Purpose: The aim of research was to test essential oil from oregano, as a new antimicrobial agent, against pathogenic bacteria using strong biofilm-producing strains of Listeria monocytogenes and Staphylococcus aureus.
Methods : Biofilm assays were performed, using 20 L. monocytogenes isolates plus the reference strain Listeria ATCC 7644 with crystal violet stain and 20 isolates of S. aureus plus the reference strains Staphylococcus epidermidis RP62A and S. aureus ATCC 25923 with safranin stain, to test biofilm production capabilities. The essential oil from Origanum vulgare subsp. hirtum was tested against the stronger biofilm-producing strains of both pathogens, in planktonic and sessile forms.
Results: MIC values of 800µg/ml were obtained in planktonic forms of both pathogens, but some strains of L.monocytogenes showed a MIC of 400µg/ml. Sub-MIC levels were used to analyze inhibition of biofilms. An average of 25% inhibition was obtained for L. monocytogenes in contrast to 68% in S. aureus. Each assay was performed in triplicate and repeated at least twice.
Significance: A slightly better activity of essential oil from oregano was observed against planktonic forms of L. monocytogenes compared to S. aureus; however, biofilm inhibition activity was more effective in S. aureus than in L. monocytogenes biofilms. New antimicrobial strategies, addressing both planktonic and sessile forms of food transmitted pathogenic bacteria, are important to counteracting the overgrowing diffusion of multidrug resistance.