Table 1 List of inclusion and exclusion criteria for subjects enrollment in the study

• Male and female subjects

• Age 18-30 years

• Good general health: free of systemic diseases such as diabetes, HIV infection or genetic disorder, ongoing malignant disease of any type that could interfere with the evaluation of the study objectives

• Good oral health: free of oral pathologies such as leukoplakia, erythroplakia, oral lichen planus (OLP)

• Availability for the 6-month duration of the study for an assigned subject

• Signed Informed Consent Form

• Pregnant or lactating women

• More than 8 missing teeth (with missing teeth, accounted for by third molar extractions, teeth extracted for orthodontic purposes, teeth extracted because of trauma, or congenitally missing teeth)

• Presence of orthodontic appliances

• Chronic dry mouth (clinically assessed)

• Significant halitosis (clinically assessed)

• Periodontitis

• Untreated carious lesions or oral abscesses

• Current or past (within 3 months before enrolment) assumption of medications that may influence oral microbiome (corticosteroids, calcium channel blockers, systemic antibiotics).

• Non-surgical and/or surgical mechanical/manual periodontal debridement within 3 months before enrolment.

• Heart diseases or blood pressure alteration that requires a medication

• Renal, hepatic, gastrointestinal disease that requires a medication

• Diabetes

• Presence of any sexually transmitted disease (STD), HIV, HCV infection

• Genetic disorders potentially interfering with the evaluation of the study objectives

• Chronic obstructive pulmonary disease and asthma

• Neoplastic lesions or paraneoplastic syndrome; tumors or significant pathology of the soft or hard tissues of the oral cavity (such as LPO, erythroplakia, leukoplakia, candidiasis)

• Current radiotherapy or chemotherapy