| Summary: | <p>Abstract</p> <p>Background</p> <p>The autoinducer-2 (AI-2) group of signalling molecules are produced by both Gram positive and Gram negative bacteria as the by-product of a metabolic transformation carried out by the LuxS enzyme. They are the only non species-specific quorum sensing compounds presently known in bacteria. The <it>luxS </it>gene coding for the AI-2 synthase enzyme was found in many important pathogens. Here, we surveyed its occurrence in a collection of 165 marine isolates belonging to abundant marine phyla using conserved degenerated PCR primers and sequencing of selected positive bands to determine if the presence of the <it>luxS </it>gene is phylogenetically conserved or dependent on the habitat.</p> <p>Results</p> <p>The <it>luxS </it>gene was not present in any of the <it>Alphaproteobacteria </it>(n = 71) and <it>Bacteroidetes </it>strains (n = 29) tested; by contrast, these bacteria harboured the <it>sahH </it>gene, coding for an alternative enzyme for the detoxification of <it>S</it>-adenosylhomocysteine (SAH) in the activated methyl cycle. Within the <it>Gammaproteobacteria </it>(n = 76), <it>luxS </it>was found in all <it>Shewanella, Vibrio </it>and <it>Alteromonas </it>isolates and some <it>Pseudoalteromonas </it>and <it>Halomonas </it>species, while <it>sahH </it>was detected in <it>Psychrobacter </it>strains. A number of <it>Gammaproteobacteria </it>(n = 27) appeared to have neither the <it>luxS </it>nor the <it>sahH </it>gene. We then studied the production of AI-2 in the genus <it>Shewanella </it>using the <it>Vibrio harveyi </it>bioassay. All ten species of <it>Shewanella </it>tested produced a pronounced peak of AI-2 towards the end of the exponential growth phase in several media investigated. The maximum of AI-2 activity was different in each <it>Shewanella </it>species, ranging from 4% to 46% of the positive control.</p> <p>Conclusion</p> <p>The data are consistent with those of fully sequenced bacterial genomes and show that the potential for <it>lu</it>xS related signalling is dependent on phylogenetic affiliation rather than ecological niche and is largest in certain groups of <it>Gammaproteobacteria </it>in the marine environment. This is the first report on AI-2 production in <it>Shewanella </it>species; its signalling role in these organisms remains to be elucidated.</p>
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