# Antibiofilm Potential of Natural Essential Oils
## 4 ABF Activity of EOs and Their Components  
### EOs as Antimicrobial Agents (Mechanisms of Action)  
EOs with strong and multi-faceted biological activity are considered as potential alternatives in clinical settings for the prevention, treatment, and control of infections transmitted by microbial BF. Most EOs modify the bacterial cell wall or membrane, causing the release of lipopolysaccharidesaccharides, which alters the ATP balance and influences pH fluctuations, protein synthesis, and internal cytoplasmic changes, such as coagulation of cytoplasmic material, DNA disruption, and QS inhibition. [PERSON] et al. [29] showed that _Tagetes minuta_ EO (TMEO) significantly inhibited virulence factors associated with QS sensing, such as pyocyanin production, protease production, and swarming motility. TMEO proved to be relatively effective in inhibiting BF even at a very low concentration (20 mg/mL), confirming that BF inhibition by EO is not based on killing bacterial cells but on the inhibition of the QS pathway. EOs also affect growth regulation, nutritional balance, and energy conversion in bacteria [23]. It should be noted that BF remains in a continuous balance between accumulation and degradation and is exposed to the action of various intra- and extracellular factors. The same EO can act synergistically or anti-synergistically, stimulating or inhibiting BF development [20]. Moreover, attention should be paid to the variability of the chemical composition and biological activity of EOs caused by various factors [30]. Microorganisms are destroyed by various compounds administered in different concentrations. The minimum inhibitory concentration (MIC) is the lowest concentration (in mg/mL) of an agent that inhibits the growth of a given strain of bacteria. The sub-minimum inhibitory concentration (sub-MIC) is defined as concentrations lower than the MIC values. Many studies have shown that sub-MICs of antibiotics can act as signal molecules and may alter their physicochemical characteristics and expression of bacterial virulence. Some scientific evidence confirms the high antibacterial and ABF activity of EOs (Table 1). The most commonly studied in this respect are clove, thyme, tea tree, oregano, and citrus fruit essential oils. They are highly effective against some viruses, whereas EO mixtures are effective against influenza and herpes simplex viruses [23]. _Thymus vulgaris_ EO (TEO) exhibits strong AM and ABF activity against _Haemophilus influenzae_, _H. paraninfluenzae_, and _P. aeruginosa_[30], which has also been confirmed by [PERSON] et al. [31], who demonstrated the potential of TEO and its components against QS and BF using a model strain _P. fluorescens_ KM121. When applied in concentrations slightly below the sub MIC of the tested TEO (20.0 \(\upmu\)L/mL), sub-MIC of carvacrol (2.0 \(\upmu\)L/mL), and sub-MIC of thymol (4.0 \(\upmu\)L/mL) caused significant reductions in the production of QS autoinducers (90%, 80%, and 78%, respectively), significantly inhibited bacterial motility, and reduced mRNA level of flagella gene (\(-\)0.672, \(-\)0.776, and \(-\)0.576 for TEO, carvacrol, and thymol, respectively). The authors posited TEO as a potential biocide against food-spoiling BF produced by _P. fluorescens_. THM-rich chemotype of TEO (TH \(>\) 50%) can be considered an effective AM and ABF agent in the in vitro experiments. It is worth emphasizing that this is the first report linking the anti-QS properties of TEO, thymol, and carvacrol with the inhibition of both the flagella gene expression and the BF formation capacity of the _P. fluorescens_ KM121 strain. In addition, [PERSON]\(\acute{\text{a}}\) et al. [32] considered TEO and _Origanum vulgare_ L. EO to be effective antifungal agents in the case of planktonic cells at MIC 0.4 mg/mL and potential therapeutic or preventive agents due to their capability to reduce pathogen resistance.