Tehran University of Medical Sciences Journal of Medical Bacteriology 2251-8649 2 3-4 2015 10 12 1 10 EN Amir Hossein Mirhashemi Dental Research Center, Dentistry Research Institute , Department of Orthodontics, Tehran University of Medical Sciences,Tehran, IR Iran. Abbas Bahador Department of Microbiology, School of Medicine, Tehran University of Medical Sciences, Tehran, IR Iran. Mohammad Zaman Kassaee Department of Chemistry, Tarbiat Modarres University, Tehran, IR Iran. Ghazaleh Daryakenari Department of Esthetic and Restorative Dentistry, Dental School, Babol University of Medical Sciences, Babol, IR Iran. Mohammad Sadegh Ahmad-Akhoundi Dental Research Center, Dentistry Research Institute , Department of Orthodontics, Tehran University of Medical Sciences,Tehran, IR Iran. Ahmad Sodagar Dental Research Center, Dentistry Research Institute , Department of Orthodontics, Tehran University of Medical Sciences, Tehran, IR Iran. 2015 10 12 2015 10 12 Background: Incidence of  white  spots due  to  demineralization of  enamel  and  gingival problems  is  an  unacceptable  result  of  orthodontic  treatment.  Plaque  accumulation  and bacterial biofilm growth are responsible for these phenomena. The resin-based dental composites used as bonding agents in orthodontics play a major role in mentioned problems. As recent researches assert the antimicrobial effects of chitosan (CS) and zinc oxide (ZnO) nanoparticles  (NPs),  it  seems  that  adding  these  nanoparticles  to  the  composite  can  be beneficial in reducing the number and function of microorganisms. The aim of this study was to  evaluate  the  antimicrobial  effects  of  ZnO-NP  and  CS-NP-containing  orthodontic composite. Methods: Antibacterial effectiveness of ZnO-NPs and CS-NPs was assessed in four groups against Streptococcus mutans, Streptococcus sanguis and Lactobacillus acidophilus grown both planktonic and as a biofilm on composites. One group as the unmodified control group and  three  groups  consisting  of  three  different  concentrations of  ZnO-NPs  and  CS-NPs mixture: 1%, 5% and 10% (1:1 w/w). 108 CFU/ml microorganism suspensions were provided with spectrophotometer. Biofilm formation was quantified by viable counts. Disc agar diffusion (DAD) test was carried out to determine antimicrobial effects of nanoparticles by measuring the inhibition diameter on brain heart infusion agar plates. Finally,viable counts of microorganisms on days 3, 15 and 30 were collected for the antimicrobial effects of eluted components from composite discs. Results: In biofilm formation test, a reduction in bacterial counts was observed with 10% nanoparticle-containing composites compared with their unmodified counterpart. In the DAD test only 10% nanoparticle-containing specimens showed statistically significant inhibition. The only noticeable datain eluted component test was on day 30 for 10% nanoparticle- containing discs, inhibiting L. acidophilus. Conclusion: It seems that a mixture of ZnO-NPs and CS-NPs has induced an antibacterial activity in resin composite; especially in 10% weight concentrations which was significantly higher than other groups. https://jmb.tums.ac.ir/index.php/jmb/article/view/43 https://jmb.tums.ac.ir/index.php/jmb/article/download/43/38