Effects of oregano essential oil and carvacrol on biofilms of Staphylococcus aureus from food-contact surfaces

Abstract

This study characterized the biofilm-forming ability of Staphylococcus aureus from food-contact surfaces and evaluated the effects of the essential oil from Origanum vulgare L. (oregano; OVEO) and carvacrol (CAR) against the planktonic and sessile cells of selected isolates, as well as their effects on biofilm formation on stainless steel over time. In microtiter plate (MtP) assay, 97.2% (70/72) of the isolates were considered biofilm-producers, while in assays using Congo Red agar, 88.9% (64/72) of the isolates presented biofilm-forming ability. The detection of icaA, icaB, icaC and icaD genes varied among the 72 S. aureus isolates; however, all isolates (n = 39) classified as strong biofilm producers in Mtp assay were positive for icaA and icaD. For eight out the ten tested isolates, the minimum inhibitory concentration (MIC) of OVEO and CAR was one-fold higher against sessile cells than planktonic cells. MICs against sessile cells were effective to eradicate pre-formed biofilms on polystyrene. Sub-MIC doses of 2.5 and 1.25 μL/mL of OVEO and 1.25, 0.62 and 0.31 μL/mL of CAR in meat broth decreased the sessile cell counts of isolates on stainless steel surfaces over time (360 h). These effects varied with the OVEO and CAR concentrations and exposure times. For all tested isolates, an increase in sessile cell counts was observed after 288 h exposure to 0.625 μL/mL of OVEO. The results showed high incidence of biofilm producers among S. aureus isolated from food-contact surfaces and efficacy of OVEO and CAR to inhibit planktonic and sessile cells. The concentrations of these antimicrobials used to control S. aureus biofilms should be cautiously considered because of possible inductive biofilm production effects.