Inclusion Criteria:  aged 18-65 years； good general health as determined by medical questionnaires; BMI <35 kg/m2;  Exclusion Criteria:  known congenital or acquired immune defects； allergies and other chronic or acute diseases requiring treatment； subjects with chronic gastrointestinal diseases; alcohol or drug misuse or both； pregnancy or lactation； vaccination against influenza within the last 12 months； use of oral antibiotics, probiotics, prebiotics, synbiotics, drugs active on gastrointestinal motility, or a laxative of any class within the last month.