Phylogenetic relationships among Gram-positive and Gram-negative bacteria on the basis of complete luxS sequences and presence of established AI-2 receptors in the respective genomes. The distance tree was generated by the NJ method with the JC formula, without choosing any outgroup. Nodal supports were assessed by 1000 bootstrap replicates. Only bootstrap values greater than 50% are shown. The scale bar represents the number of substitutions per site. The presence of recognized AI-2 receptor genes (luxP, lsrB) is indicated between parentheses. The asterisk shows the truncated Lsr-receptor complex of X. bovienii. With the exception of the E. billingiae, which was produced in this work (accession number DQ977724), all luxS sequences were retrieved at the NCBI database or in published genome projects (see Methods for accession numbers).