**3. Essential oils**

Several possible strategies could treat infections associated with biofilms: substances capable of destroying the biofilm matrix (e.g., dispersion B), substances capable of destroying resistant cells, quorum-quenching enzymes that interfere with the quorum sensing phenomenon, substances that cause self-destruction of the biofilm, and then, in particular, strategies to strengthen the action of antimicrobials. The treatment of biofilms with antibiotics often causes only partial killing, allowing the surviving bacteria, present in the depth of the biofilm, to act as a true nucleus of propulsion for the spread of the infection after the interruption of the antibiotic therapy. Antibiotics can be inactivated by the production of specific enzymes within the biofilm. In some extreme cases, even the sessile population must not be surgically removed from the body. Another aspect to take into consideration is the age of biofilm: the younger is the biofilm, the easier is its eradication.

The need to identify substances/active ingredients able to replace synthetic drugs in the fight against pathogens, in particular against those more resistant to conventional treatments, also directed research toward (or better to say, to the rediscovery of) the "natural world," source of bioactive compounds used by traditional medicine since ancient times. Moreover, these substances have always exhibited a great spectrum of action that can be considered of great benefit, also due to the chemical structural differences of the active compounds. In such context, substances of vegetal origin, such as essential oils, have always been successfully used in traditional medicine and stimulated, practically always but particularly in recent decades, the scientific world to discover and identify substances, intended as a mixture or as single components that are able to fight pathogenic microorganisms. From this point of view, the study of

plants is very interesting and offers many interesting ideas and results: the same kind of plant can provide a pool of substances with a wide and very diverse spectrum of action [49]. Within the same genus, in fact, there are species with a different chemical composition, which therefore can provide bioactive substances (hydroalcoholic fraction or essential oils) different for the qualitative and quantitative profile. Moreover, the same plant species can diversify and present a different chemical composition depending also on the environmental and climatic conditions in which it grows, the stage of maturity, and method of extraction. Essential oils are substances that appear liquid, aromatic, and limpid and are obtained from different portions of plants through different extraction procedures, such as crushing, distillation, fermentation, the so-called enfleurage, or the use of organic solvents. The International Organization for Standardization (ISO) (ISO/D1S9235.2) defines an essential oil as a product made by distillation with either water or steam or by mechanical processing or by dry distillation of natural materials. About 300 essential oils, within the more than 3000 known types, are available on the market. The antimicrobial and antifungal properties of essential oils have been known since ancient times; however, the first "scientific" demonstrations of this activity date back only to the 1950s, when both Guenther [50] and Boyle [51] described in detail the activity of natural preservatives exhibited by different essential oils derived from plants and spices. The increase in interest from the economic world has meant that, by increasing the research on these substances, other properties were discovered [52], among which, for example, those antivirals [53, 54]. Chemical characterization of essential oils, conducted through chromatographic approaches (GC and GC/MS), has allowed obtaining detailed information on their composition. Essential oils are generally formed by volatile substances, also called volatile organic compounds (VOCs), molecules characterized by a high lipophilicity and a high vapor pressure. Within each essential oil, one can identify one or more quantitatively more abundant molecules and a series (more or less numerous) of other molecules, sometimes present only in traces. Moreover, within the essential oils, other types of molecules can be identified, such as phenolic compounds [55], alkaloids, saponins, and sesquiterpenes, which contribute to the antimicrobial activity of the oil. Further than other properties, EOs protect the plant against some pathogenic microorganisms. Through their smell, they are capable of exercising repulsive action against insect or, concurrently, to attract others to favor the dispersion of pollens and seeds. The same smell can also negatively affect the appetite drive of some herbivores. Some essential oils are reported to be very effective allelopathic agents. Thus, EOs can play a role in mediating the interactions of plants with the environment in a way that, although improperly, we could almost define as similar (however in a certain way opposite) to that exhibited by QSMs that allow these last to communicate in microorganisms with each other and with the environment. Essential oils can be classified according to the chemical constituent contained in greater concentration. Following such criterion, we have, among others:


**173**

sition, so to best argue about [65, 66].

*Essential Oils and Microbial Communication DOI: http://dx.doi.org/10.5772/intechopen.85638*

disulfide in *Allium*)

acetate in *Lavandula angustifolia*)

*Artemisia*, *Thuja*, and in *Salvia officinalis*)

*lus*, carvacrol in *Satureja* and in other Labiatae)

The composition and the relative differences among the EOs lead to different biological activities that EOs can exhibit [54]. This also means that some species of plants, which exhibit different chemotypes, are characterized by a different composition and rate among the EO components too, to lead a change in EO biological properties. Thus, the final effect of an EO against a specific pathogen can give rise from a synergistic mechanism of its components or from just a unique compound that, although present in less percentage, can enhance the antimicrobial activity of the entire EO. In general, plant EOs and their components have a broad spectrum of inhibitory activities both against Gram-positive and Gram-negative pathogens [56, 57]. Citronellol can exert a broad inhibitory activity against the formation of biofilm. In fact, it acts against the planktonic forms of different Gram-positive (*Listeria monocytogenes*, *Staphylococcus aureus*, *Staphylococcus epidermidis*) and Gramnegative species (*E.coli*, *Pseudomonas aeruginosa*), inhibiting their capability to form biofilm when used at percentage ranging between 3.5% and 7% wt [58, 59]. However, the antibacterial effectiveness can vary depending not only on the EO but also on the bacteria. Therefore, some EOs, such as those of sandalwood (*Santalum album*), manuka (*Leptospermum scoparium*), and vetiver (*Chrysopogon zizanioides*), can act against some Gram-positive bacteria, but result ineffective against Gram-negative [60, 61]. Concurrently, sa microorganism can be more or less to the activity of different EOs. Thus, *Cymbopogon citratus* (lemon grass), *Syzygium aromaticum* (clove), and *Laurus nobilis* (bay laurel) EOs as well as *Thymus vulgaris* (thyme), *Rosmarinus officinalis* (rosemary), and *Mentha piperita* (peppermint) ones are capable to act against *St. aureus* at concentrations of ≤0.05%. On the other hand, *Ocimum basilicum* (basil) and *Eucalyptus globulus* (eucalyptus) EOs exhibit the same level of activity against this microorganism only if used at 1% concentration [60–62]. Different types of cardamom EOs affect differently the growth, the Qs, and the capability to form biofilm of several bacteria [63]. Thyme, *Origanum vulgare* (oregano), *Melaleuca alternifolia* (tea tree), *Cinnamomum verum* (cinnamon), lemon grass, bay laurel, *Backhousia citriodora* (lemon myrtle), clove, and *Aniba rosaeodora* (rosewood) EOs result the most active antimicrobials, at concentration and MICs also less than 1% [60, 64]. Most of these EOs, in particular bay laurel, clove, lemon grass, oregano, and thyme oils, inhibit growth of *E.coli* at concentrations of 0.02, 0.04, 0.06, 0.05, and 0.05%, respectively. In few cases, a major constituent molecule could exhibit a more effective activity compared to the EO. For example, carvacrol and eugenol, present in clove EO or terpinen-4-ol, which is the main component of tea tree EO, can show greater efficacy than the relative oil. This highlights also that, in the study of a biological activity of an EO, it is important to provide also the chemical compo-

• EOs with high content of ketones (carvone in *Carum carvi*, thujone in

• EOs with a predominant amount of phenols (eugenol in *Dianthus caryophyl-*

• EOs that have a prevalent content of sulfured compounds (bisulfide, allyl

• EOs with a prevalent content in esters and alcohols (linalool and linalyl

• EOs having predominantly peroxides (ascaridol in *Chenopodium*)

*Essential Oils - Oils of Nature*

plants is very interesting and offers many interesting ideas and results: the same kind of plant can provide a pool of substances with a wide and very diverse spectrum of action [49]. Within the same genus, in fact, there are species with a different chemical composition, which therefore can provide bioactive substances (hydroalcoholic fraction or essential oils) different for the qualitative and quantitative profile. Moreover, the same plant species can diversify and present a different chemical composition depending also on the environmental and climatic conditions in which it grows, the stage of maturity, and method of extraction. Essential oils are substances that appear liquid, aromatic, and limpid and are obtained from different portions of plants through different extraction procedures, such as crushing, distillation, fermentation, the so-called enfleurage, or the use of organic solvents. The International Organization for Standardization (ISO) (ISO/D1S9235.2) defines an essential oil as a product made by distillation with either water or steam or by mechanical processing or by dry distillation of natural materials. About 300 essential oils, within the more than 3000 known types, are available on the market. The antimicrobial and antifungal properties of essential oils have been known since ancient times; however, the first "scientific" demonstrations of this activity date back only to the 1950s, when both Guenther [50] and Boyle [51] described in detail the activity of natural preservatives exhibited by different essential oils derived from plants and spices. The increase in interest from the economic world has meant that, by increasing the research on these substances, other properties were discovered [52], among which, for example, those antivirals [53, 54]. Chemical characterization of essential oils, conducted through chromatographic approaches (GC and GC/MS), has allowed obtaining detailed information on their composition. Essential oils are generally formed by volatile substances, also called volatile organic compounds (VOCs), molecules characterized by a high lipophilicity and a high vapor pressure. Within each essential oil, one can identify one or more quantitatively more abundant molecules and a series (more or less numerous) of other molecules, sometimes present only in traces. Moreover, within the essential oils, other types of molecules can be identified, such as phenolic compounds [55], alkaloids, saponins, and sesquiterpenes, which contribute to the antimicrobial activity of the oil. Further than other properties, EOs protect the plant against some pathogenic microorganisms. Through their smell, they are capable of exercising repulsive action against insect or, concurrently, to attract others to favor the dispersion of pollens and seeds. The same smell can also negatively affect the appetite drive of some herbivores. Some essential oils are reported to be very effective allelopathic agents. Thus, EOs can play a role in mediating the interactions of plants with the environment in a way that, although improperly, we could almost define as similar (however in a certain way opposite) to that exhibited by QSMs that allow these last to communicate in microorganisms with each other and with the environment. Essential oils can be classified according to the chemical constituent contained in greater concentration. Following

• EOs with a predominance of mono or sesquiterpene hydrocarbons (e.g.,

*Cinnamomum verum*, benzoic aldehyde in *Prunus dulcis* var. *dulcis* and in

• EOs containing predominantly alcohols (geraniol in *Geranium*, santalol in

• EOs with a prevalence of aldehydes (e.g., cinnamic aldehyde in

*Santalum album*, linalool in *Coriandrum sativum*)

**172**

such criterion, we have, among others:

*Citrus*, *Juniperus* L)

var. *amara*)


The composition and the relative differences among the EOs lead to different biological activities that EOs can exhibit [54]. This also means that some species of plants, which exhibit different chemotypes, are characterized by a different composition and rate among the EO components too, to lead a change in EO biological properties. Thus, the final effect of an EO against a specific pathogen can give rise from a synergistic mechanism of its components or from just a unique compound that, although present in less percentage, can enhance the antimicrobial activity of the entire EO. In general, plant EOs and their components have a broad spectrum of inhibitory activities both against Gram-positive and Gram-negative pathogens [56, 57]. Citronellol can exert a broad inhibitory activity against the formation of biofilm. In fact, it acts against the planktonic forms of different Gram-positive (*Listeria monocytogenes*, *Staphylococcus aureus*, *Staphylococcus epidermidis*) and Gramnegative species (*E.coli*, *Pseudomonas aeruginosa*), inhibiting their capability to form biofilm when used at percentage ranging between 3.5% and 7% wt [58, 59]. However, the antibacterial effectiveness can vary depending not only on the EO but also on the bacteria. Therefore, some EOs, such as those of sandalwood (*Santalum album*), manuka (*Leptospermum scoparium*), and vetiver (*Chrysopogon zizanioides*), can act against some Gram-positive bacteria, but result ineffective against Gram-negative [60, 61]. Concurrently, sa microorganism can be more or less to the activity of different EOs. Thus, *Cymbopogon citratus* (lemon grass), *Syzygium aromaticum* (clove), and *Laurus nobilis* (bay laurel) EOs as well as *Thymus vulgaris* (thyme), *Rosmarinus officinalis* (rosemary), and *Mentha piperita* (peppermint) ones are capable to act against *St. aureus* at concentrations of ≤0.05%. On the other hand, *Ocimum basilicum* (basil) and *Eucalyptus globulus* (eucalyptus) EOs exhibit the same level of activity against this microorganism only if used at 1% concentration [60–62]. Different types of cardamom EOs affect differently the growth, the Qs, and the capability to form biofilm of several bacteria [63]. Thyme, *Origanum vulgare* (oregano), *Melaleuca alternifolia* (tea tree), *Cinnamomum verum* (cinnamon), lemon grass, bay laurel, *Backhousia citriodora* (lemon myrtle), clove, and *Aniba rosaeodora* (rosewood) EOs result the most active antimicrobials, at concentration and MICs also less than 1% [60, 64]. Most of these EOs, in particular bay laurel, clove, lemon grass, oregano, and thyme oils, inhibit growth of *E.coli* at concentrations of 0.02, 0.04, 0.06, 0.05, and 0.05%, respectively. In few cases, a major constituent molecule could exhibit a more effective activity compared to the EO. For example, carvacrol and eugenol, present in clove EO or terpinen-4-ol, which is the main component of tea tree EO, can show greater efficacy than the relative oil. This highlights also that, in the study of a biological activity of an EO, it is important to provide also the chemical composition, so to best argue about [65, 66].
