Antimicrobial Activity of Gold-Titanates on Gram-positive Cariogenic Bacteria
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Gram–positive cariogenic bacteria are important etiological agents in dental caries; therefore, strategies to inhibit these bacteria to reduce the incident of this disease have intensified. Antimicrobial activity of gold–titanates against several bacteria has recently been explored. Monosodium titanate (MST), nanomonosodium titanate (nMST), and amorphous peroxo–titanate (APT) are inorganic compounds with high binding affinity for specific metal ions or compounds such as gold (Au), palladium (Pd), and platinum (Pt). Titanates have been used as an ion exchanger binding to the metal ions and act as a carrier. In this study, we investigated antibacterial activities of titanates and gold–titanates against <italic>Actinomyces viscosus</italic>, <italic>Lactobacillus casei</italic>, and <italic>Streptococcus mutans</italic>, which are important Gram–positive cariogenic bacteria as well as <italic>Streptococcus gordonii</italic>, non–pathogenic oral bacteria. Total bacterial proteins were extracted and measured to represent total bacterial cell mass after 24 h incubation. We found all gold–titanates and APT alone significantly reduced bacterial protein content for <italic>L. casei</italic> and <italic>A. viscosus</italic> while only MST–Au(III) and nMST–Au(III) affected <italic>S. mutans</italic>. Total cell mass of <italic>S.gordonii</italic> was significantly decreased when exposed to gold–titanates vs. to titanates alone. Overall, nMST–Au(III) showed the most effectiveness against all bacteria at 400 mg/L. We further examined the effect of gold–titanate nanoparticles (NPs) nMST–Au(III) concentration (10,200,400 mg/L) on <italic>L. casei</italic> and <italic>S. mutans</italic> cell viability over time via Live/Dead (L/D) direct cell fluorescent staining and colony forming units (CFUs). The L/D staining showed all three concentrations of nMST–Au(III) affected <italic>L. casei</italic> growth but only 200 and 400 mg/L nMST–Au(III) interrupted <italic>S. mutans</italic> growth. The growth curves based on CFUs/mL showed all nMST–Au(III) concentrations affected growth of both <italic>L. casei</italic> and <italic>S. mutans</italic>. Transmission electron microscopy (TEM) was used to determine specific locations on all four bacteria (<italic>A. viscosus</italic>, <italic>L. casei</italic>, <italic>S. gordonii</italic> and <italic>S. mutans</italic>) affected by the nMST–Au(III). TEM images showed gold–titanate NPs attached to the bacterial cell wall and were internalized into all bacteria. Small–scale transcriptomics relating to <italic>S. mutans</italic> selected metabolic functions were monitored to quantify the mechanism of antibacterial activity of gold–titanate NPs, using quantitative reverse transcription–polymerase chain reaction (QRT–PCR). We found the mRNA expression of <italic>S. mutans</italic> genes responded to nMST–Au(III) in different manner when exposed to various concentrations and times. Genes related to bacterial stress response were up–regulated when exposed to medium and high concentrations at both 6 and 10 h. These suggest that gold–titanate NPs cause environmental stress to <italic>S. mutans</italic>. Gold–titanate NPs demonstrated potential antimicrobial activity against Gram–positive cariogenic bacteria. These results support further development of gold–titanate NPs as a potential novel material to prevent dental caries.
- Dentistry