These results could be explained by several studies, which demonstrated the role of E. faecalis and its virulence factors (such as extracellular proteases and cytolysin) in local inflammation and alveolar bone destruction in apical periodontitis (Souto and Colombo, 2008; Guerreiro-Tanomaru et al., 2013). This evidence concerns the gene PRF1 and periapical periodontitis.