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). Figure 2 Phenotypic characteristics of S. mutans after short-term and long-term hyperosmotic stimuli. (A) Representative Scanning Electronic Microscopy

images of S. mutans biofilm on glass surfaces. Images www.selleckchem.com/products/incb28060.html shown were taken at 1000 ×, 5000 × and 10000 × magnification. (B) Representative 3D rendering images of S. mutans biofilms without NaCl for 24 h (upper left), versus with 0.4 M NaCl for either 15 min

(upper right) or 24 h (lower left). Bacterial cells and EPS are in situ labelled. Green, the bacteria (SYTO 9); red, the EPS (Alexa Fluor 647). At the right of each panel, the two channels are displayed separately, while the merged image is displayed at the left. Lateral (side) views of each biofilm are displayed at the bottom. Quantitative determination of S. mutans biofilms (lower right) confocal image stacks analyzed by the image-processing software COMSTAT. Results were averaged from 3 independent experiments and are presented as mean ± standard LY2874455 supplier deviation. *, P ≤ 0.05. To better understand the underlying molecular machineries, we performed whole-genome microarray analysis to profile the transcriptomic changes

of S. mutans upon short term exposure (15 min) to 0.4 M of NaCl. We identified 40 genes with ≥ 2 fold changes, among which 14 genes were up-regulated and 26 genes were down-regulated (Table 1 and Additional file 1). Specific genes were further quantified by quantitative RT-PCR, and the results showed acceptable consistency with the microarray data (Figure 3). In agreement with the observed biofilm dispersal phenotype, a significant down-regulation of glycosyltransferase B encoding gene (gtfB) was identified (Table 1 and Figure 3). Glycosyltransferase B is the major enzyme responsible for the oxyclozanide EPS synthesis, mediating the cellular adherence and biofilm formation of S. mutans[16]. By down-regulating gtfB expression under hyperosmotic conditions, bacterial cells are more ready to “break their biofilm bonds”, leading to a less condensed microbial community with reduced biomass. In addition, we also found that a competence-stimulating peptide (CSP) encoding gene, comC was down-regulated upon 15 min exposure to 0.4 M of NaCl (Table 1). The CSP is a GF120918 mw member of bacterial quorum sensing system. It has been reported to be involved in competence development, acid tolerance and biofilm formation of S. mutans[17]. Importantly, recent findings from Lévesque’s group have demonstrated that high level of CSP may act as an “alarmone”, triggering “guard cells” autolysis and release of eDNA necessary for the genetic diversity and survival of whole community [18, 19].