Pseudomonas aeruginosa is an opportunistic pathogen of plants and animals, which produces virulence factors in order to infect or colonize its eukaryotic hosts. Cyclodipeptides (CDPs) produced by P. aeruginosa exhibit cytotoxic properties toward human tumor cells. In this study, we evaluated the effect of a CDP mix, comprised of cyclo(L-Pro-L-Tyr), cyclo(L-Pro-L-Val), and cyclo(L-Pro-L-Phe) that were isolated from P. aeruginosa, on two human cancer cell lines. Our results demonstrated that the CDP mix promoted cell death in cultures of the HeLa cervical adenocarcinoma and Caco-2 colorectal adenocarcinoma cell lines in a dose-dependent manner, with a 50% inhibitory concentration (IC50) of 0.53 and 0.66 mg/mL, for HeLa and Caco-2 cells, respectively. Flow cytometric analysis, using annexin V and propidium iodide as apoptosis and necrosis indicators, respectively, clearly showed that HeLa and Caco-2 cells exhibited apoptotic characteristics when treated with the CDP mix at a concentration <0.001 mg/mL. IC50 values for apoptotic cells in HeLa and Caco-2 cells were 6.5 × 10−5 and 1.8 × 10−4 mg/mL, respectively. Our results indicate that an apoptotic pathway is involved in the inhibition of cell proliferation caused by the P. aeruginosa CDP mix.

Diverse molecules mediate cross-kingdom communication between bacteria and their eukaryotic partners and determine pathogenic or symbiotic relationships. N-acyl-L-homoserine lactone-dependent quorum-sensing signaling represses the biosynthesis of bacterial cyclodipeptides (CDPs) that act as auxin signal mimics in the host plant Arabidopsis thaliana. In this work, we performed bioinformatics, biochemical, and plant growth analyses to identify non-ribosomal peptide synthase (NRPS) proteins of Pseudomonas aeruginosa, which are involved in CDP synthesis. A reverse genetics strategy allowed the identification of the genes encoding putative multi-modular-NRPS (MM-NRPS). Mutations in these genes affected the synthesis of the CDPs cyclo(L-Pro-L-Val), cyclo(L-Pro-L-Leu), and cyclo(L-Pro-L-Tyr), while showing wild-type-like levels of virulence factors, such as violacein, elastase, and pyocyanin. When analyzing the bioactivity of purified, naturally produced CDPs, it was found that cyclo(L-Pro-L-Tyr) and cyclo(L-Pro-L-Val) were capable of antagonizing quorum-sensing-LasR (QS-LasR)-dependent signaling in a contrasting manner in the cell-free supernatants of the selected NRPS mutants, which showed QS induction. Using a bacteria-plant interaction system, we further show that the pvdJ, ambB, and pchE P. aeruginosa mutants failed to repress primary root growth, but improved root branching in A. thaliana seedlings. These results indicated that the CDP production in P. aeruginosa depended on the functional MM-NRPS, which influences quorum-sensing of bacteria and plays a role in root architecture remodeling.