3.1.1. Antibacterial Effect

A total of 15 studies reporting the antibacterial effects of SDF on bacteria are summarized in Table 1. Among them, 14 studies measured the antibacterial properties using monospecies bacteria such as *Lactobacillus acidophilus*, *Streptococcus sobrinus*, *Enterococcus faecalis*, and *Actinomyces naeslundii*. However, subgingival microbial biofilm specimens were used on 24 adults with severe periodontitis [22]. The majority of the included studies that explored bactericidal properties were operated in vitro on dentin or enamel samples, with no difference in the effect regarding the sample material. Only one research study was conducted in vivo [23]. The longest study duration was 21 days, whereas the shortest one was 14 min. They also revealed that biofilm treated with SDF had fewer bacteria compared with that treated with water or other interventions. However, one study did not detect any significant difference among the study groups [24]. Among the included studies, eight out of nine studies reported that SDF inhibited the growth of *Streptococcus mutans* through colony-forming unit counts. Two studies revealed that SDF also had an antibacterial function in *Lactobacillus acidophilus* and *Actinomyces naeslundii*. In addition, only one out of the two studies reported that SDF reduced the amount of *Enterococcus faecalis* [25] while the other reported reduced *Lactobacillus rhamnosus* [26]. One study reported that a 38% and a 19% SDF solution inhibited cultivable bacteria without any significant difference [27]. Only *P. micra* and *S. constellatus* belong to red- and orange-complex species recovered from SDF-treated specimens.

**Figure 1.** Flowchart of the study selection process: identification, selection, eligibility, included.

Furthermore, some studies reported that the live-to-dead ratios of *Streptococcus mutans*, *Lactobacillus acidophilus*, and *Actinomyces naeslundii* in biofilm were significantly lower after the application of SDF [23–27]. Only Al-Madi and co-workers reported that a 5.25% sodium hypochlorite solution had a significantly higher live-to-dead ratio of *Enterococcus faecalis* compared with the study's SDF group [27]. Only one study reported that the antibacterial effects of nano silver fluoride varnish and the potency of propolis fluoride varnish were comparable with a 38% SDF varnish. Another study compared the minimum inhibitory concentration and the minimum bactericidal concentration of propolis fluoride and nano silver fluoride for the inhibition of biofilm formation [28].

#### 3.1.2. Microbiota Change in Community Diversity and/or Composition within Biofilm

A total of four studies reported on the microbiota diversity before and after SDF treatment. In terms of intervention with SDF solution, only a 38% concentration was adopted in all four studies (Table 2). The majority of the included studies that explored bactericidal properties were operated in vivo, with one study conducted in vitro; it used extracted caries teeth in an artificial mouth model [29]. Both saliva and plaque were collected in two studies [29,30], whereas only plaque was collected in the other two studies [31,32]. Two studies were conducted on preschool children, whereas one was conducted on primary school students [29], and another was conducted on adults with a mean age of 47 years [32]. The shortest study duration was 24 h, whereas the longest was 12 weeks. Among the included studies, three out of the four used the polymerase chain reaction amplification of 16S ribosomal ribonucleic acid genes and MiSeq sequencing in diversity analyses. One

study used ribosomal ribonucleic acid sequencing and quantitative polymerase chain reaction quantification [31]. The Shannon index was the most frequently used index for diversity assessment.

All four studies reported no significant change in diversity. However, one study found a significant decrease in the diversity of oral bacteria plaque within active caries, but not in arrested caries, 12 weeks after SDF treatment [30]. Furthermore, the same paper reported an alteration in the species of the bacteria both before and after the SDF treatment. *Streptococcus mutans* and *Streptococcus sobrinus* increased significantly after two and 12 weeks of the SDF treatment in the plaque within active caries. Meanwhile, *Lactobacillus* sp. and *Rothia* sp. increased significantly after two weeks. Another study reported that the carbohydrate transportation and metabolic functions in plaque were significantly reduced at 24 h and one week post-intervention [29].
