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

Authors


Paul Williams. E-mail paul.williams@nottingham. ac.uk; Tel. (+44) 115 951 5047; Fax (+44) 115 951 5110.

Abstract

In cell-free Pseudomonas aeruginosa culture supernatants, we identified two compounds capable of activating an N-acylhomoserine lactone (AHL) biosensor. Mass spectrometry and NMR spectroscopy revealed that these compounds were not AHLs but the diketopiperazines (DKPs), cyclo(ΔAla-l-Val) and cyclo(l-Pro-l-Tyr) respectively. These compounds were also found in cell-free supernatants from Proteus mirabilis, Citrobacter freundii and Enterobacter agglomerans [cyclo(ΔAla-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 cyclo(l-Phe-l-Pro). Dose–response curves using a LuxR-based AHL biosensor indicated that cyclo(ΔAla-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-oxohexanoyl)-l-homoserine lactone (3-oxo-C6-HSL). Competition studies showed that cyclo(ΔAla-l-Val), cyclo(l-Pro-l-Tyr) and cyclo(l-Phe-l-Pro) antagonize the 3-oxo-C6-HSL-mediated induction of bioluminescence, suggesting that these DKPs may compete for the same LuxR-binding site. Similarly, DKPs were found to be capable of activating or antagonizing other LuxR-based quorum-sensing systems, such as the N-butanoylhomoserine lactone-dependent swarming motility of Serratia liquefaciens. Although the physiological role of these DKPs has yet to be established, their activity suggests the existence of cross talk among bacterial signalling systems.

Ancillary