The specificity of the observed modulations in gene expression was validated by monitoring

the impact of HQNO on the expression of the housekeeping gene gyrB. The expression of gyrB was not modulated in the different conditions tested (Fig. 4F). These results suggest that HQNO induces the expression of sarA by a SigB-dependent mechanism. Overall, these results suggest that exposure of S. aureus to HQNO reproduces the transcriptional signature found in SCVs [12, 15, 19, 20, 41] and stimulates biofilm production by having opposite effects on the activity of SigB (up) and agr (down) as well as on the expression of sarA (up by a SigB-dependent mechanism). P. aeruginosa stimulates biofilm formation and increases the activity of SigB of a

S. YH25448 aureus CF isolate In order to ascertain that the effect of HQNO on S. aureus is representative of what may happen when P. aeruginosa and S. aureus are in close proximity during a co-infection, we conducted Eltanexor cell line experiments in which S. aureus was exposed to supernatants from overnight cultures of P. aeruginosa as well as experiments using a double chamber co-culture model. We used the E. coli strain K12 in control experiments to ensure that the observed effect was specific to P. aeruginosa and was not only caused by the close proximity of a Gram-negative bacterium or non specific alterations of the growth medium. We used E. coli because it is known that this bacterium does not produce HQNO (E. Déziel, unpublished data). Fig.5A shows that P. aeruginosa PAO1 inhibits the growth of the S. aureus strain CF1A-L whereas this phenomenon was not observed with E. coli K12. The supernatant collected from an overnight culture PD0332991 of PAO1 significantly inhibited the growth of S. aureus. This growth inhibition was accompanied by a significant increase in biofilm production (Fig.

5B). Fig. 5C shows that when S. aureus CF1A-L was co-cultured with PAO1 for 6 h, significantly more SCVs were recovered than that seen when the co-culture was done with E. coli K12. Of striking interest, the co-cultivation of S. aureus CF1A-L with P. aeruginosa PAO1 specifically and significantly increased the expression of asp23. Oxymatrine These results confirm that P. aeruginosa has the potential to specifically inhibit the growth, stimulate biofilm production, favor the emergence of the SCV phenotype and increase the activity of SigB in non-SCV S. aureus strains. Figure 5 P. aeruginosa stimulates biofilm formation and increases the activity of SigB of a S. aureus CF isolate. (A) CFU/ml recovered after 48 h of growth of CF1A-L (open bar) and CF1A-L in the presence of supernatants from overnight cultures of P. aeruginosa PAO1 (black bar) or of E. coli K12 (hatched bar). The picture shows the specific inhibitory effect of P. aeruginosa on the growth of S. aureus. (B) Relative biofilm production by CF1A-L grown in the presence of supernatants from overnight cultures of P. aeruginosa or E. coli.