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Essential Oils (EOs) are mixtures of monoterpenes (MT), sesquiterpenes (ST) and/or phenylpropanoids(PhP) (Zygadlo 2011, Raut and Karuppayil 2014), with MT and ST being biosynthesized frommevalonate and/or deoxyxylulose (methyl D-erythritol 4-phosphate) pathways, while PhP come froma biosynthetic route of sikimic acid. The chemical diversity of EOs is high with aliphatic, cyclic andbicyclic structures, and is increased with the presence of different functional groups such as hydroxyls,carbonyls, carboxyls or esteres (Sell 2010, Zygadlo 2011). The great variety of bioactivities andthe many mechanisms of action or targets are associated with the large molecular diversity of EOs(Koroch et al. 2007, Franz 2010, Sadgrove et al. 2015). In fact, several botanical families produce EOs,for examples Zingiberaceae, Rutaceae, Poaceae, Apiaceae, Asteraceae, Geraniaceae, Lamiaceae,Lauraceae, Myrtaceae, Pinaceae, Piperaceae, Verbenaceae and Santalaceae, with the EOs beingbiosynthetized and stored in special structures, such as glandular hairs or esquizogene channels (Franz2010, Zygadlo 2011, Raut and Karuppayil 2014).The physician Arnald de Villanova was the first to obtain EOs by hydrodistillation. After EOscould be isolated, they had an impact on the medicine of the 13th century, and from this time arespecified in various European pharmacopoeia (Guenther 1948, Sadgrove et al. 2015). However, thefirst studies on EO composition were not carried out until the 19th century by the French chemistM.J. Dumas (Kubeczka 2010). The chemical composition of EOs depend on climatic conditions,type soil (Hussain et al. 2008, Oliva et al. 2010, Lopez et al. 2012, Ben El Hadj Ali et al. 2015),genetic material, chemotype (Jordán et al. 2013), culture techniques (Rioba et al. 2015), and theparts of the plants (leaves, fruits, flowers) where the EOs are obtained (Saïdana et al. 2008, Maggi etal. 2009, de Almeida et al. 2013, Alipour et al. 2014, Popovic et al. 2015, Villa-Ruano et al. 2015).Hydrodistillation, steam distillation, solvent extraction, water microwave assisted hydrodistillation,and more recently, supercritical fluid extraction with CO2, are the techniques most frequently usedto obtain EOs (Zygadlo 2011), but each method used has produced differences in the chemicalcomposition of the final product (Wenqiang et al. 2007, Okoh et al. 2010).Nowadays, there is a wealth of information available related to the biological activities of EOsand their components (Koroch et al. 2007, Lang and Buchbauer 2012, Nazzaro et al. 2013, Raut etal. 2014, Hassanien et al. 2015, Sadgrove et al. 2015), which is the starting point for the developmentof many commercial products (Brud 2010). At present, there is great interest in using EOs in theindustrial development of hygiene products, particularly due to their antimicrobial properties (Langand Buchbauer 2012, Dreger and Wielgus 2013, Calo et al. 2015, Prakash et al. 2015). In addition,since 2008, there has been an increase in bacterial diseases worldwide (Raut et al. 2014), with a rise ofabove 30% in the consumption of antibiotics, with monolactams, cephalosporins and fluoroquinolonesbeing the antibiotics most in demand (Van Boeckel et al. 2014, Gelband et al. 2015, Price et al.2015). This increase in the consumption of antibiotics has been accompanied by a correspondinggrowth in the bacterial population that is β-lactamase positive and not susceptible to third generationcephalosporins (Guzman-Blanco et al. 2014). In the context of this global problem MT and ST haveshown the ability to synergize and optimize the efficiency of antibiotics, for examples, geraniolincreased the performance of β-lactams, quinolones and chloramphenicol (Lorenzi et al. 2009), whilevancomycin was synergized by aromadendrene and 1,8-cineole on vancomycin-resistant enterococci.The susceptibility of Staphylococcus aureus to several antibiotics (ciprofloxacin, clindamycin,erythromycin, polymycin, gentamicin, tetracycline, oxacillin, methacillin and vancomycin) was alsoimproved by EOs of Cassia, Peru balsam, red thyme (Kavanaugh and Ribbeck 2012), nerolidol,farnesol and bisabolol (Wolska et al. 2012). Furthermore, cinnamaldehyde increased the potencyof clindamycin (Abreu et al. 2012), while carvacrol, thymol and eugenol enhanced the activity ofmultiple antibiotics (Langeveld et al. 2014). Hence, EOs and their components have been shown to besynergist agents of several antibiotics (Brud 2010, Abreu et al. 2012, Wolska et al. 2012, Langeveldet al. 2014, Price et al. 2015).The antibacterial activity of EOs, however, is not always attributed to its main component (Bakkaliet al. 2008), because a combination of different constituents may exhibit antagonistic, additive orsynergistic effects. Concerning this, the concentration values of phenols (thymol+carvacrol) insome Thymus sp. did not show a direct correlation with the values of antibacterial activity (Nikolićet al. 2014). Thus, the toxic effects of phenols included in an EO may not be directly related to theirconcentrations, because there are compounds in EOs with inhibitory activity. Therefore, it would beinteresting to be able to identify the compounds that generate opposing effects and thereby reduce theirpresence in the EOs. While it seems evident that the presence of phenols, such as eugenol or thymol,in an EO ensure a significant degree of antibacterial activity, a high concentration of hydrocarbonsin an EO is an indicator of poor antimicrobial activity (Tables 4.1 and 4.2). Moreover, aldehydes andketones are electrophiles that produce adducts with biological nucleophiles, hence strong toxic activitywas also shown for ketones and aldehydes such as menthone (Soković et al. 2010), thymoquinone(Harzallah et al. 2011), p-anisaldehyde, cinnamaldehyde and cuminaldehyde (Andrade-Ochoa et al.2015), the magnitude of antibacterial activity of carbonyl compounds was related to both partitioncoefficient (Log P) and electrophilicity (Schultz and Yarbrough 2004, Lopachin and Gavin 2014).The antibacterial capacity of EOs is also conditioned by the strain used in bioassays, with in general,Gram positive bacteria being more sensitive than Gram negative bacteria for the same EOs.We collected information concerning the antibacterial activity of EOs and their componentssince 2011 (Tables 4.1 and 4.2), with compounds or EOs having MIC values less than 30 μg/mLbeing categorized as showing excellent antibacterial activity. Those with MIC values between30 and 200 μg/mL had good antibacterial activity, while EOs with MIC values above 200 μg/L wereconsidered to be inactive.