Quorum Sensing in <i>Chromobacterium subtsugae</i> (Previously – <i>C</i>. <i>violaceum</i>) Is Inhibited by Gamma-Lactones, the Minor Components of Eucalyptus Leaf Extract

Abstract

Abstract—The goal of the study was to investigate the mechanism of quorum sensing (QS) inhibition in the model bacterium Chromobacterium subtsugae 026 (previously – C. violaceum 026) by Eucalyptus viminalis Labill leaf extract, and to identify the plant-derived components that provide this biological activity. The raw extract showed pronounced anti-QS activity within a broad concentrations range which were an order of magnitude lower than required for bacterial growth suppression (the MIC50/EC50 ratio of 43.6). Chemical analysis of the extract revealed six small plant-derived molecules: pyrogallol, 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-1, p-cymene, 4-((1E)-3-hydroxy-1-propenyl)-2-methoxyphenol, gallic and palmitic acids, whose inhibitory effect on QS in Chromobacterium spp. has previously been shown by in vitro experiments or predicted by in silico models. In addition, minor components γ-caprolactone and γ-octanolactone, structurally similar to acylated homoserine lactones, QS autoinducers in Chromobacterium spp., were found in the extract. Chemically synthesized analogs of these molecules showed anti-QS activity in the C. subtsugae 026 bioassay (ЕС50 = 354.2 and 145.6 µg/mL, respectively). Enrichment of the E. viminalis leaf extract with γ-caprolactone or γ-octanolactone significantly increased the anti-QS activity of these compositions, which was determined by isobolographic analysis as additive or superadditive effects.