Two hundred milligrams of faeces are used for bacterial DNA extraction using the QIAamp DNA Stool Mini Kit (QIAGEN, West Sussex, UK). Extracted DNA is stored at −80 °C. The purity and concentration of extracted DNA are determined by measuring the ratio of the absorbance at 260 and 280 nm (Infinite® 200 PRO NanoQuant, Tecan, Mannedorf, Switzerland).