## 4 Discussion
Through the advent of next-generation sequencing (NGS), which is a sequencing technology used to rapidly determine the order of nucleotides in whole genomes or targeted regions of DNA or RNA, it has been possible to provide quantitatively and qualitatively enhanced classification of the oral microbiota [47]. The first study which used NGS to compare the peri-implant and periodontal microbiota was by [PERSON] [33]; it was concluded that 85% of the individuals analyzed shared \(<\)8% bacteria between peri-implant and periodontal sites. It was shown that the peri-implant microbiota appears to be, both in health and disease, quantitatively and qualitatively lower than the periodontal microbiota. In addition, the authors highlighted the presence, at the peri-implant site, of bacterial genera that are not present at the periodontal site: for example, the genera Burkholderia, Anaerobicvorax, Anaerobicococcus, Aerofilium and Exiguobacterium. The predominant genera in the peri-implant microbiota were Butyrivivibrio, Campylobacter, Eubacterium, Prevotella, Selenomonas, Streptococcus, Actinomyces, Leptotrichia, Propionibacterium, Peptococcus, Lactococcus, and Treponema. Implant sites with peri-implantitis had lower concentrations of _Prevotella_ and _Leptotrichia_ and higher concentrations of _Actinomyces_, _Peptococcus, Campylobacter, Streptococcus nonmutans_, _Butyrivivibrio_, _Pseudoramic-ter alactolyticus_, and _Streptococcus mutans_ than healthy peri-implant sites [31]. Finally, this study found the presence of a higher amount of _Staphylococcus pettenkoferi_ and _Staphylococcus hominis_ in sites with peri-implantitis compared to sites with periodontitis [33]. In a later study, also based on the use of NGS, an increased concentration of _Prevotella nigrescens_ was shown in sites with peri-implantitis, while bacteria such as _Peptostreptococcaceae_ spp. and _Desulfomicrobium__orale_ were significantly higher in periodontitis. In addition, the greater the severity of peri-implantitis, the higher the concentration of _Treponema_ sp_HMT-257_, which is correlated with radiographic bone resorption, subsequent increase in peri-implant pocket, and suppuration [34]. Another study showed a gradual differentiation of the microbial community from peri-implant health to peri-implant mucositis and finally to peri-implantitis. An increased concentration of periodontal bacteria such as _Porphyomonas gingivalis_, _Tannerella forythia_, and _Prevotella intermedia_ was detected at sites with peri-implant mucositis, whereas in the presence of peri-implantitis, the study observed the presence of quantitatively rich microbial communities, with an increased concentration of bacteria from the genus _Eubacterium_ spp. [35]. Moreover, if the subject is a smoker, in healthy peri-implant sites the peri-implant microbiota is qualitatively less diversified, but there are more bacteria typical of peri-implant disease; instead, in sites with peri-implant mucositis, there is a quantitative reduction of bacterial species typically present in a healthy peri-implant site, also reducing its bacterial diversification; finally, it has been demonstrated that there are no qualitatively and quantitatively significant changes in the progression from peri-implant mucositis to peri-implantitis [36]. Bacteria from the classes Gammaproteobacteria (genus Vibrio), Epsilonproteobacteria (genus Campylobacter), and Bacilli (genus Granulicatella) were identified in greater amounts in the peri-implant crevicular fluid of healthy sites, whereas the classes Gammaproteobacteria (genus Acinetobacter and Moraxella) and Actinobacteria (genus Micrococcus) mainly in sites with peri-implantitis [37]. Bacteria belonging to the genus Filifactor, typically found at sites with chronic periodontitis, Dialister, Mogibacterium, Propionibacterium, Acinetobacter, Staphylococcus, Paludibacter, and Bradyrhizobium were identified only at healthy peri-implant sites [38].