*3.1. Composition of EOs and MICs*

The compositions of the EOs were previously determined by the producers, and detailed data on them were also reported in previous studies [32,34]. The major components of the EOs were terpinen-4-ol (33.5%) for marjoram, trans-cinnamaldehyde (93.1%) for cinnamon, and thymol (51.8%) for thyme.

MIC values ranged between 0.25 and 20 mg/mL and the bacterium most sensitive to the agents was found to be *E. coli* (Table 1). Among the substances investigated, cinnamon and the components cinnamaldehyde and thymol presented the lowest MIC values. The latter result supports the generally accepted finding that phenolics have the best antimicrobial activity among EO compounds [25,35,36].


**Table 1.** Minimum inhibitory concentration (MIC) of essential oils (EOs) and their components against the food-related bacteria investigated.

<sup>1</sup> Results from Kerekes et al. (2013). <sup>2</sup> Results from Kerekes et al. (2016).

#### *3.2. Anti-Biofilm-Forming E*ff*ect of EOs and Their Major Components*

#### 3.2.1. Monocultures

Table 2 summarizes the effects of the tested EOs on monoculture biofilms. For *E. coli*, each components and the thyme EO showed significant inhibitory effect when compared to the control. Cinnamon, despite its notable antibacterial effect, did not reduce the biofilm-forming ability of *E. coli*, but its main component and thyme did. Thymol exhibited the best effect against biofilm formation.


**Table 2.** Effect of essential oils and components (in MIC/2 concentration) on monoculture biofilm formation of food-related bacteria.

<sup>1</sup> Data are presented as mean ± standard deviation of six replicates. Values within a row with different letters are significantly different (*p* < 0.05). <sup>2</sup> Results from Kerekes et al. (2013).

All EOs and their investigated components had considerable anti-biofilm-forming effects on *L. monocytogenes*, as reflected in significant differences compared with the control. Cinnamon and trans-cinnamaldehyde were the best inhibitors and there were no significant differences between the anti-biofilm-forming capacity of the EOs and their main components (*p* > 0.05). Similar results could be seen in the case of *P. putida*, for which cinnamon, thyme, and their major components inhibited biofilm formation, but no significant differences were observed among these groups. Marjoram and terpinene-4-ol also had a strong effect against biofilm formation of all bacteria studied.

For *S. aureus* biofilms, all EOs and components tested proved to be effective against their formation. Absorbance of the treated samples differed significantly from that in the control (*p* < 0.001), but not from each other (Table 2).
