Sub-lethal concentrations of carvacrol (from oregano) inhibit bacterial quorum sensing and formation of biofilms

The formation of biofilm by bacteria confers resistance to biocides presents problems in both medical and veterinary clinical settings. Here we report the activity of carvacrol, one of the major antimicrobial components of oregano oil, on the formation of biofilms and it's activity on existing biofilms. Carvacrol was able to inhibit the formation of biofilms of Chromobacterium violaceum, Salmonella enterica subsp. Typhimurium DT104, and Staphylococcus aureus, while it showed no effect on formation of Pseudomonas aeruginosa biofilms. This inhibitory effect of Carvacrol was observed at sub-MIC concentrations (< 0.5 mM) where no effect was seen on total bacterial numbers, indicating that carvacrol's direct antimicrobial effect was not causing the observed inhibition of biofilm formation. In contrast carvacrol had (up to 8mM) no or very low 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 Chromobacterium violaceum was studied. Sub-MIC concentrations of carvacrol reduced quorum sensing at concentrations coinciding with carvacrol's inhibiting effect on biofilm formation (Fig 1). These results indicate that carvacrol's activity in inhibition of biofilm formation could be linked to its activity on quorum sensing.

Fig. 1: Carvacrol inhibits quorum sensing (QS) in C. violaceum at the same concentrations at which it deters biofilms.