title: The natural antimicrobial carvacrol inhibits quorum sensing in Chromobacterium violaceum and reduces bacterial biofilm formation at sub-lethal concentrations.
abstract: The formation of biofilm by bacteria confers resistance to biocides and presents problems in medical and veterinary clinical settings. Here we report the effect of carvacrol, one of the major antimicrobial components of oregano oil, on the formation of biofilms and its activity on existing biofilms. Assays were carried out in polystyrene microplates to observe (a) the effect of 0-0.8 mM carvacrol on the formation of biofilms by selected bacterial pathogens over 24 h and (b) the effect of 0-8 mM carvacrol on the stability of pre-formed biofilms. Carvacrol was able to inhibit the formation of biofilms of Chromobacterium violaceum ATCC 12472, Salmonella enterica subsp. Typhimurium DT104, and Staphylococcus aureus 0074, while it showed no effect on formation of Pseudomonas aeruginosa (field isolate) biofilms. This inhibitory effect of carvacrol was observed at sub-lethal concentrations (<0.5 mM) where no effect was seen on total bacterial numbers, indicating that carvacrol's bactericidal effect was not causing the observed inhibition of biofilm formation. In contrast, carvacrol had (up to 8 mM) very little or no activity against existing biofilms of the bacteria described, showing that formation of the biofilm also confers protection against this compound. Since quorum sensing is an essential part of biofilm formation, the effect of carvacrol on quorum sensing of C. violaceum was also studied. Sub-MIC concentrations of carvacrol reduced expression of cviI (a gene coding for the N-acyl-L-homoserine lactone synthase), production of violacein (pigmentation) and chitinase activity (both regulated by quorum sensing) at concentrations coinciding with carvacrol's inhibiting effect on biofilm formation. These results indicate that carvacrol's activity in inhibition of biofilm formation may be related to the disruption of quorum sensing.
authors.affiliation: Institute for Risk Assessment Sciences, Veterinary Public Health Division, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
authors.forename: Sara A
authors.initials: SA
authors.lastname: Burt
authors.affiliation: Institute for Risk Assessment Sciences, Veterinary Public Health Division, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
authors.forename: Victoria T A
authors.initials: VT
authors.lastname: Ojo-Fakunle
authors.affiliation: Institute for Risk Assessment Sciences, Veterinary Public Health Division, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
authors.forename: Jenifer
authors.initials: J
authors.lastname: Woertman
authors.affiliation: Department of Infectious Diseases and Immunology, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
authors.forename: Edwin J A
authors.initials: EJ
authors.lastname: Veldhuizen
mesh_terms: D000890:Anti-Infective Agents / Q000494:pharmacology*; D018441:Biofilms / Q000187:drug effects*; D001688:Biological Products / Q000494:pharmacology*; D002861:Chromobacterium / Q000187:drug effects* / Q000502:physiology*; D000081007:Cymenes; D020869:Gene Expression Profiling; D015964:Gene Expression Regulation, Bacterial / Q000187:drug effects; D008826:Microbial Sensitivity Tests; D050296:Microbial Viability / Q000187:drug effects; D039821:Monoterpenes / Q000494:pharmacology*; D053038:Quorum Sensing / Q000187:drug effects*; D012486:Salmonella typhimurium / Q000187:drug effects / Q000502:physiology; D013211:Staphylococcus aureus / Q000187:drug effects / Q000502:physiology
chemical_list: D000890:Anti-Infective Agents; D001688:Biological Products; D000081007:Cymenes; D039821:Monoterpenes; C073316:carvacrol