# Antibiofilm Potential of Natural Essential Oils
## 4 ABF Activity of EOs and Their Components
### EOs Compounds as Antimicrobial Agents
#### 4.2.2 Terpene Compounds  
Terpenes have been found to exert antimicrobial effects on antibiotic-sensitive and resistant bacteria, mainly through their ability to promote cell rupture and inhibit protein and DNA synthesis [47]. Linalool and eucalypt (Figure 2) have been reported to reduce BF-formation in both reference and clinical isolates, _S. aureus_ and _P. aeruginosa_[48,49]. [PERSON] et al. [50] demonstrated the anti-staphylococcal activity and antibacterial efficacy of eucalypt, in which the minimum inhibitory concentrations were equal to 7.23 mg/mL. This compound also enhanced membrane permeability, with a 5.36-fold increase in nucleic acid and protein leakage compared to untreated strains, and caused an increase in ROS generation along with a decrease in the activity of oxidative stress enzymes. The authors suggest that eucalypt has the ability to damage membrane integrity and induce ROS-mediated oxidative stress in MRSA cells, which may be helpful in reversing antibiotic  
Figure 2: BF-active EOs components: Phenolic compounds: thymol, eugenol, carvacrol, and terpene compounds: linalool, limonene, geranyl acetate, terpen-4-ol, eucalypt, \(\beta\)-caryophyllene, \(\alpha\)-pinene, p-cymene, and bornyl acetate.  
resistance. Other studies [15] indicate that limonene at a concentration of 0.1% mg/mL caused 41% inhibition of BF formation by _P. aeruginosa_ and 33% inhibition of AHL signaling. [PERSON] et al. [20] evaluated the modulation of BF growth exerted by 61 commercial EOs and found that limonene and other hydrophobic components (\(\alpha\)-pinene and p-cymene) could act as enhancers (either positive or negative) for other EO components. Linalool was found to be the component most likely involved in the strong inhibition of BF formation, followed by eucalypt, linalyl anthranilate, geranyl acetate, bornyl acetate, cis-geraniol, sabinene, and cis-3-pinanone. LIN and geranyl acetate were the major compounds of the palmarosa EO, which showed a strong BF-reducing potential (Figure 2). Furthermore, when combined mainly with eucalypt and geranyl acetate, linalool present in other EOs with a stronger BF-reducing capability probably exhibited synergistic action [20]. \(\beta\)-caryophyllene, \(\alpha\)-pinene, limonene, and p-cymene have been found important in diminishing BF formation, but could also negatively affect its inhibition. In turn, \(\beta\)-pinene and carvacrol have been demonstrated to only negatively modulate BF inhibition [20]. [PERSON] et al. [51] evaluated the AM and ABF activities of terpinen-4-ol against _S. aureus_ and _Klebsiella pneumoniae_, and their reference strains MTCC-740 and MTCC-109. The minimum inhibitory concentrations (MICs) of terpinen-4-ol against _S. aureus_ and _K. pneumoniae_ were determined at 50 and 25 mM, respectively. At the MIC level, terpinen-4-ol showed AM activity against both reference strains, whereas at the sub-MIC concentration (i.e., \(\forall\)2 MIC), it caused a significant reduction (at the level of 67.51 \(\pm\) 1.29%) in exopolysaccharideacharide (EPS) synthesis and a significant reduction in BF thickness. Based on these findings, the authors considered terpinen-4-ol as a potential drug candidate in combating chronic infections associated with BF and drug resistance.