# The Novel Role of Solvents in Non-Surgical Endodontic Retreatment
## 8 Antimicrobial/Antibiofilm Activity  
AP is currently recognized as a biofilm-induced disease [60]. This causal link explains the increased resistance of endodontic intra-radicular infections to conventional disinfection procedures associated with the number of unprepared areas where root canal microorganisms, in planktonic and especially biofilm form, may persist [11,60]. These are considered to be the main causes of treatment failure. Moreover, the awareness that bacterial biofilms occur with particular relevance in the apical portion is crucial for the treatment, indicating the importance of primary and post-treatment AP therapeutics [61].  
Research focusing on the antimicrobial properties of conventional gutta-percha solvents, such as halothane, eucalypt, and OOil, has not been deep. The reported assays are almost exclusively against planktonic bacteria, such as _Enterococcus faecalis_ (_E. faecalis_) and _Staphylococcus aureus_ (_S. aureus_). In general, findings agree upon a stronger degree of antibacterial activity that is associated with the most cytotoxic solvents; OOil, for example, shows no antibacterial activity against the species mentioned [62]. Ex vivo studies emphasize that chloroform reduces intracanal levels of cultivable _E. faecalis_ during endodontic retreatment [63]. By also stressing the role of _E. faecalis_ as being the prime etiological agent of post-treatment infection, [PERSON] et al. [64] highlighted that RC Solve, a derivative of OOil, had superior antibacterial activity compared to xylene and EndoSolv E, which has tetrachloroethylene as its major compound. That study considered the minimal inhibitory concentration against _E. faecalis_ ATCC and a clinical isolate from a failed root canal.  
Maximum antimicrobial activity against _E. faecalis_ biofilm has been reported with the association of a surfactant, such as cetrimide, and with chloroform, eucalypt, or OOil. Although the combinations with cetrimide achieved a 100% kill rate, cytotoxicity assessments or the dissolution efficacy of the suggested associations were missing [65]. Biofilm removal strategies include its disruption via chemo-mechanical preparation with specific shaping techniques, and antimicrobial irrigating solutions/dressings. Increased concern over its resistance to conventional antimicrobial drugs should be considered [66,67].  
Supplementary procedures for activating the final irrigating protocol with NaCl with recent devices, such as ultrasonics or XP-endo Finisher R, have been suggested [5,6]. However, microorganisms are reported to regrow after NaOCl treatment. Although the final exposure with the chelating EDTA had an additional antimicrobial effect, authors claim there is a flaw in its ability to completely eliminate resistant biofilms, such as _C. albicans_, the most prevalent fungi isolated from persistent endodontic infections [14,68]. A recent study highlighted that the association of MEK/OOil could eradicate _C. albicans_ biofilm cells remaining after the conventional NaOCl and EDTA final irrigating protocol [14]. To our knowledge, this is one of the first reports regarding the antibiofilm activity of solvents over endodontic microorganisms, refractory to the NaOCl and EDTA protocol, while exhibiting excellent dissolution ability over the most common filling materials.