Bacterial Toxicity Comparison Between Silver, Titanium Dioxide and Hydroxyapatite Nanoparticles
Objectives: This study aimed to determine the toxicity to Streptococcus sanguinis of a range of engineered nanomaterials (ENMs) compared to their equivalent bulk materials or metal salts, and against a positive control of chlorhexidine.
Methods: Bacterial suspensions of S. sanguinis (107 CFU ml-1) were exposed to dilutions series (400-3.125 mg l-1) of each material using the minimum inhibitory concentration assay (MIC). The lactate production by S. sanguinis was also measured. The materials tested included Ag, TiO2 and hydroxyapatite (HA) nanoparticles (NPs). The antibacterial activity of these ENMs was compared to that of AgNO3, TiO2 bulk and HA microparticles respectively. Chlorhexidine was used as positive control. All test solutions were prepared in physiological saline (modified Krebs solution) and normal saline (0.85 % NaCl). The materials under investigation were characterised in solution and the aggregate size was measured using nanoparticle tracking analysis (NTA).
Results: Additions of the ENMs to physiological saline caused some aggregation. Silver nitrate was effective at growth inhibition at all the dilutions tested compared to unexposed controls. Ag NPs caused growth inhibition at 100 mg l-1 or less. The MIC for complete growth inhibition for TiO2 NPs was 50 mg l-1, with the nano form being more toxic than bulk TiO2. HA particles (nano or micro) were less toxic. Apparent lactate production by S. sanguinis was abolished by all concentrations of AgNO3, but only at 400 mg l-1 for Ag NPs. The different forms of neither TiO2 nor HA had any effect on lactate production.
Conclusions: Data showed that Ag NPs were a better antibacterial than TiO2, and that HA particles were not overtly toxic to S. sanguinus in the conditions used here. All ENMs that demonstrated antibacterial action were found to be more effective in saline compared to physiological saline.