Activity of Origanum vulgare L. Essential Oil and Carvacrol against Planktonic and Biofilm Cells of Staphylococcus aureus Strains Isolated from Hospital Food Service Environments

Introduction: The ability of Staphylococcus aureus to form biofilms enhances it survival in food processing and food service environments. The increased pressure for safe and natural foods has motivated research of the antimicrobial efficacy of natural compounds, such as Origanum vulgare L. essential oil (OVEO) and its major constituent carvacrol (CAR). There is a lack of information regarding the effects of these substances against S. aureus cells grown in biofilms.

Purpose: The aim of this study was to determine the minimum inhibitory concentration (MIC) of OVEO and CAR against planktonic and biofilm S. aureus cells.

Methods: Ten strong biofilm producing strains of S. aureus were previously isolated from hospital food service environments. MIC values of OVEO and CAR were determined in triplicate by microdilution in Tryptone Soya Broth with glucose 1g/100ml, using a microplate incubator/reader (OD 490_nm). OVEO and CAR were used in concentrations from 0.3 to 40 μl/ml. Biofilms were produced by culturing cells for 24 h before addition of antimicrobial substances. S. epidermidis ATTCC 12228 and S. aureus ATTCC 25923 were used as negative and positive control (strong producer) for biofilm formation, respectively

Results: MIC value of OVEO was 5 µl/ml against planktonic cells for eleven strains (n = 9; and control strains) and 10 µl/ml for one S. aureus strain. MIC value of CAR ranged from 2.5 µl/ml (n = 8; control negative strain) to 5 µl/ml (n = 2; control positive strain) against planktonic S. aureus cells. The MIC values of OVEO (10 µl/ml or 20 µl/ml) and CAR (5 µl/ml or 10 µl/ml) were always doubled for S. aureus cells in the biofilm state. 

Significance: OVEO and CAR are effective in inhibiting growth of cells of S. aureus in planktonic and biofilms states, and cells in biofilms were more resistant to tested antimicrobials.