 A sterile micro brush will be used to collect the tongue coatings The tongue coating sample will be taken after stroking the surface of the tongue for three times from the back to the front side on one half of the anterior two-thirds of the tongue for each time point of sampling. After the sample collection, the tip of the micro brush will be cut and placed inside an Eppendorf vial. The samples will be stored at -800C until DNA extraction is performed. 