Culture of Ruminococcus torques
R. torques (ATCC BAA-2281) were cultured under strict anaerobic conditions using the Anoxomat III Jar system (Advanced Instruments, Norwood, MA) in modified reinforced clostridial medium following ATCC protocol. Cultures were washed and concentrated anaerobically in sterile, anaerobic PBS containing 25% (vol/vol) glycerol to a final concentration of 1\(\:\times\:\)1010 CFU/mL, then immediately frozen and stored at − 80°C. Before oral administration, stocks were thawed and diluted anaerobically in sterile PBS with 2.5% glycerol to a final concentration of 1\(\:\times\:\)109 CFU/mL.
Mice, diets, and treatment
As shown in Fig. 1A, sixteen weaning female C57BL/6J mice were housed under conventional specific pathogen-free (SPF) conditions and fed either a Se-deficient or Se-adequate torula yeast-based purified diet for 26 weeks, as previously described previously [16]. At week 21, mice (n = 4 per group) received a daily oral gavage of either 2\(\:\times\:\)108 CFU live R. torques or a control vehicle (2.5% glycerol in 200 µL) for 4 weeks. Mice were handled aseptically in a controlled environment with 12-hour light/dark cycle (lights on from 6 p.m. to 6 a.m.), with ad libitum access to food and water. Body weight and food intake were monitored weekly. Fresh fecal samples were collected before and after R. torques treatment, snap-frozen in liquid nitrogen, and stored at − 80°C. Two days after the insulin sensitivity assay, mice were fasted for 6 hours, anesthetized with carbon dioxide, and euthanized by cardiac exsanguination. Liver, skeletal muscle, and cecal contents were collected, rapidly frozen in liquid nitrogen, and stored at − 80°C for further analyses. All procedures were approved by the Institutional Animal Care and Use Committee of Mississippi State University.
Glucose tolerance and insulin sensitivity
At weeks 21 and 26 of dietary intervention, mice fasted for 8 hours were intraperitoneally injected with glucose (1 g/kg body weight) or insulin (0.25 U/kg body weight) (Sigma Aldrich, St. Louis, MO). Blood glucose concentrations were measured using a glucose meter (Bayer Contour Next EZ, Ascensia Diabetes Care US, Inc., Parsippany, NJ) from a drop of tail vein samples collected at 0 (baseline), 0.25, 0.5, 1, 1.5, 2 hours post-injection. Insulin sensitivity was assessed 2 days after glucose tolerance testing. The area under the curve (AUC) was calculated to quantify glucose and insulin responses.
Bacterial genomic DNA extraction and Quantitative PCR (qPCR) analysis
Bacterial DNA was extracted from cecal contents and feces using the QIAamp PowerFecal Pro DNA Kit (#51804, QIAGEN, Germantown, MD) following the manufacturer’s instructions. Universal primers targeting the V4 region of the bacterial 16S rRNA gene were used for amplification (primer sequences listed in Supplemental Table 1). qPCR was performed in 10 µL reactions using PowerUp™ SYBR™ Green Master Mix (#A25741, Applied Biosystems, Waltham, MA) on a QuantStudio 5 Real-Time PCR System (#A34322, Applied Biosystems) with the following thermal profile: 95°C for 2 min, followed by 40 cycles of 95°C for 5 sec and 60°C for 30 sec. Total bacterial load in fecal samples was quantified using the ΔCT method. Due to normalization of ΔCT values by subtraction against the control group, two-way ANOVA was not appropriate for analyzing ΔΔCT data, as a designated control could not be applied across two factors. Instead, unpaired t-tests were used for statistical analysis.
Immunoblotting
Tissues were homogenized in RIPA lysis buffer with protease inhibitors (# sc-24948, Santa Cruz Biotech, Dallas, TX) and centrifuged at 12,000 × g for 10 min at 4°C. Supernatants (30 µg protein per lane) were separated by 14% SDS-PAGE, then transferred to polyvinylidene difluoride membranes. Membranes were incubated overnight at 4°C with primary antibodies (listed in Supplemental Table 2), followed by HRP-conjugated secondary antibodies for 2 hours at room temperature. Signals were developed using Clarity Western ECL substrate, and images were captured and quantified using a Chemidoc-XS system with the volume tool in Image Lab Software (Bio-Rad Lab, Hercules, CA). Protein levels were normalized to albumin, β-tubulin, or total AKT.
Statistical analysis
Data are presented as means ± SEM. Most datasets were analyzed by two-way ANOVA followed by Tukey’s post hoc test, except for qPCR-based assays, which were evaluated using unpaired t-tests. Statistical analyses were performed using SAS (version 9.4) and GraphPad Prism (version 8.0). A significance level of α = 0.05 was used for all tests.