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Ahmad, A., Viljoen, A.M., Chenai, H.Y. 2014. The impact of plant volatiles on bacterial quorum sensing. Letters in Applied Microbiology 60: 8-19.

Studies describing the use of essential oil constituents as antimicrobial agents have steadily increased; however, some phyto-constituents are often over-looked due to unfavourable MIC values. Virulence depends on transcriptional factors which are regulated by cell-to-cell communication called quorum sensing (QS). This study was undertaken to evaluate the antimicrobial and anti-QS properties of 29 compounds commonly found in essential oils using two bioreporter strains. QS-inhibitory activity was assessed qualitatively by agar-diffusion and quantitatively by spectrophotometric assays. MICs of all the tested compounds ranged from 0.032 – >5 mg ml-1. Twenty-two compounds displayed varying levels of quorum sensing inhibitory activity with zones of violacein inhibition ranging from 9 – 16 mm. Majority of tested molecules inhibited violacein and pyocyanin production in Chromobacterium violaceum and Pseudomonas aeruginosa while seven compounds increased violacein and pyocyanin production. Interestingly, it was observed that the (+)-enantiomers of carvone, limonene and borneol increased violacein and pyocyanin production while their levorotary analogues inhibited this production. a-Terpineol and cis-3-nonen-1-ol, exhibited >90% violacein inhibition, suggesting their potential as quorum sensing inhibitors. This preliminary study indicates that plant volatiles have the potential to impede or promote bacterial communication and further studies need to be undertaken to explore the contribution of structural analogues and stereochemistry of molecules in this process.

bacteria purple - TUT, Pretoria, Gauteng, South Africa [2010 ©]