The potential of plant volatiles in blocking bacterial communication

In the last two decades, studies describing the use of essential oils and their volatile constituents as antimicrobial agents have steadily increased. However, additional number of essential oils and their compounds are often over-looked and dismissed due to their high MIC values. Virulance depends on transcriptional factors which are regulated by cell to cell communication called quorum sensing (QS). Interruption of QS may lead to the development of therapeutic agents to control disease-causing pathogens. In addition, it is preferential to develop anti-virulent agents rather than antimicrobial agents as the latter results in selection pressure on microbial communities to acquire resistance. This study was undertaken to evaluate the anti-microbial as well as anti-QS properties of 25 mono- and sesquiterpenes commonly found in essential oils using a bioreporter strain Chromobacterium violaceum (ATCC 12472). Minimum inhibitory concentrations were determined using the CLSI micro-dilution assay. QS inhibitory activity was assessed qualitatively by the agar diffusion assay and quantitatively using a spectrophotometry assay. MICs of all the tested compounds ranged between 0.032 mg/ml and 1 mg/ml. Out of 25 tested compounds, 22 inhibited violacein production while three (α-pinene, β-pinene and boroneyl acetate) increased violacein production. All the 22 compounds inhibiting QS showed varying levels of activity with zones of violacein inhibition ranging from 9 to 16 mm. α-Terpineol and nonen-1-ol at 0.125 mg/ml exhibit the most promising QS-inhibition (> 90% violacein inhibition). At all the quorum sensing inhibitory concentrations, no significant reduction (or increase) in bacterial growth was observed. This preliminary study indicates that plant volatiles have the potential to inhibit bacterial communication and further studies need to be undertaken to explore the contribution of structural analogues and the role of specific moieties in this process.