2.2. Collection of stool sample and DNA extraction
Samples were placed in a freezer at −70 °C immediately after
arrival, which was at most 30 min after sample collection, and they
were stored until analysis. Frozen fecal samples were thawed at room
temperature, and an aliquot of approximately 200 mg (wet weight) of
each sample was added to a 2-mL sterile tube containing 1.4 mL of ASL
buffer from the TIANamp stool DNA kit (Tiangen, Beijing, China). DNA
was isolated according to the manufacturer's instructions with an
initial bead-beating step of 30 s at 5000 rpm. Extracts were then
stored at −20 °C.