title: The impact of plant volatiles on bacterial quorum sensing.
abstract: Studies describing the use of essential oil constituents as antimicrobial agents have steadily increased; however, some phyto-constituents are often overlooked due to unfavourable minimum inhibitory concentration (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 to >5 mg ml(-1). Twenty-two compounds displayed varying levels of QS inhibitory activity with zones of violacein inhibition ranging from 9 to 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. α-Terpineol and cis-3-nonen-1-ol exhibited >90% violacein inhibition, suggesting their potential as QS 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.
  
  
  CONCLUSIONS
  Antimicrobial and antiquorum sensing (QS) properties of 29 common essential oil compounds were evaluated. Interruption of QS may lead to the development of therapeutic, antivirulence agents to control disease-causing pathogens which are preferable over antimicrobial agents as the latter drives selection pressure on microbial communities to acquire resistance. Twenty-two compounds inhibited QS, while seven promoted the QS to a variable extent in Chromobacterium violaceum and Pseudomonas aeruginosa. Preliminary results suggest that QS-inhibitory compounds of natural origin may inspire the formulation of new generation of antimicrobial agents to control infectious pathogens.
authors.affiliation: Department of Pharmaceutical Sciences, Tshwane University of Technology, Pretoria, South Africa.
authors.forename: A
authors.initials: A
authors.lastname: Ahmad
authors.forename: A M
authors.initials: AM
authors.lastname: Viljoen
authors.forename: H Y
authors.initials: HY
authors.lastname: Chenia
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002861:Chromobacterium / Q000187:drug effects* / Q000472:pathogenicity / Q000502:physiology; D052978:Disk Diffusion Antimicrobial Tests; D007211:Indoles / Q000378:metabolism; D008826:Microbial Sensitivity Tests; D009822:Oils, Volatile / Q000494:pharmacology*; D010944:Plants / Q000737:chemistry; D011550:Pseudomonas aeruginosa / Q000187:drug effects* / Q000472:pathogenicity / Q000502:physiology; D011710:Pyocyanine / Q000096:biosynthesis; D053038:Quorum Sensing / Q000187:drug effects*; D014774:Virulence; D037521:Virulence Factors / Q000096:biosynthesis
chemical_list: D000900:Anti-Bacterial Agents; D007211:Indoles; D009822:Oils, Volatile; D037521:Virulence Factors; D011710:Pyocyanine; C063155:violacein
keywords: Chromobacterium violaceum; Pseudomonas aeruginosa; monoterpenes; natural compounds; pyocyanin; quorum sensing; violacein