ORIGINAL ARTICLE Silver nanoparticles: influence of stabilizing agent and diameter on antifungal activity against Candida albicans and Candida glabrata biofilms D.R. Monteiro 1,2 , S. Silva 2 , M. Negri 2,3 , L.F. Gorup 4 , E.R. de Camargo 4 , R. Oliveira 2 , D.B. Barbosa 1 and M. Henriques 2 1 Department of Dental Materials and Prosthodontics, Arac ¸ atuba Dental School, Universidade Estadual Paulista (UNESP), Arac ¸ atuba, Sa ˜ o Paulo, Brazil 2 Department of Biological Engineering, Institute for Biotechnology and Bioengineering, University of Minho, Braga, Portugal 3 Faculdade INGA ´ , Maringa ´ , Parana ´ , Brazil 4 LIEC-Department of Chemistry, Federal University of Sa ˜ o Carlos (UFSCar), Sa ˜ o Carlos, Sa ˜ o Paulo, Brazil Introduction The emergence of a pathogenic state known as denture stomatitis, which is associated with biofilm formation by Candida species, particularly Candida albicans and Can- dida glabrata (Coco et al. 2008), is common in complete denture wearers. Candida infections display increased resistance to antifungal therapy, leading to treatment failure and to recalcitrant infections (Watamoto et al. 2009). This fact has stimulated the possibility of using silver nanoparticles (SN) to control Candida biofilm formation. Keywords biofilms, Candida albicans, Candida glabrata, infection, silver nanoparticles. Correspondence Mariana Henriques, Institute for Biotechnology and Bioengineering, Department of Biological Engineering, University of Minho, 4710-057 Braga, Portugal. E-mail: mcrh@deb.uminho.pt 2011 ⁄ 2024: received 27 November 2011, revised 30 January 2012 and accepted 30 January 2012 doi:10.1111/j.1472-765X.2012.03219.x Abstract Aim: The purpose of this work was to evaluate the size-dependent antifungal activity of different silver nanoparticles (SN) colloidal suspensions against Candida albicans and Candida glabrata mature biofilms. Methods and Results: The research presented herein used SN of three different average sizes (5, 10 and 60 nm), which were synthesized by the reduction of silver nitrate through sodium citrate and which were stabilized with ammonia or polyvinylpyrrolidone. Minimal inhibitory concentration (MIC) assays were performed using the microdilution methodology. The antibiofilm activity of SN was determined by total biomass quantification (by crystal violet staining) and colony forming units enumeration. MIC results showed that all SN colloi- dal suspensions were fungicidal against the tested strains at very low concentra- tions (0Æ4–3Æ3 lg ml )1 ). With regard to biomass quantification, SN colloidal suspensions were very effective only against C. glabrata biofilms, achieving biomass reductions around 90% at a silver concentration of 108 lg ml )1 . In general, all SN suspensions promoted significant log 10 reduction of the mean number of cultivable biofilm cells after exposure to silver concentrations at or higher than 108 lg ml )1 . Moreover, the results showed that the particle size and the type of stabilizing agent used did not interfere in the antifungal activity of SN against Candida biofilms. Conclusions: This study suggests that SN have antifungal therapeutic potential, but further studies are still required namely regarding formulation and delivery means. Significance and Impact of the Study: SN may contribute to the development of new strategies for the improvement of oral health and quality of life particu- larly of the complete denture wearers. Letters in Applied Microbiology ISSN 0266-8254 ª 2012 The Authors Letters in Applied Microbiology 54, 383–391 ª 2012 The Society for Applied Microbiology 383