Diketopiperazines as Cross-Communication Quorum-Sensing Signals between Cronobacter sakazakii and Bacillus cereus

[Image: see text] Herein, we reveal a second quorum-sensing system produced by Cronobacter sakazakii. A cyclo(l-Pro–l-Leu) diketopiperazine, detected in pure and mixed cultures of C. sakazakii and Bacillus cereus explains the coexistence of both in the same industrial environments. The molecule was identified by gas chromatography–mass spectrometry (GC–MS), (1)H, and (13)C NMR, including 2D NMR (correlation spectroscopy, heteronuclear multiple bond correlation, and heteronuclear single quantum correlation), and the absolute configuration was compared with that of four synthetic standards produced by solid phase peptide synthesis using a chiral column on a GC–flame ionization detection. This article provides a new method to determine the absolute configuration of cyclo(Pro–Leu) diketopiperazine replacing the joint use of (1)H NMR and Marfey’s method.