Dodecanoyl-L-homoserine lactone

Certain bacteria can coordinate group behaviors via a chemical communication system known as quorum sensing (QS). Gram-negative bacteria typically use N-acyl l-homoserine lactone (AHL) signals and their cognate intracellular LuxR-type receptors for QS. The opportunistic pathogen Pseudomonas aeruginosa has a relatively complex QS circuit in which two of its LuxR-type receptors, LasR and QscR, are activated by the same natural signal, N-(3-oxo)-dodecanoyl l-homoserine lactone. Intriguingly, once active, LasR activates virulence pathways in P. aeruginosa, while activated QscR can inactivate LasR and thus repress virulence. We have a limited understanding of the structural features of AHLs that engender either agonistic activity in both receptors or receptor-selective activity. Compounds with the latter activity profile could prove especially useful tools to tease out the roles of these two receptors in virulence regulation. A small collection of AHL analogs was assembled and screened in cell-based reporter assays for activity in both LasR and QscR. We identified several structural motifs that bias ligand activation towards each of the two receptors. These findings will inform the development of new synthetic ligands for LasR and QscR with improved potencies and selectivities.

In contrast to the plethora of antibacterial agents, only a handful of antifungals are currently available to treat Candida albicans biofilm-associated infections. Additional novel antibiofilm strategies to eliminate C. albicans biofilm infections are needed. This study aims to improve the efficacy of a widely used azole, fluconazole by co-delivering it with a Pseudomonas aeruginosa quorum sensing molecule (QSM), N-3-oxo-dodecanoyl-L-homoserine lactone (C12AHL) in a liposomal formulation. C12AHL is known to inhibit C. albicans' morphological transition and biofilm formation. Four different formulations of liposomes with fluconazole (L-F), with C12AHL (L-H), with fluconazole and C12AHL (L-HF), and a drug-free control (L-C) were prepared using a thin-film hydration followed by extrusion method, and characterised. The effect of liposomes on colonising (90 min-24 h) and preformed (24 h) C. albicans biofilms were assessed using a standard biofilm assay. Biofilm viability (XTT reduction assay), biomass (Safranin-O staining) and architecture (confocal laser scanning microscopy, CLSM) were determined. Similar efficiencies of fluconazole entrapment were noticed in L-HF and L-F (11.74% vs 10.2%), however, L-HF released greater quantities of fluconazole compared to L-F during 24 h (4.27% vs 0.97%, P < 0.05). The entrapment and release of C12AHL was similar for L-H and L-HF liposomes (33.3% vs 33% and 88.9% vs 92.3% respectively). L-HF treated colonising, and preformed biofilms exhibited >80%, and 60% reduction in their respective viabilities at a fluconazole concentration as low as 5.5 µg/mL compared to 12% and 36%, respective reductions observed in L-F treated biofilms (P < 0.05). CLSM confirmed biofilm disruption, lack of hyphae, and reduction in biomass when treated with L-HF compared to other liposomal preparations. Liposomal co-delivery of C12AHL and fluconazole appears to suppress C. albicans biofilms through efficacious disruption of the biofilm, killing of constituent yeasts, and diminishing their virulence at a significantly lower antifungal dose. Therefore, liposomal co-formulation of C12AHL and fluconazole appears to be a promising approach to improve the efficacy of this common triazole against biofilm-mediated candidal infections.

We report the production and degradation of quorum sensing N-acyl-homoserine lactones by bacteria isolated from Malaysian montane forest soil. Phylogenetic analysis indicated that these isolates clustered closely to the genera of Arthrobacter, Bacillus and Pseudomonas. Quorum quenching activity was detected in six isolates of these three genera by using a series of bioassays and rapid resolution liquid chromatography analysis. Biosensor screening and high resolution liquid chromatography-mass spectrometry analysis revealed the production of N-dodecanoyl-L-homoserine lactone (C12-HSL) by Pseudomonas frederiksbergensis (isolate BT9). In addition to degradation of a wide range of N-acyl-homoserine lactones, Arthrobacter and Pseudomonas spp. also degraded p-coumaroyl-homoserine lactone. To the best of our knowledge, this is the first documentation of Arthrobacter and Pseudomonas spp. capable of degrading p-coumaroyl-homoserine lactone and the production of C12-HSL by P. frederiksbergensis.