## 4 Discussion  
Persistent microbial biofilms and filling residuals in \"hard-to-reach\" areas of the complex root canal system, particularly in the apical region, are more likely to develop post-treatment AP, even in well-treated teeth [36]. Despite the recent advances in endodontic materials and technologies, there is still a need to promote new solutions, overcoming the flaws of the standard disinfecting protocol while fighting the increasing emergence of pathogenic bacterial and fungi resistance. An ideal solvent should also be able to have antimicrobial efficacy, accounting for additional disinfection over the adhered biofilms, exposed by filling materials' dissolution [29]. Thus, an intro investigation was developed in order to evaluate the antimicrobial and antib biofilm activity of MEK/OOil and MEK/TCE, two promising solvent proposals for filling residuals dissolution, on _C. albicans_ SC5314 planktonic cells and biofilm. The conventional irrigation of NaOCl and EDTA was used as a reference of the standard recommended clinical procedure. A preliminary disk diffusion assay commonly used to screen the in vitro antimicrobial activity of new agents was performed. Apparently, all the tested solvents and mixtures exhibited a prelude antifungal effect. Despite the well-known limitations of disc diffusion assay, it easily allowed to compare the solvents tested (Table 1), highlighting the great diffusion ability of OOil and its superior antifungal activity against _C. albicans_. The results are in agreement with other reports, confirming the antimicrobial effect of OOil and TCE, tested in different microorganisms [29; 37]. A relevant antifungal activity of both solvent mixtures was also demonstrated, particularly when OOil was present. The investigation revealed that a 24 h _C. albicans_ biofilm exposed to the sequence NaOCl and EDTA still presented some yeast viable cells, which were largely reduced after the exposure to either solvent mixture, MEK/TCE, or MEK/OOil (Figure 1). Despite only including one strain (_C. albicans_ SC5314), the results corroborate other investigations with several laboratory and oral clinical isolates [5]. MEK/OOil had a particularly excellent antifungal and antibiofilm activity since it was totally effective, overcoming the \"flaw\" of the conventional NaOCl and EDTA sequence, emphasized by [PERSON] et al. [5]. The quantification of biofilm viable cell counts was further reinforced by the SEM results (Figure 1). The superior performance of solvent mixtures presenting high filling dissolution and low cytotoxicity, previously reported by our team [31], was still reinforced by their antimicrobial activity over planktonic and _C. albicans_ biofilm. Our hypothesis is that the synergy obtained by solvent association, improving filling material dissolution [31] could also be applied here, resulting in a superior anti-biofilm effect of MEK/TCE regarding MEK, despite the apparently null antibiofilm activity of the isolated TCE. To our knowledge, this is the first study with evidence of antifungal and antibiofilm activity of endodontic solvent associations, with a different insight into retreatment procedures. Some limitations should be stressed, apart from those inherent to in vitro studies, such as the influence of the biofilm age or structure [5; 12; 28] on the efficacy of different endodontic irrigating solutions. The higher antibiofilm effect of both mixtures, particularly MEK/OOil, was clear compared with the standard protocol. The additional retreatment step, incorporating novel endodontic solvents with the dual role of dissolving and disinfecting, might be relevant over persistent biofilms, such as _Candida albicans_', even whenever filling removal is incomplete. Concerns about the deleterious effect of irrigating/solvent solutions on dentin's mechanical properties [13; 38; 39], risking tooth integrity or enabling easier bacterial colonization [40], have been emphasized.