Antimicrobial activity of silver nanoparticle colloids of different sizes and shapes against Streptococcus mutans

Silver nanoparticles stabilized with chitosan biopolymer are a new antibacterial agent for treatment of caries. To determine whether the size and morphology of silver nanoparticles in colloidal solution altered their antimicrobial activity, four colored colloids were characterized by ultraviolet-visible (UV-Vis) spectroscopy and transmission electron microscopy (TEM), and their antimicrobial activity [minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC)] against reference stocks of Streptococcus mutans ATCC 25175 compared with chlorhexidine and silver diamine fluoride as controls. TEM revealed that the seed solution (SS) contained only spherical nanoparticles with size of 8.7 +/- 3.1 nm whereas the other colloids had predominantly spherical nanoparticle shape, although colloid 1 (S1) contained 2.5% triangular nanoparticles. Colloid 2 (S2) had 15.9% triangular and 6.8% cylindrical nanoparticles, while 23.3% of the silver nanoparticles in colloid 3 (S3) were triangular and 1.5% were cylindrical. All the silver nanoparticle colloids showed antimicrobial activity with no statistically significant differences (p > 0.05) in MIC or MBC. The colloids showed antimicrobial activity higher than silver diamine fluoride (p < 0.05) and equivalent to chlorhexidine (p > 0.05). The size and shape of the nanoparticles did not affect the antimicrobial properties of the colored colloids. Thus, the antimicrobial activity of the colloids containing silver nanoparticles against Streptococcus mutans was considered equivalent to that of chlorhexidine, the gold-standard antibiotic in dentistry.