Study subjects will be recruited from juveniles and young adults, age 12-25, displaying high (≥105 CFU) salivary counts of caries promoting mutans streptococci and seeking orthodontic therapy with fixed appliances. Evaluated parameters are the presence of enamel caries on the buccal aspects of teeth 15-25 according to the criteria of the International Caries Detection and Assessment System (ICDAS), the presence of gingival inflammation assessed by the Gingival Index (GI) the presence of microbial plaque on the evaluated teeth assessed by the Plaque Index (PlI) and the extent of mutans streptococci colonization (MS) on the evaluated teeth and in saliva using standardized cultivation techniques. Primary endpoint of the study is the number and severity of enamel caries lesions according to the ICDAS classification. Exclusion criteria are the presence of ICDAS lesion ≥ 3 on the buccal surfaces of teeth 15-25, the presence of gingival lesions > GI 2 and the regular use of salivary flow-reducing medications.