Abstract
The prominent antibacterial and quorum sensing (QS) inhibition activity of aromatic plants can be used as a novel intervention strategy for attenuating bacterial pathogenicity. In the present work, a total of 29 chemical components were identified in the essential oil (EO) of Melaleuca bracteata leaves by gas chromatography-mass spectrometry (GC-MS). The principal component was methyleugenol, followed by methyl trans-cinnamate, with relative contents of 90.46% and 4.25%, respectively. Meanwhile, the antibacterial activity and the QS inhibitory activity of M. bracteata EO were first evaluated here. Antibacterial activity assay and MIC detection against seven pathogens (Dickeya dadantii Onc5, Staphylococcus aureus ATCC25933, Pseudomonas spp., Escherichia coli ATCC25922, Serratia marcescens MG1, Pseudomonas aeruginosa PAO1 and Chromobacterium violaceum ATCC31532) demonstrated that S. aureus ATCC25933 and S. marcescens MG1 had the higher sensitivity to M. bracteata EO, while P. aeruginosa PAO1 displayed the strongest resistance to M. bracteata EO. An anti-QS (anti-quorum sensing) assay revealed that at sub-minimal inhibitory concentrations (sub-MICs), M. bracteata EO strongly interfered with the phenotype, including violacein production, biofilm biomass, and swarming motility, as well as N-hexanoyl-L-homoserine lactone (C6-HSL) production (i.e., a signaling molecule in C. violaceum ATCC31532) of C. violaceum. Detection of C6-HSL indicated that M. bracteata EO was capable of not only inhibiting C6-HSL production in C. violaceum, but also degrading the C6-HSL. Importantly, changes of exogenous C6-HSL production in C. violaceum CV026 revealed a possible interaction between M. bracteata EO and a regulatory protein (cviR). Additionally, quantitative real-time polymerase chain reaction (RT-qPCR) analysis demonstrated that the expression of QS-related genes (cviI, cviR, vioABCDE, hmsNR, lasA-B, pilE1, pilE3, and hcnB) was significantly suppressed. Conclusively, these results indicated that M. bracteata EO can act as a potential antibacterial agent and QS inhibitor (QSI) against pathogens, preventing and controlling bacterial contamination.
Highlights
It is well documented that the large-scale use of chemical antimicrobials and antibiotics causes resistance in pathogenic microorganisms
We evaluated the antimicrobial activity of M. bracteata essential oil (EO) against seven pathogens, and its potential anti-quorum sensing (QS) activity was detected for the first time with C. violaceum ATCC31532
The results showed that M. bracteata EO had stronger inhibition against S. aureus ATCC25933 and S. marcescens MG1, and P. aeruginosa PAO1 was generally more resistant than the other organisms tested in this paper, which were well-matched with the minimum inhibitory concentrations (MICs) results
Summary
It is well documented that the large-scale use of chemical antimicrobials and antibiotics causes resistance in pathogenic microorganisms. It is not a wise solution that the use of these antimicrobials be kept to a minimum or until pathogens are halted. Natural antimicrobials from aromatic plants have attracted attention as alternatives to chemical ones [2]. Essential oils (EOs), the secondary metabolites of aromatic plants, are used to prevent bacterial infections due to their prominent antibacterial activity and quorum sensing (QS) inhibition. They are safe and nontoxic compounds, meeting the requirements for green antibacterial agents [3]
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