Although the exact role of quorum sensing (QS) in various stages of biofilm formation, maturation, and dispersal and in biofilm resistance is not entirely clear, the use of QS inhibitors (QSI) has been proposed as a potential antibiofilm strategy. We have investigated whether QSI enhance the susceptibility of bacterial biofilms to treatment with conventional antimicrobial agents. The QSI used in our study target the acylhomoserine lactone-based QS system present in Pseudomonas aeruginosa and Burkholderia cepacia complex organisms (baicalin hydrate, cinnamaldehyde) or the peptide-based system present in Staphylococcus aureus (hamamelitannin). The effect of tobramycin (P. aeruginosa, B. cepacia complex) and clindamycin or vancomycin (S. aureus), alone or in combination with QSI, was evaluated in various in vitro and in vivo biofilm model systems, including two invertebrate models and one mouse pulmonary infection model. In vitro the combined use of an antibiotic and a QSI generally resulted in increased killing compared to killing by an antibiotic alone, although reductions were strain and model dependent. A significantly higher fraction of infected Galleria mellonella larvae and Caenorhabditis elegans survived infection following combined treatment, compared to treatment with an antibiotic alone. Finally, the combined use of tobramycin and baicalin hydrate reduced the microbial load in the lungs of BALB/c mice infected with Burkholderia cenocepacia more than tobramycin treatment alone. Our data suggest that QSI may increase the success of antibiotic treatment by increasing the susceptibility of bacterial biofilms and/or by increasing host survival following infection.Biofilm-associated infections are often very difficult to treat with conventional antibiotics (7,17,28,38). Hence, novel targets are needed to combat biofilm infections. One of them could be the bacterial communication system (quorum sensing [QS]). Bacteria coordinate their cell-density-dependent gene expression by excreting small signaling molecules (26). When a certain extracellular threshold concentration is reached, these molecules bind to a receptor, thereby activating the QS system. The typical QS system in Gram-negative bacteria consists of three components: a LuxI synthase homolog, acyl-homoserinelactone (AHL) signaling molecules, and a LuxR receptor homolog (10). Gram-positive bacteria generally use small peptide signaling molecules, which are transported out of the cell and bind to a membrane-associated two-component receptor (42). Binding to the receptor activates a signal transduction system leading to the transcription of QS-regulated genes. A third QS system using autoinducer 2 (AI-2) as signaling molecule is widespread in both Gram-positive and Gram-negative bacteria and is considered to be responsible for interspecies communication (43). QS has been shown to regulate biofilm formation in several bacterial species (15,20). AHL QS mutants of Burkholderia cenocepacia and Pseudomonas aeruginosa form biofilms with a drastically altered str...