For BA analysis, samples (≈40 mg of caecal content) were homogenised with acetonitrile using zirconia/silica beads (0.1 mm diameter). After discarding stool particles, the supernatant was evaporated in a vacuum centrifuge and solubilised in a volume of methanol to a final concentration of 1 μL mg−1 of gut content. Chromatographic separation was performed on Agilent 1290 Infinity UHPLC using a 150 mm × 2.1 mm internal diameter (i.d.) Phenomenex Kinetex® C18 core-shell column, packed with 2.6-μm particles. HPLC was carried out with mobile phase A (0.1% formic acid in aqueous solution) and mobile phase B (0.1% formic acid in acetonitrile) at a total flow rate of 0.5 mL min−1. Gradient program was increased linearly from 5% mobile phase B and 95% mobile phase A to 100% mobile phase B for 9.5 min. Bile acid identities were established in negative ion mode using a mass MSMS instrument (Agilent QTOF 6540) and the following pure standards: cholic acid (C1129, SIGMA), deoxycholic acid (D4297, SIGMA), lithocholic acid (L6250, SIGMA), chenodeoxycholic acid (C1050000, European Pharmacopoeia Reference Standard), cholic acid 7-sulphate (9002532, Cayman Chemical), α-muricholic acid (C1890-000, Steraloids), β-muricholic acid (sc-477731, Santa Cruz), ω-muricholic acid (C1888-000, Steraloids), ursodeoxycholic acid (C1020-000, Steraloids), hyodeoxycholic acid (H0535, TCI), taurocholic acid (sc-220189, Santa Cruz) and taurodeoxycholic acid (15935, Cayman Chemical). Peak integration and analysis was performed using ProFinder (software version B.06.00, Agilent Technologies) and a customised spectral library.
For SCFA profiling, samples were spiked with 5 nmol of 13C-sodium acetate (279293, SIGMA) and 5 nmol of 2-ethyl butyric acid (109959, SIGMA) as internal standards and were homogenised in isopropanol. After centrifugation, 1 μL of the supernatant was injected into a HP 6890 Series GC System, equipped with an Agilent 5973 Network Mass Selective Detector in splitless mode. Samples were separated on a Stabilwax®-DA (Shimadzu) column (30 m × 0.25 mm i.d.) coated with a 0.25-μm-thick film. The carrier gas was helium at a flow rate of 1 mL min−1. The initial oven temperature of 90 °C was held for 2 min, then increased to 240 °C at 5 °C min−1 and maintained for additional 2 min. The temperature of the quadrupole, MS source and inlet were 150, 230 and 250 °C, respectively. Identities and retention times of the SCFA were established using the volatile-free acid mix (46975-U, Supelco). Peaks were automatically integrated using MSD ChemStation (version D.03.00.611). SCFA concentration was estimated using the internal references 13C-sodium acetate (for acetic acid) or 2-ethyl butyric acid (for all the others SCFA tested). Data were calculated as nanomoles per microlitre serum or per milligram caecal content from at least three biological replicates within each different group.
For SCFA profiling, samples were spiked with 5 nmol of 13C-sodium acetate (279293, SIGMA) and 5 nmol of 2-ethyl butyric acid (109959, SIGMA) as internal standards and were homogenised in isopropanol. After centrifugation, 1 μL of the supernatant was injected into a HP 6890 Series GC System, equipped with an Agilent 5973 Network Mass Selective Detector in splitless mode. Samples were separated on a Stabilwax®-DA (Shimadzu) column (30 m × 0.25 mm i.d.) coated with a 0.25-μm-thick film. The carrier gas was helium at a flow rate of 1 mL min−1. The initial oven temperature of 90 °C was held for 2 min, then increased to 240 °C at 5 °C min−1 and maintained for additional 2 min. The temperature of the quadrupole, MS source and inlet were 150, 230 and 250 °C, respectively. Identities and retention times of the SCFA were established using the volatile-free acid mix (46975-U, Supelco). Peaks were automatically integrated using MSD ChemStation (version D.03.00.611). SCFA concentration was estimated using the internal references 13C-sodium acetate (for acetic acid) or 2-ethyl butyric acid (for all the others SCFA tested). Data were calculated as nanomoles per microlitre serum or per milligram caecal content from at least three biological replicates within each different group.
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