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.
Highlights
The formation of biofilms by bacteria confers resistance to biocides and represents a serious problem in medical and veterinary clinical settings [1]
Bacterial biofilm activity is regulated by quorum sensing (QS), a system used by both Gram positive and Gram negative bacteria based on the secretion and/ or detection of external signal molecules; QS influences motility and the expression of flagella [2,3]
Sixteen hour bacterial cultures were adjusted to OD (590 nm) 0.02 and aliquots of 50 ml were added to the test wells, leaving one column for sterile broth only
Summary
The formation of biofilms by bacteria confers resistance to biocides and represents a serious problem in medical and veterinary clinical settings [1]. Some essential oils have been shown to inhibit QS in bacteria and this has been proposed as a mechanism of their antibacterial activity [5]. Previous studies have shown that sub-lethal concentrations of carvacrol, a component of oregano essential oil, reduce motility and invasiveness in bacteria [6,7]. This antimicrobial compound can reduce biofilm formation in staphylococci and Salmonella strains [8,9]. The mechanism by which carvacrol inhibits biofilm accretion has not yet been fully established
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