Fecal collection
All fecal samples were collected within 48 hours using Zymo DNA/RNA ShieldTM fecal collection tubes (Zymo Research, U.S.). At least 1 g or 1 ml of neonates’ feces were collected, mixed, and then stored at 2-25°C before being transferred into a -80°C freezer for storageg. The date and time of fecal sample collection were recorded to determine the neonatal age at the time of sample collection.