Accumulation of QSSMs both in supernatant and in cellular extracts of PAO1, ΔmexL, and ΔmexK strains grown in LB medium. Thin-layer chromatography analysis of both cell extracts (CEs) and supernatants (SNs) of PAO1, ΔmexL, and ΔmexK cultures grown to late exponential (AHLs; OD600 = 1.7) or early stationary phase (AQs; OD600 = 2.5) coupled to the growth of (A) LasR, (B) RhlR, or (C) PqsR-based biosensor strain. Since C4-HSL freely diffuses through the plasma membrane and hence should reach an equilibrium between the extracellular and intracellular levels [48], only supernatants were measured. No differences in 3-oxo-C12-HSL accumulation between the strains were observed. Although a decrease in C4-HSL accumulation was observed, this was found in both ΔmexL and ΔmexK strains, evidencing that MexJK has a minor impact on this phenotype. The lower PQS and HHQ accumulation in SN and CE of ΔmexL with respect to PAO1 and ΔmexK, was the most evident change associated with MexJK overproduction.