The contradictory results may be due to the differences in the bacterial species or strains and the antibiotics used in studies, which is evident

from our results (Table 2). It should also be noted that DSF-family signals were shown to play dual roles in regulation of biofilm formation as they positively control the biofilm development in some bacterial species, and they could also disperse the biofilms of other bacterial species [15, 19, 21, 37]. Our results suggest that DSF and related molecules may influence the bacterial antibiotic see more susceptibility by multiple ways, including modulation of the biofilm formation, antibiotic resistant activity and bacterial persistence (Figure 4; Additional file 1: Table S1). In addition, we also examined the possibility KPT-8602 chemical structure of DSF and related molecules acting as biosurfactants to influence bacterial susceptibility to antibiotics by using rhamnolipid, which is a well characterized biosurfactants, as a control in MIC and growth analysis. We found

that rhamnolipid could also increase the antibiotic susceptibility of B. cereus at the final concentration of 50 μM (data not shown), but it also inhibits bacterial growth at this concentration and its toxicity on B. cereus cells was at least 5-fold higher than DSF (Additional file 1: Figure S3), which complicates the comparison. With all considered, at this stage we could not rule out the possibility that DSF and related molecules may have biosurfactant property and this property may contribute to their synergistic effects with antibiotics. Furthermore, several lines of evidence from this study and previous reports seem to suggest that Calpain the signalling activity of DSF and its structurally related molecules may contribute to their ability in changing bacterial antibiotic susceptibility. Firstly, it was reported that BDSF signalling system positively controls the antibiotic

resistance in B. cenocepacia, and selleck inhibitor addition of 50 μM DSF signal increased the antibiotic resistance of P. aeruginosa to polymyxins [21, 23], indicating that DSF-family signals are possibly widely involved in regulation of bacterial antibiotic resistance. Secondly, different from rhamnolipid which has a strong hydrophilic head group glycosyl, DSF and related molecules only have a very weak hydrophilic activity, suggesting that they could not be good surfactants. This notion appears to be supported by the different inhibitory activity of DSF and rhamnolipid on the growth of B. cereus (Additional file 1: Figure S3). Thirdly, our findings showed that addition of 50 μM DSF signal showed no cytotoxicity to HeLa cells, didn’t affect the B. cereus virulence (Figure 3), but could significantly change the expression patterns of many genes in B. cereus, some of which are known to be associate with antibiotics resistance or tolerance (Additional file 1: Table S1). Fourthly, the synergistic activity of DSF is antibiotic specific.