Quorum-sensing cross talk: isolation and chemical characterization of cyclic dipeptides from Pseudomonas aeruginosa and other Gram-negative bacteria

In cell-free Pseudomonas aeruginosa culture supernatants, we identified two compounds capable of activating an N-acylhomoserine lactone (AHL) biosenso r, Mass spectrometry and NMR spectroscopy revealed that these compounds wer e not AHLs but the dike-topiperazines (DKPs), cyclo(Delta Ala-L-Val) and cy clo(L-Pro-L-Tyr) respectively. These compounds were also found in cell-free supernatants from Proteus mirabilis, Citrobacter freundii and Enterobacter agglomerans [cyclo(Delta Ala-L-Val) only]. Although both DKPs were absent from Pseudomonas fluorescens and Pseudomonas alcaligenes, we isolated, from both pseudomonads, a third DKP, which was chemically characterized as cycl o(L-Phe-L-Pro), Dose-response curves using a LuxR-based AHL biosensor indic ated that cyclo(Delta Ala-L-Val), cyclo(L-Pro-L-Tyr) and cyclo(L-Phe-L-Pro) activate the biosensor in a concentration-dependent manner, albeit at much higher concentrations than the natural activator N-(3-oxohexanoyi)-L-homos erine lactone (3-oxo-C6-HSL), Competition studies showed that cyclo(Delta A la-L-Val), cyclo(L-Pro-L-Tyr) and cyclo(L-Phe-L-Pro) antagonize the 3-oxo-C 6-HSL-mediated induction of bioluminescence, suggesting that these DKPs may compete for the same LuxR-binding site. Similarly, DKPs were found to be c apable of activating or antagonizing other LuxR-based quorum-sensing system s, such as the N-butanoylhomoserine lactone-dependent swarming motility of Serratia liquefaciens. Although the physiological role of these DKPs has ye t to be established, their activity suggests the existence of cross talk am ong bacterial signalling systems.