## 1 Introduction  
Multi-bracket appliances have long been established as tools for treating malocclusions. However, difficulty in oral cleaning due to the complex structure of orthodontic appliances allows plaque accumulation. This in turn increases the number of caries-pathogenic and periodonopathogenic bacteria [1; 2; 3], which have adverse effects and risks associated with orthodontic treatment, including the development of white spots, caries, and progression of periodontal disease [4; 5].  
_Streptococcus mutans (S. mutans)_, a typical caries-causing bacterium, increases after wearing fixed orthodontic appliances [6; 7]. Among periodonontopathogenic bacteria, _Porphomonas_, _Tannerella_, _Prevotella_, _Capnocytophaga_, and _Selenomonas_ also increase before and after the use of fixed orthodontic appliances [8]. Therefore, the state of the oral microbiota, which changes with the use of fixed orthodontic appliances, is similar to that of the microbiota during the transition to severe periodontitis [8]. Such changes in oral conditions and periodontal disease induced by orthodontic therapy are only partially reversible, even 3 months after appliance removal [9]. Therefore, it is important to control dysbiosis of the oral microbiota that occurs while wearing multi-bracket appliances during treatment.  
Recent advances in analytical techniques have led to extensive research on the interrelationships between oral and intestinal environments [10; 11]. Previously, digestive juices were believed to sterilize oral bacteria ingested through swallowing. However, not only has the presence of oral bacteria in the gut microbiota has been revealed, but it has been shown that pathogenic bacteria in the oral cavity can disrupt the balance of gut microbiota [12; 13]. It has also been suggested that failure to maintain oral hygiene leads to elevated systemic inflammation and an increased risk of extraoral diseases [14]. Orthodontic treatment, especially with multi-bracket appliances, involves long-term device fixation in the oral cavity and significant changes to the oral environment. Therefore, understanding the changes in the gut microbiota associated with orthodontic treatment is a significant challenge in positioning orthodontic therapy as part of comprehensive systemic management. Additionally, multi-bracket appliances may restrict the chewing of hard or sticky foods owing to bracket dropout, which may result in restricted food choices, in turn affecting the gut microbiota.  
The risk of caries and periodontitis associated with wearing fixed orthodontic appliances is evident from changes in the oral microbiota before and after appliance wear [6; 8; 15; 16]. However, a solution for risk reduction has yet to be identified. The importance of oral cleaning in orthodontic patients has also been suggested [16; 17; 18]. However, changes in the oral microbiota over time, with or without oral cleaning and oral hygiene instructions, in multi-bracket appliance wearers have yet to be determined. It is also unclear whether changes occur over time in the gut microbiota of multi-bracket device wearers. Therefore, there is an increasing need for further clarification of the relationship between fixed orthodontic appliances, such as multi-bracket appliances, and gut microbiota, as well as the associated systemic effects in the field of orthodontics, in addition to the changes in the oral environment related to orthodontic appliances, which have been the focus of much attention.  
In this study, we investigated whether continuous professional mechanical tooth cleaning (PMTC), including tooth-brushing instructions (TBI), could be used to reduce the risk of dental caries and periodontitis during orthodontic treatment with multi-bracket appliances. The analysis was based on changes in the oral microbiota of the patients after more than 6 months of treatment. Second, we evaluated the dynamics of the gut microbiota of multi-bracket appliance wearers and examined the relationship between orthodontic treatment and the gut microbiota.