# A Concise Review of Silver Diamine Fluoride on Oral Biofilm
## 4 Discussion  
This review is the first review concerning the effect of SDF applied directly to biofilm. It is based on the count of bacteria and fungi as well as the diversity within the biofilm. A large number of reviews have reported the effectiveness of SDF for preventing and arresting caries in clinical studies, and its effect was reported as extraordinary, mostly due to its antibacterial function and reineralization property [20]. Although no limitation was set regarding the publication year during the search, the earliest study found was published in 2010. This shows a new trend where research is being turned into laboratory work exploring the reason for SDF's effectiveness in preventing and arresting caries. The majority (15/21, 71%) of the included studies were published between 2016 and 2020. SDF solutions featuring different concentrations [22; 34] or combined with different materials [17; 35; 36] were adopted to investigate their potential use for antibacterial purposes. SDF has an antibacterial function because both the silver ions and the fluoride contained in SDF appear to have the ability to inhibit the formation of variogenic biofilm [37]. A 38% SDF solution contains approximately 253,870 ppm silver and 44,800 ppm fluoride ions [38]. The microorganisms can be killed and silver ions can interfere with metabolic processes [12]. A three-pronged approach exists for silver ions to kill the microbiota: damage the cell wall structure of bacteria, influence metabolic processes and inhibit enzyme activities, and, finally, inhibit the replication of bacterial deoxyribonucleic acid [12]. In addition, it has been suggested that silver ions at a concentration of 20 ppm can inhibit the growth of _Streptococcus mutans_[39]. It has also been reported that the antimicrobial effect stems from the silver ions, especially at low concentrations [40]. Furthermore, the antimicrobial function of a high concentration of fluoride cannot be ignored [41]. The reason for this is that a high concentration of fluoride can influence enzymes' carbohydrate metabolism and sugar uptake and bind the bacterial cellular components, resulting in the inhibition of biofilm formation [37]. However, only one study reported no significant difference in the count of _Lactobacillus rhamnus_ among the 38% SDF group and the other groups (35% chlorhexidine varnish, 5% sodium fluoride varnish, 500 ppm sodium fluoride solution + 0.1% chlorhexidine solution, and the blank control) [26]. The possible reason for this may be that the formation of the SDF adopted in the study was a varnish rather than a solution, which indicates that the reduction in bioactive silver ions was not the same as that in the 38% SDF solution. According to this review, even though SDF inhibits some oral microbiota species, the diversity of the biofilm does not seem to change before and after SDF application. One possible reason for this is that SDF only reduces the number of curious species, for example, _Streptococcus mutans_ and _Lactobacillus_, rather than inhibiting all of the microbiota species. In the caries process, _Streptococci_, _Lactobacillus_, and _Actinomycetes_ are treated as the initial bacterial invasion species. Among them, _Streptococcus mutans_ is one of the most important pathogens associated with the initiation and progression of caries [42]. _Lactobacillus acidophilus_ and _Lactobacillus rhamnus_ are routinely found in deep and superficial carions lesions, which indicates that they are the most abundant bacteria species [43; 44]. _Actinomyces naslundii_ is related to root caries, which have the potential to invade dental tubules [15]. After the results of the studies were analyzed, it was found that the possible mode of action of SDF can be related to its antibacterial properties on cariogenic bacteria rather than a change in the diversity of the biofilm. The microbiota appeared to reach a new balance, but with fewer carious species [45]. According to this review, SDF also has antifungal properties.