Regulation by AI-2 
Beside AHL, other putative quorum sensing signalling molecules have been identified. One of them is autoinductor 2 (AI-2), furanosyl borate diester, which is synthesized by the luxS product [39,40] of which homologues are present in P. luminescens and Y. enterocolitica (plu1253, ye0839). It has been shown that the luxS pathway negatively controls the expression of genes for carbapenem antibiotic biosynthesis in P. luminescens [41]. Overall, more than 300 AI-2 regulated genes involved in regulation, metabolic activity, stress response and pathogenicity are known in P. luminescens [42]. For example, the expression of tcdA1 and tccC1 encoding subunits of the insecticidal toxin complexes and the production of mcf2 encoding the "Makes caterpillar floppy" toxin was identified to be luxS dependent. The DeltaluxS mutant also showed decreased expression of virulence factors such as the cytotoxic protein CcdB, the hemolysin secretion protein HlyD (Plu0635), and the toxin ABC transporter subunits RtxD/RtxB. Homologues of these proteins are present in Y. enterocolitica (see chapter 2), suggesting their possible regulation by AI-2 also in this bacterium. Furthermore, the P. luminescens luxS-negative strain exhibited decreased biofilm formation, increased type IV/V pilus-dependent twitching motility, and attenuated virulence against insect larvae [42]. Taken together, a similar and AI-2 dependent mechanism for the regulation of insect colonization and insect pathogenicity might be used by both organisms. Whether they sense self-produced or external AI-2, or a combination of both, indicating a quorum sensing mechanism or a regulation similar to AHL as described above, remains to be elucidated.
