Cytotoxicity, interaction with dentine and efficacy on multispecies biofilms of a modified salt solution intended for endodontic disinfection in a new in vitro biofilm model S. V. van der Waal 1,2 , N. Scheres 1 , J. J. de Soet 1 , P. R. Wesselink 2 & W. Crielaard 1 Departments of 1 Preventive Dentistry and 2 Cariology, Endodontology and Pedodontology, Academic Centre for Dentistry, Amsterdam, the Netherlands Abstract van der Waal SV, Scheres N, de Soet JJ, Wesselink PR, Crielaard W. Cytotoxicity, interaction with dentine and efficacy on multispecies biofilms of a modified salt solution intended for endodontic disinfection in a new in vitro biofilm model. International Endodontic Journal. Aim To investigate the cytotoxicity of a modified salt solution (MSS) and evaluate the antimicrobial proper- ties of MSS on in vitro biofilm models. Methodology In a metabolic assay, fibroblasts derived from periodontal ligaments (PDL) of human extracted teeth were cultured and challenged with MSS or controls. Then, in active attachment biofilm models, the efficacy of MSS in the presence of dentine powder and in eliminating mature biofilms was inves- tigated. In the dentine assay, a biofilm of Enterococcus faecalis was employed. For the final assay, microor- ganisms were retrieved from infected root canals and cultured to produce biofilms. After the treatments with MSS or the controls, the biofilms were collected, serially diluted and plated. The colony-forming units were counted. One-way ANOVA was used to analyse the differences between the groups. A P < 0.05 was considered significant. Results The PDL fibroblasts remained metabolically active after challenges with MSS. Dentine powder did not alter the efficacy of MSS (P > 0.05). In endodon- tic biofilms, the culturable bacteria were equally reduced by MSS, 2% chlorhexidine (CHX) or 2% sodium hypochlorite (NaOCl) (P > 0.05). Conclusions Modified salt solution is noncytotoxic in vitro and has good antimicrobial properties equal to CHX and NaOCl. Although the results are promising, ex vivo and in vivo studies are needed before its use as an interappointment root canal dressing can be con- sidered. Keywords: biofilm, cytotoxicity, disinfection, end- odontic, modified salt solution, potassium sorbate. Received 23 December 2013; accepted 2 April 2014 Introduction Apical periodontitis is the inflammatory response to a microbial biofilm infection of the root canal system (Kakehashi et al. 1965, Ricucci & Siqueira 2010). Biofilms are clusters of microorganisms attached to a surface. The microorganisms are embedded in a self- produced slimy matrix. Although an infected root canal space is disinfected with irrigants such as sodium hypochlorite (NaOCl) or chlorhexidine (CHX), these cannot eradicate all endodontic microorganisms (Nair et al. 2005, Ricucci & Siqueira 2010). A pro- spective study has reported the success rate of root canal treatments to be 80% (Ng et al. 2011), which means that one in five apical periodontitis lesions fails to heal. Current disinfectants are safe when used within the confinement of the root canal, but are toxic to the Correspondence: Suzette V. van der Waal, Room 13N-15, Gustav Mahlerlaan 3004, 1081 LA Amsterdam, The Netherlands (Tel.: 0031 20 5980880; Fax: 0031 20 5980333; e-mail: s.vd.waal@acta.nl). International Endodontic Journal © 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd doi:10.1111/iej.12294 1