Acylated homoserine lactones (AHLs) play a widespread role in intercellular communication among bacteria. The Australian macroalga Delisea pulchra produces secondary metabolites which have structural similarities to AHL molecules. We report here that these metabolites inhibited AHL-controlled processes in prokaryotes. Our results suggest that the interaction between higher organisms and their surface-associated bacteria may be mediated by interference with bacterial regulatory systems.Acylated homoserine lactones (AHLs) serve as signals in bacterial communication. AHLs and their derivatives regulate bioluminescence, Ti plasmid transfer, production of virulence factors and antibiotics (for reviews, see references 15 and 24), and swarming motility (14). These bacterial processes are fundamental to the interaction of bacteria with each other, their environment, and, particularly, higher organisms. It might therefore be expected that plants or animals would have evolved strategies to interfere with bacterial AHL-mediated processes. The seaweed Delisea pulchra (Rhodophyta) produces a number of halogenated furanones (9), which are structurally similar to the bacterial AHLs ( Fig. 1) and have strong biological activity (7), including antifouling and antimicrobial properties (8,23). We hypothesized that these metabolites could interfere with bacterial processes which involve AHLdriven quorum-sensing systems. This hypothesis was tested in terms of responses known to be regulated by AHLs; swarming motility in Serratia liquefaciens (14) and bioluminescence produced by the marine bacterial strains Vibrio fischeri and Vibrio harveyi (15).Bacterial swarming is a multicellular, density-dependent behavior that is induced in response to recognition of surfaces with a certain viscosity. Cells differentiate into a multinucleated, elongated, and hyperflagellated form, orient themselves lengthwise in close contact with each other, and then move rapidly in a coordinated fashion over the surface of the growth substratum. Swarming has been described for a variety of bacteria, including members of the genera Serratia, Proteus, Vibrio, Bacillus, Escherichia, and Salmonella (1,2,17). For Proteus mirabilis and Vibrio parahaemolyticus, the ability to differentiate into the swarmer cell state plays an important role in bacterial virulence, surface adsorption, and colonization (3-5).S. liquefaciens is a suitable model organism, because members of the genus can colonize a wide variety of surfaces in water, soil, plants, insects, fish, and humans (16). The formation of a swarming colony of S. liquefaciens was recently shown to involve two genetic switches, the first of which encodes a quorum-sensing control mechanism employing at least two extracellular signal molecules, N-butanoyl-L-homoserine lactone (BHL) and N-hexanoyl-L-homoserine lactone ( Fig. 1 and reference 14). The second involves the flhDC master operon, which regulates expression of the flagellar regulon and governs control over swim and swarm cell differentiation (13).Development ...
