Isolation and characterization of quorum sensing interfering compounds produced by Medicago sativa

Quorum sensing is a population density dependent mechanism of gene regulation widely found in Gram-positive and Gram-negative bacterial strains. Bacteria use population density as a signal to activate the co-ordinated expression of certain phenotypes that are required for communal behavior. Quorum sensing is more common in bacteria involved in pathogenic, symbiotic or commensal relationships with eukaryotes. Interestingly, recent research shows that many eukaryotes produce compounds that have an influence on bacterial quorum sensing. This work describes the isolation and characterization of multiple quorum sensing interfering (QSI) compounds from the seed exudates of Medicago sativa (alfalfa) that either activate or inhibit quorum sensing dependent behavior of several bacterial strains including Chromobacterium violaceum, Agrobacterium tumefaciens and Sinorhizobium meliloti. M. sativa shares a nitrogen-fixing symbiotic relationship with S. meliloti, while the other two bacterial strains are commonly found in soil.; Chapter 1 is an extensive review of the general quorum sensing mechanisms in Gram-negative bacteria with emphasis on the different methods of interference with bacterial quorum sensing. Chapter 2 describes the isolation, identification and characterization of a quorum-sensing inhibitory compound, L-canavanine, isolated from alfalfa seed exudates. We demonstrate the QSI activity of L-canavanine on quorum-sensing regulated phenotypes in C. violaceum and S. meliloti.; In Chapter 3, we describe the isolation of four additional groups of QSI compounds that specifically activate quorum-sensing in A. tumefaciens . We have partially characterized the biochemical nature of these compounds and tentatively term them as AHL-mimics as they appear to mimic the activity of bacterial AHLs. Chapter 4 discusses the creation of a S. meliloti quorum sensing indicator strain that specifically detects long-chain AHLs produced by different bacterial strains.; We use the S. meliloti indicator strain described in Chapter 4, to isolate additional QSI compounds from alfalfa seed exudates that specifically activate the quorum sensing system of S. meliloti . We characterize three QSI compounds that are clearly different from the ones described in Chapters 2 and 3. In Chapter 6, we summarize the various QSI compounds isolated from alfalfa seed exudates in this work and discuss the potential role of these QSI compounds in the symbiotic relationship between M. sativa and S. meliloti.