*Article* **Anti-Biofilm E**ff**ect of Selected Essential Oils and Main Components on Mono- and Polymicrobic Bacterial Cultures**

**Erika Beáta Kerekes 1,\*, Anita Vidács 2, Miklós Takó <sup>1</sup> , Tamás Petkovits 1, Csaba Vágvölgyi <sup>1</sup> , Györgyi Horváth 3, Viktória Lilla Balázs <sup>3</sup> and Judit Krisch <sup>2</sup>**


Received: 26 July 2019; Accepted: 10 September 2019; Published: 12 September 2019

**Abstract:** Biofilms are surface-associated microbial communities resistant to sanitizers and antimicrobials. Various interactions that can contribute to increased resistance occur between the populations in biofilms. These relationships are the focus of a range of studies dealing with biofilm-associated infections and food spoilage. The present study investigated the effects of cinnamon (*Cinnamomum zeylanicum*), marjoram (*Origanum majorana*), and thyme (*Thymus vulgaris*) essential oils (EOs) and their main components, i.e., trans-cinnamaldehyde, terpinen-4-ol, and thymol, respectively, on single- and dual-species biofilms of *Escherichia coli*, *Listeria monocytogenes*, *Pseudomonas putida,* and *Staphylococcus aureus*. In dual-species biofilms, *L. monocytogenes* was paired with each of the other three bacteria. Minimum inhibitory concentration (MIC) values for the individual bacteria ranged between 0.25 and 20 mg/mL, and trans-cinnamaldehyde and cinnamon showed the highest growth inhibitory effect. Single-species biofilms of *L. monocytogenes*, *P. putida,* and *S. aureus* were inhibited by the tested EOs and their components at sub-lethal concentrations. Scanning electron microscopy images showed that the three-dimensional structure of mature biofilms embedded in the exopolysaccharide matrix disappeared or was limited to micro-colonies with a simplified structure. In most dual-species biofilms, to eliminate living cells from the matrix, concentrations exceeding the MIC determined for individual bacteria were required.

**Keywords:** antibacterial activity; biofilm; polymicrobial biofilm; essential oil; food spoilage
