**2.3 DNA extraction**

Total DNA was extracted from the contents of each intestinal segment (duodenum, jejunum, ileum, and cecum) according to the manufacturer's instructions using a QIAamp DNA Stool Mini Kit (QIAGEN, CA, Hamburg, Germany). The optical density of the 16 DNA samples was measured using a Nano-Drop 2000 (Thermo Electron Corporation, Waltham, MA, USA) at wavelengths of 260 and 280 nm to assess their integrity. Using 1.0% agarose gel electrophoresis (including ethidium bromide), the integrity of the DNA extracts was visually evaluated.
