**5. Streptococcus bacteriophages**

The most important species that play a key role in dental plaque formation are oral Streptococci. The oral streptococci mainly constitute 12 species including *Streptococcus salivarius*, *S. agnisosus*, *S. mutans*, *S. constellates*, *S. cristareus*, *Streptococcus gordonii*, *S. mitis*, *Streptococcus oralis*, *S. parasanguis*, *Streptococcus pneumoniae*, *S. sanguis*, *S. sobrinus*. The initial colonizers of the tooth are *S. salivarius*, *S. sanguis*, *S. oralis*, and *S.gordonii*; however, *S. sobrinus* and *S. mutans* are more involved in dental infections [24]. Initial studies reported the isolation of lytic bacteriophages from human saliva [25]. The complete genome sequence of *S. mutans* lytic bacteriophage M102 was revealed [26]. In 2008, Van de Ploeg reported the complete genome sequence of prophage 15 infecting *S. gordonii*, which was a lysogenic phage [24]. A diverse group of around 50 bacteriophages that infect *S. mitis*, *S. mutans*, *S. oralis*, *S. salivarius*, and *S. sobrinus* have been identified and reported [27]. Unlike the *S. mutans* phages that are seen as lytic phages, the phages for *S. mitis* have been found as temperate. These temperate phages have the property to transfer host DNA into other bacterial strains. Seven phage-related gene clusters were detected in the genome of *S. mitis* B6, SM1, and ϕB6 prophages were isolated and sequenced [28]. Virulent pneumophages DP-1 and CP-1 were able to infect *S. mitis* and are also able to infect and replicate in commensal streptococci [29]. As *S. pneumoniae* and *S. mitis* carry numerous temperate phages in their genomes they are closely related and these virulent cross-infecting streptococcal phages and their enzymes are being used to biocontrol of oral infections [30].
