Given the mortality and morbidity associated with PJI and the challenges in treating it, there has been increased interest in risk factors that can be modified before surgery. In this study, we used a novel mouse model to consider the role of the gut microbiome as a risk factor for PJI. QUESTIONS/PURPOSES: (1) Does the state of the gut microbiota before surgery influence the likelihood of developing an established infection in a mouse model of PJI? (2) How does the state of the gut microbiota before surgery influence the local and systemic response to the presence of an established infection in a mouse model of PJI? METHODS: Male C57Bl/6 mice were divided into two groups: those with modified microbiome [INCREMENT]microbiome (n = 40) and untreated mice (n = 42). In [INCREMENT]microbiome mice, the gut flora were modified using oral neomycin and ampicillin from 4 weeks to 16 weeks of age. Mice received a titanium tibial implant to mimic a joint implant and a local inoculation of Staphylococcus aureus in the synovial space (10 colony forming units [CFUs]). The proportion of animals developing an established infection in each group was determined by CFU count. The local and systemic response to established infection was determined using CFU counts in surrounding joint tissues, analysis of gait, radiographs, body weight, serum markers of inflammation, and immune cell profiles and was compared with animals that received the inoculation but resisted infection. RESULTS: A greater proportion of animals with disrupted gut microbiota had infection (29 of 40 [73%]) than did untreated animals (21 of 42 [50%]; odds ratio, 2.63, 95% CI, 1.04-6.61; p = 0.035). The immune response to established infection in mice with altered microbiota was muted; serum amyloid A, a marker of systemic infection in mice, was greater than in mice with disrupted gut microbiota with infection (689 µg/dL; range, 68-2437 µg/dL, p < 0.05); infection associated increases in monocytes and neutrophils in the spleen and local lymph node in untreated mice but not were not observed in mice with disrupted gut microbiota. CONCLUSIONS: The findings from this in vivo mouse model suggest that the gut microbiota may influence susceptibility to PJI. CLINICAL RELEVANCE: These preclinical findings support the idea that the state of the gut microbiome before surgery may influence the development of PJI and justify further preclinical and clinical studies to develop appropriate microbiome-based interventions.

Immunomodulatory nanogels overcome restricted immunity in a murine model of gut microbiome-mediated metabolic syndrome.

Biomaterials-based nanovaccines, such as those made of poly(lactic-co-glycolic acid) (PLGA), can induce stronger immunity than soluble antigens in healthy wild-type mouse models. However, whether metabolic syndrome can influence the immunological responses of nanovaccines remains poorly understood. Here, we first show that alteration in the sensing of the gut microbiome through Toll-like receptor 5 (TLR5) and the resulting metabolic syndrome in TLR5 -/- mice diminish the germinal center immune response induced by PLGA nanovaccines. The PLGA nanovaccines, unexpectedly, further changed gut microbiota. By chronically treating mice with antibiotics, we show that disrupting gut microbiome leads to poor vaccine response in an obesity-independent manner. We next demonstrate that the low immune response can be rescued by an immunomodulatory Pyr-pHEMA nanogel vaccine, which functions through TLR2 stimulation, enhanced trafficking, and induced stronger germinal center response than alum-supplemented PLGA nanovaccines. The study highlights the potential for immunomodulation under gut-mediated metabolic syndrome conditions using advanced nanomaterials.

Alterations to the Gut Microbiome Impair Bone Strength and Tissue Material Properties.

Alterations in the gut microbiome have been associated with changes in bone mass and microstructure, but the effects of the microbiome on bone biomechanical properties are not known. Here we examined bone strength under two conditions of altered microbiota: (1) an inbred mouse strain known to develop an altered gut microbiome due to deficits in the immune system (the Toll-like receptor 5-deficient mouse [TLR5KO]); and (2) disruption of the gut microbiota (ΔMicrobiota) through chronic treatment with selected antibiotics (ampicillin and neomycin). The bone phenotypes of TLR5KO and WT (C57Bl/6) mice were examined after disruption of the microbiota from 4 weeks to 16 weeks of age as well as without treatment (n = 7 to 16/group, 39 animals total). Femur bending strength was less in ΔMicrobiota mice than in untreated animals and the reduction in strength was not fully explained by differences in bone cross-sectional geometry, implicating impaired bone tissue material properties. Small differences in whole-bone bending strength were observed between WT and TLR5KO mice after accounting for differences in bone morphology. No differences in trabecular bone volume fraction were associated with genotype or disruption of gut microbiota. Treatment altered the gut microbiota by depleting organisms from the phyla Bacteroidetes and enriching for Proteobacteria, as determined from sequencing of fecal 16S rRNA genes. Differences in splenic immune cell populations were also observed; B and T cell populations were depleted in TLR5KO mice and in ΔMicrobiota mice (p < 0.001), suggesting an association between alterations in bone tissue material properties and immune cell populations. We conclude that alterations in the gut microbiota for extended periods during growth may lead to impaired whole-bone mechanical properties in ways that are not explained by bone geometry. © 2017 American Society for Bone and Mineral Research.

Links Between the Microbiome and Bone.

The human microbiome has been shown to influence a number of chronic conditions associated with impaired bone mass and bone quality, including obesity, diabetes, and inflammatory bowel disease. The connection between the microbiome and bone health, however, has not been well studied. The few studies available demonstrate that the microbiome can have a large effect on bone remodeling and bone mass. The gut microbiome is the largest reservoir of microbial organisms in the body and consists of more than a thousand different species interacting with one another in a stable, dynamic equilibrium. How the microbiome can affect organs distant from the gut is not well understood but is believed to occur through regulation of nutrition, regulation of the immune system, and/or translocation of bacterial products across the gut endothelial barrier. Here we review each of these mechanisms and discuss their potential effect on bone remodeling and bone mass. We discuss how preclinical studies of bone-microbiome interactions are challenging because the microbiome is sensitive to genetic background, housing environment, and vendor source. Additionally, although the microbiome exhibits a robust response to external stimuli, it rapidly returns to its original steady state after a disturbance, making it difficult to sustain controlled changes in the microbiome over time periods required to detect alterations in bone remodeling, mass, or structure. Despite these challenges, an understanding of the mechanisms by which the gut microbiome affects bone has the potential to provide insights into the dissociation between fracture risk and bone mineral density in patients including those with obesity, diabetes, or inflammatory bowel disease. In addition, alteration of the gut microbiome has the potential to serve as a biomarker of bone metabolic activity as well as a target for therapies to improve bone structure and quality using pharmaceutical agents or pre- or probiotics. © 2016 American Society for Bone and Mineral Research.