# Susceptibility of the Oral Commensal Bacterium Streptococcus sanguinis to ZnO Nanoparticles

## TL;DR

This study shows how zinc oxide nanoparticles affect the oral bacteria Streptococcus sanguinis, both killing it and disrupting biofilms.

## Contribution

The study reveals the antimicrobial and antibiofilm mechanisms of ZnO NPs against S. sanguinis, including oxidative stress and membrane dysfunction.

## Key findings

- ZnO NPs rapidly inhibit S. sanguinis metabolism and ATP levels within 2–4 hours.
- ZnO NPs induce reactive oxygen species and lipid peroxidation, partially mitigated by α-tocopherol.
- Prolonged exposure to ZnO NPs causes membrane damage and disrupts biofilm structure.

## Abstract

Streptococcus sanguinis (S. sanguinis) is an oral commensal and early colonizer of the tooth surface that contributes to dental biofilm homeostasis. Zinc oxide nanoparticles (ZnO NPs) are often incorporated into dental restorative materials to enhance mechanical performance and confer antibacterial properties; however, their effects on S. sanguinis have not been thoroughly studied. Here, we investigated the antimicrobial and antibiofilm efficacy of ZnO NPs against this bacterial species. ZnO NPs exhibited a minimal inhibitory concentration (MIC) of 100 µg/mL and caused rapid, dose-dependent suppression of intracellular ATP levels and overall metabolic activity within 2–4 h of exposure. ZnO NPs induced reactive oxygen species (ROS) production in a dose-dependent manner. The free radical scavenger α-tocopherol partly prevented the antibacterial effect of ZnO NPs, suggesting that lipid peroxidation contributes to ZnO NP-mediated toxicity, although it is not the sole mechanism involved. Short-term exposure (2 h) to ZnO NPs did not significantly affect membrane integrity or cellular morphology, whereas prolonged treatment (24 h) resulted in pronounced membrane permeabilization, membrane hyperpolarization, and cellular swelling. Computational morphometric analyses of high-resolution scanning electron microscopy (HR-SEM) images of planktonic growing bacteria after a 24 h treatment confirmed a significant, dose-dependent increase in cell surface area and surface roughness. Importantly, ZnO NPs also reduced the metabolic activity and compromised the structural integrity of mature, preformed biofilms. Collectively, these findings demonstrate that ZnO NPs exert antimicrobial and antibiofilm effects against S. sanguinis through early metabolic inhibition associated with oxidative stress followed by progressive membrane dysfunction.