Effect of osmotic stress on S. mutans planktonic and biofilm cells. (A) 0.4 M was the sub-inhibitory sodium chloride concentration (the highest concentration without significantly inhibiting the growth of bacteria) for S. mutans growth. (B) Biofilm formation was compromised under hyperosmotic conditions. (C) Short-term sub-inhibitory hyperosmotic stress disintegrated the pre-established biofilm. (D) Representative confocal laser scanning microscopy images (left panel) of live (green)/dead (red) stain of S. mutans biofilm after exposure to 0.4 M NaCl for 15 min. The upper right panel shows the percentage of viable cells versus total biofilm cells. (E) Colony forming unit of S. mutans biofilm after exposure to 0.4 M NaCl for 15 min (CFU/ml). Results were averaged from 3 independent experiments and are presented as mean ± standard deviation. *, P ≤ 0.05; N.S, not significant (P > 0.05).