The plates were shaken at 200 rpm for 4 h at 37 C. LasR agonism and antagonism assays in the presence of PAN were performed by adding PAN from a 50 mg/mL H2O stock to cultures immediately prior to plating to yield a final PAN concentration of 25 g/mL. Supplementary Material Supporting InformationClick here to view.(4.2M, pdf) Acknowledgements Financial support for this work was provided by the NIH (AI063326), Burroughs Wellcome Fund, and Johnson & Johnson. aeruginosa (mexAB-oprM) mutant, suggesting that MexAB-OprM also recognizes these compounds as substrates. We also demonstrate that the potency of 5,6-dimethyl-2-aminobenzimidazole, recently shown to be a QS and biofilm inhibitor in P. aeruginosa, is not affected by the presence or absence of the MexAB-OprM pump. These results Ramipril have implications for the use of non-native AHLs and related derivatives as QS modulators in P. aeruginosa and other bacteria, and provide a potential design strategy for the development of new QS modulators that are resistant to active efflux. is an opportunistic pathogen responsible for life-threatening infections in immunocompromised patients, such as those suffering from AIDS, burn wounds, or cystic fibrosis. These infections are often refractory to treatment with common antibiotics due to the emergence of multidrug-resistant (MDR) strains of and other bacterial pathogens has attracted significant attention over the past ~20 years. The efficacy of such compounds as QS inhibitors in is the focus of the current study. QS is widespread in bacteria and allows the coordination of gene expression with bacterial population density. This intercellular communication pathway is mediated by small molecules or peptides (i.e., autoinducers) that vary in concentration as a function of cell number. At high cell densities, the signals reach a sufficient concentration to bind and Ramipril activate QS receptors, which subsequently regulate transcription of primarily group-beneficial genes. Proteobacteria use has two LuxI/LuxR pairs, LasI/LasR and RhlI/RhlR, which produce and sense and regulates the production of elastase B, exotoxin A, and the biosynthesis machinery for a number of metabolites related to host tissue breakdown. Furthermore, clinical isolates of strains lacking a functional system are less virulent in animal infection models, suggesting that successful LasR inhibition could significantly attenuate virulence. Our laboratory and others have synthesized and examined a range of non-native AHLs as LasR and QscR modulators in Ramipril reporter strain to measure LasR-mediated transcriptional activation. However, the potencies of these compounds in LasR reporter strains are generally muted in comparison. Meijler and co-workers observed similar effects in their studies of both covalent and non-covalent inhibitors of LasR in related and reporter strains. In general, the efficacy of small molecule drugs is often lower in relative to many other Gram-negative bacteria due to decreased membrane permeability, enhanced active efflux, or a combination of both factors, which prompted us to consider the possibility that these features could also influence the potency of our synthetic LasR modulators. In 1999, Iglewski and co-workers showed that OdDHL passively diffuses across the cell membrane (albeit at a ~10-fold slower rate than the shorter-chain autoinducer BHL) and that the presence Ramipril of the efflux pump MexAB-OprM significantly reduces the intracellular concentration of OdDHL, suggesting that MexAB-OprM recognizes OdDHL as a substrate. In concurrent work, Poole and co-workers demonstrated that a mutant capable of Plxnd1 MexAB-OprM overexpression produced reduced levels of QS-regulated virulence factors, presumably due to low levels of intracellular OdDHL. MexAB-OprM is a member of the resistance-nodulation-division (RND) family of efflux pumps, which are a main class of pumps in Gram-negative bacteria known to contribute to intrinsic and acquired resistance to exogenous compounds. Given that RND pumps often possess broad substrate profiles, we reasoned that active efflux could play a role in reducing the potency of our AHL-derived LasR inhibitors in (MexAB-OprM reduces the potency of OdDHL We began our study by comparing the potency of OdDHL in a mutant lacking both the AHL synthases LasI and RhlI (PAO-JP2; i.e., pump-active) and a mutant lacking both AHL synthases and the MexAB-OprM pump (PAO-JG21; i.e., pump-mutant). Both strains contained a functional LasR receptor and reported LasR activity via a pstrains were obtained. As we hypothesized, OdDHL was a more potent activator of LasR in the pump-mutant strain relative to the pump-active strain (Figure 1). The EC50 value shifted from 95 nM in the pump-active PAO-JP2 to 6.6.