Bacterial quorum sensing (QS) is anticipated as a new potential target for the development of antimicrobial drugs. An anti-QS substance against Chromobacterium violaceum CV026 and Pseudomonas aeruginosa PA01 has been isolated and purified from the crude extracts of the marine fungus Penicillium chrysogenum DXY-1, and the accurate structure was identified as cyclo(l-Tyr-l-Pro). This cyclic dipeptide at sub-minimum inhibitory concentration can decrease the QS-regulated violacein production of C. violaceum CV026 by 79% and QS-mediated pyocyanin production, proteases, and elastase activity of P. aeruginosa PA01 by 41%, 20%, and 32%, respectively. In addition, it can also destroy the biofilm formation and decrease QS gene expression of P. aeruginosa PA01. Molecular docking was further performed, and the obtained data indicated that this dipeptide blocks the effect of QS autoinducers through competitive binding to the same pocket of the receptor proteins. We expect this anti-QS cyclic dipeptide to be a potential pro-drug treating drug-resistant P. aeruginosa infections, and these findings could relieve the alarming problem of microbial resistance to antimicrobial drugs.

Quorum sensing (QS) can regulate the pathogenicity of bacteria and the production of some virulence factors. It is a promising target for screening to find anti-virulence agents in the coming post-antibiotics era. Cyclo (L-Trp-L-Ser), one variety of cyclic dipeptides (CDPs), isolated from a marine bacterium Rheinheimera aquimaris, exhibited anti-QS activity against Chromobacterium violaceum CV026 and Pseudomonas aeruginosa PAO1. Unlike the CDPs composed of phenylalanine or tyrosine, the anti-QS activity has been widely studied; however, cyclo (L-Trp-L-Ser) and derivatives, containing one tryptophan unit and one non-aromatic amino acid, have not been systematically explored. Herein, the cyclo (L-Trp-L-Ser) and seven derivatives were synthesized and evaluated. All tryptophane-contained CDPs were able to decrease the production of violacein in C.violaceum CV026 and predicted as binding within the same pocket of receptor protein CviR, but in lower binding energy compared with the natural ligand C₆HSL. As for P. aeruginosa PAO1, owning more complicated QS systems, these CDPs also exhibited inhibitory effects on pyocyanin production, swimming motility, biofilm formation, and adhesion. These investigations suggested a promising way to keep the tryptophan untouched and make modifications on the non-aromatic unit to increase the anti-QS activity and decrease the cytotoxicity, thus developing a novel CDP-based anti-virulence agent.

We investigated the influence of hydroxyl groups on the anti-quorum-sensing (anti-QS) and anti-biofilm activity of structurally similar cyclic dipeptides, namely cyclo(L-Pro-L-Tyr), cyclo(L-Hyp-L-Tyr), and cyclo(L-Pro-L-Phe), against Pseudomonas aeruginosa PAO1. Cyclo(L-Pro-L-Phe), lacking hydroxyl groups, displayed higher virulence factor inhibition and cytotoxicity, but showed less inhibitory ability in biofilm formation. Cyclo(L-Pro-L-Tyr) and cyclo(L-Hyp-L-Tyr) suppressed genes in both the las and rh l systems, whereas cyclo(L-Pro-L-Phe) mainly downregulated rhlI and pqsR expression. These cyclic dipeptides interacted with the QS-related protein LasR, with similar binding efficiency to the autoinducer 3OC12-HSL, except for cyclo(L-Pro-L-Phe) which had lower affinity. In addition, the introduction of hydroxyl groups significantly improved the self-assembly ability of these peptides. Both cyclo(L-Pro-L-Tyr) and cyclo(L-Hyp-L-Tyr) formed assembly particles at the highest tested concentration. The findings revealed the structure-function relationship of this kind of cyclic dipeptides and provided basis for our follow-up research in the design and modification of anti-QS compounds.