# Microbiological Changes during Orthodontic Aligner Therapy: A Prospective Clinical Trial
## 4 Discussion
The authors found a slightly decreasing microbial diversity with a significant change of microbial structure during the first three months of CAT. However, the patients were free from periodontitis and showed relatively stable levels of periodontal microorganisms and core microorganisms. Our study suggests that using clear aligners causes no significant alterations in the bacterial count and bacterial percentage of neither red and orange complexes nor of _A. actinomycetemcomitans_, thus, avoiding a risk of periodontitis, at least during the first two months of the therapy. In particular, _A. actinomycetemcomitans_, one of the microorganisms most related to periodontal disease, was not detected at all in any group at any time. However, we cannot conclude that CAT is not associated with the growth of this pathogen, considering that a study conducted by [PERSON] et al. [26] assessed its increase, despite not being statistically significant. This might be due to the longer follow up considered by the authors (three months instead of two) as well as to the fact that microbiological samples were collected at the beginning of the study without submitting patients to a professional oral hygiene, differently from our protocol. Similar to _A. actinomycetemcomitans_, the microorganism _T. denticola_ was not detected in any patient of this study. Considering that this bacterium belongs to the red complex of pathogens related to periodontitis, its absence further suggests that CAT does not represent a risk for the development of the disease. However, it must be taken into account that this result might be influenced again by the relative short follow up. As regards the other pathogens (_P. gingivalis, T. forsythia, P. intermedia_ and _F. nucleatum_), a limited number of colonies were detected in both the trial and the control patients, with differences between T\({}_{0}\) and T\({}_{1}\) that were not statistically significant, anyway. Our data confirm the evidence of previous studies that did not assess the significant variations of these four microorganisms [10; 26]. A significant difference assessed by us that deserves to be discussed concerns the total bacterial count, which encompasses not only the pathogens more strictly related to the periodontal disease, but also those that normally constitute the oral microbiota of healthy subjects. This value significantly varied from T\({}_{0}\) to T\({}_{1}\) both in the control and the trial group (intragroup significant differences). The increase was equal to 291% for controls and to 151% for trial, but the intergroup difference was not significant, allowing us, again, to consider clear aligners sure for the periodontium of the patients treated. These variations of the total bacterial count were obviously expected, considering the protocol chosen for the study: in fact, the first microbiological samples collection was carried out two weeks from the professional oral hygiene, which justifies the reductions in the total number of bacteria (independently of their pathogenetic action). Conversely, for the samples collection at T\({}_{1}\), the patients were not submitted to a further hygiene treatment, therefore, explaining the higher values assessed for the total bacterial count. However, we cannot exclude the possibility that the bacterial increase might be independent from the protocol used and that the intergroup difference between trials and controls might become statistically significant on a longer follow up. Surely, it should be taken into account that periodontal diseases have multifactorial aetiology, encompassing not only the bacterial reservoir, but also the immune factors related to the host [27]. A major limitation of the present report is associated to the relatively short follow up considered. In order to confirm the results obtained here, further studies should be conducted including patients belonging to different countries and on a longer period, covering the entire length of the orthodontic treatment.