chanisms underlying the downregulation of SLAMF3 have yet to be identified. Bacterial populations synthesize and exchange chemical signals which coordinate and synchronize gene expression in a celldensity dependent manner. Such regulatory pathways are called quorum-sensing and involve diverse QS-signals, including Nacylhomoserine lactones . The canonical proteins required for the synthesis of AHLs belong to the LuxI family, and those for AHL-sensing to the LuxR family. The AHLmediated QS is widespread among Proteobacteria, controlling -for instance -the expression of genes involved in bacterial virulence in animal and plant hosts, horizontal gene transfer by plasmid conjugation, as well as bacterial competitiveness in the environment through production of antibiotics. Natural and BI 2536 synthetic compounds which alter QS signalling and thereby disrupt QS-regulated gene expression are called QS inhibitors. Considering the central role played by QS in the expression of virulence genes in pathogenic bacteria, the search for QSIs has driven many efforts. Over the past several years, numerous QSIs with diverse structures have been identified using different approaches such as the synthesis of structural analogues, experimental and virtual screening of chemo-libraries and purification of natural QSIs from diverse organisms, especially plants. The natural QSIs contribute to host defense against bacteria and both natural and synthetic QSIs have been proposed as promising molecules because they may act synergistically with NBI-56418 antibiotics to limit bacterial infection. In this work, we screened a chemo-library for the presence of QSIs and validated the QSI activity of the identified compounds using two bacterial species, the plant pathogen Agrobacterium tumefaciens in which QS regulates the horizontal transfer of the tumor-inducing plasmid, and the opportunistic pathogen Pseudomonas aeruginosa, in which QS controls the expression of virulence factors. This paper reports the identification of novel natural and synthetic QSIs, and also experimentally demonstrates QSI-activity of three human sexual hormones: estrone, estriol, and estradiol. This article reports the identification of novel QSIs such as the natural plant compound hordenine and the synthetic indoline-2-carboxamides, and
