**3. Effects of essential oils as antibacterial agents**

Various studies showed that essential oils also have antibacterial properties against a wide range of bacterial strain such as *Listeria monocytogenes*, *L. innocua*, *Salmonella typhimurium*, *Escherichia coli*, *Shigella dysenteria*, *Bacillus cereus*, *Staphylococcus aureus*, and *Salmonella typhimurium* [42]. Direct inhibition correlation due to presence of thymol and carvacrol in the essential oils of thyme and oregano can inhibit some pathogenic bacterial strains such as *E. coli*, *Salmonella enteritidis*, *Salmonella choleraesuis*, and *Salmonella typhimurium* [43]. The same correlation was also confirmed for oils rich in carvacrol alone. Eugenol and carvacrol showed an inhibitory effect against the growth of four strains of *Escherichia coli* O157:H7 and *Listeria monocytogenes* [44]. The carvacrol showed strong antibacterial activity due to presence of phenolic hydroxyl group. Some essential

**159**

*Essential Oil as Antimicrobial Agents: Efficacy, Stability, and Safety Issues for Food Application*

oils demonstrated antibacterial activity against zoonotic enteropathogens including *Salmonella spp*., *Escherichia coli* O157, *Campylobacter jejuni*, and *Clostridium perfringens*. Thus, these oils could possibly be used as an alternative to antibiotics in animal feed [45]. Essential oils with high concentrations of thymol and carvacrol e.g., oregano, savory and thyme, usually inhibit Gram-positive more than Gram-negative pathogenic bacteria. However the essential oil of *Achillea clavennae* exhibited strong antibacterial activity against the Gram-negative *Haemophilus influenzae* and *Pseudomonas aeruginosa* respiratory pathogens, while Gram-positive *Streptococcus pyogenes* was the most resistant to the oil. Most antiseptic agents can damage the skin, leading to a change in microbial flora, and an increased shedding of the original protective bacterial flora of the hand leads to an increased risk of transmission of pathogenic microorganisms [46]. Reports suggest that repeated use of formulations containing tea tree essential oil (TTO) does not lead to dermatological problems, nor affect the original protective bacterial flora of the skin [47], so the antibacterial activity of some skin-wash formulas containing TTO as well as pure TTO was evaluated against *Staphylococcus aureus*, *Acinetobacter baumannii*, *Escherichia coli*, and *Pseudomonas aeruginosa.* The antibacterial activity of tea tree essential oil has recently been reviewed. It was found that antibacterial property of TTO is mainly due to presence of terpinen-4-ol. The essential oil of oregano is found to effective against *Pseudomonas aeruginosa* and *Escherichia coli* [48]. *Ocimum gratissimum* essential oil can also inhibit extracellular protease and the expression of O-lipopolysaccharide rhamnose in virulence and multidrug-resistant strains of 22 Shigellae. Thus, the oil may find a use as a therapeutic measure against shigellosis. Methicillin-resistant *Staphylococcus aureus* can also be inhibited by the application of peppermint and spearmint essential oils. Essential oils could be used as antibacterial agents against some respiratory tract pathogens. The oil of *Achillea clavennae* showed its maximum activity against *Klebsiella pneumoniae* and penicillin-susceptible and penicillin resistant *Streptococcus pneumoniae*. The oil also exhibited strong activity against *Haemophilus influenzae* and *Pseudomonas aeruginosa*. An increased density of Helicobacter pylori in the gastric mucosa is associated with severe gastritis and an increased incidence of peptic ulcers [49]. The activities of 60 essential oils against *H. pylori* P1 were evaluated: 30 oils were able to affect the growth in vitro, and 15 showed strong activity. Among the individual constituents of these oils, carvacrol, isoeugenol, nerol, citral and sabinene exhibited the strongest anti-*H. pylori* effects. Further investigations are underway regarding the ability of essential oils to control *H. pylori* infections [50]. Croton cajucara Benth essential oil was found to be toxic for some pathogenic bacteria and fungi associated with oral cavity disease and may be useful for controlling the microbial population in patients with fixed orthodontic appliances. A 6-month controlled clinical study demonstrated that a mouth rinse containing essential oils showed a comparable antiplaque and anti-gingivitis activity to that containing the synthetic antibacterial agent, chlorhexidine [51]. Mouth rinses containing essential oils (specially phenolic rich types) with chlorhexidine gluconate are commonly used as preprocedural preparations to prevent possible disease transmission, decrease chances of postoperative infection, decrease oral bacterial load and decrease aerosolization of bacteria Mouth washes containing essential oils could also be used as a part of plaque control routine since they can penetrate the plaque biofilm, kill pathogenicplaque—forming microorganisms by disrupting their cell walls and inhibiting their enzymatic activity. In addition, essential oils in mouth washes prevent bacterial aggregation, slow the multiplication and extract bacterial endotoxins. The mechanisms by which essential oils can inhibit microorganisms involve different modes of action, and in part may be due to their hydrophobicity. As a result, they get partitioned into the lipid bilayer of the cell membrane, rendering it more permeable, leading to leakage

*DOI: http://dx.doi.org/10.5772/intechopen.92305*

## *Essential Oil as Antimicrobial Agents: Efficacy, Stability, and Safety Issues for Food Application DOI: http://dx.doi.org/10.5772/intechopen.92305*

oils demonstrated antibacterial activity against zoonotic enteropathogens including *Salmonella spp*., *Escherichia coli* O157, *Campylobacter jejuni*, and *Clostridium perfringens*. Thus, these oils could possibly be used as an alternative to antibiotics in animal feed [45]. Essential oils with high concentrations of thymol and carvacrol e.g., oregano, savory and thyme, usually inhibit Gram-positive more than Gram-negative pathogenic bacteria. However the essential oil of *Achillea clavennae* exhibited strong antibacterial activity against the Gram-negative *Haemophilus influenzae* and *Pseudomonas aeruginosa* respiratory pathogens, while Gram-positive *Streptococcus pyogenes* was the most resistant to the oil. Most antiseptic agents can damage the skin, leading to a change in microbial flora, and an increased shedding of the original protective bacterial flora of the hand leads to an increased risk of transmission of pathogenic microorganisms [46]. Reports suggest that repeated use of formulations containing tea tree essential oil (TTO) does not lead to dermatological problems, nor affect the original protective bacterial flora of the skin [47], so the antibacterial activity of some skin-wash formulas containing TTO as well as pure TTO was evaluated against *Staphylococcus aureus*, *Acinetobacter baumannii*, *Escherichia coli*, and *Pseudomonas aeruginosa.* The antibacterial activity of tea tree essential oil has recently been reviewed. It was found that antibacterial property of TTO is mainly due to presence of terpinen-4-ol. The essential oil of oregano is found to effective against *Pseudomonas aeruginosa* and *Escherichia coli* [48]. *Ocimum gratissimum* essential oil can also inhibit extracellular protease and the expression of O-lipopolysaccharide rhamnose in virulence and multidrug-resistant strains of 22 Shigellae. Thus, the oil may find a use as a therapeutic measure against shigellosis. Methicillin-resistant *Staphylococcus aureus* can also be inhibited by the application of peppermint and spearmint essential oils. Essential oils could be used as antibacterial agents against some respiratory tract pathogens. The oil of *Achillea clavennae* showed its maximum activity against *Klebsiella pneumoniae* and penicillin-susceptible and penicillin resistant *Streptococcus pneumoniae*. The oil also exhibited strong activity against *Haemophilus influenzae* and *Pseudomonas aeruginosa*. An increased density of Helicobacter pylori in the gastric mucosa is associated with severe gastritis and an increased incidence of peptic ulcers [49]. The activities of 60 essential oils against *H. pylori* P1 were evaluated: 30 oils were able to affect the growth in vitro, and 15 showed strong activity. Among the individual constituents of these oils, carvacrol, isoeugenol, nerol, citral and sabinene exhibited the strongest anti-*H. pylori* effects. Further investigations are underway regarding the ability of essential oils to control *H. pylori* infections [50]. Croton cajucara Benth essential oil was found to be toxic for some pathogenic bacteria and fungi associated with oral cavity disease and may be useful for controlling the microbial population in patients with fixed orthodontic appliances. A 6-month controlled clinical study demonstrated that a mouth rinse containing essential oils showed a comparable antiplaque and anti-gingivitis activity to that containing the synthetic antibacterial agent, chlorhexidine [51]. Mouth rinses containing essential oils (specially phenolic rich types) with chlorhexidine gluconate are commonly used as preprocedural preparations to prevent possible disease transmission, decrease chances of postoperative infection, decrease oral bacterial load and decrease aerosolization of bacteria Mouth washes containing essential oils could also be used as a part of plaque control routine since they can penetrate the plaque biofilm, kill pathogenicplaque—forming microorganisms by disrupting their cell walls and inhibiting their enzymatic activity. In addition, essential oils in mouth washes prevent bacterial aggregation, slow the multiplication and extract bacterial endotoxins. The mechanisms by which essential oils can inhibit microorganisms involve different modes of action, and in part may be due to their hydrophobicity. As a result, they get partitioned into the lipid bilayer of the cell membrane, rendering it more permeable, leading to leakage

*Essential Oils - Bioactive Compounds, New Perspectives and Applications*

common to the rose and geranium essential oils.

mint, petitgrain, pine, rosemary, sage, thyme.

Aromatic compounds comprise:

1.**Aldehyde**: e.g., cinnamaldehyde

2.**Alcohol**: e.g., cinnamic alcohol

3.**Phenols**: e.g., chavicol and eugenol

(Apiaceae, Lamiaceae, Myrtaceae, Rutaceae).

of diverse plants or of torrefied, grilled or roasted products.

**3. Effects of essential oils as antibacterial agents**

eugenol as minor constituents, also clove oil, fennel, etc.).

**2.2 Aromatic compounds**

β-pinene from *Pinus caribaea* and from *Pinus pinaster*. Another example is linalool from coriander is (+); however, linalool from lavender oil is (−). In some cases, it is the racemic form which is the most frequently encountered. (±)citronellol is widespread, the form (+) is characteristic of *Eucalyptus citriodor*, the form (−) is

The sesquiterpenes are formed from the assembly of three isoprene units (C15). The extension of the chain increases the number of cyclisations which allows a great variety of structures. The structure and function of the sesquiterpenes are similar to those of the monoterpenes. Examples of plants containing these compounds are angelica, bergamot, caraway, celery, citronella, coriander, eucalyptus, geranium, juniper, lavandin, lavander, lemon, lemongrass, mandarin, mint, orange, pepper-

Derived from phenylpropane, the aromatic compounds occur less frequently than the terpenes. The biosynthetic pathways concerning terpenes and phenylpropanic derivatives generally are separated in plants but may coexist in some, with one major pathway taking over (e.g., cinnamom oil with cinnamaldehyde as major and

4.**Methoxyderivatives**: e.g., anethole, elemicine, estragole and methyleugenols

The principal plant sources for these compounds are anise, cinnamon, clove, fennel, nutmeg, parsley, sassafras, star anise, tarragon, and some botanical families

Nitrogenous or sulfured components such as glucosinolates or isothiocyanate derivatives (garlic and mustard oils) are also characteristic as secondary metabolites

Various studies showed that essential oils also have antibacterial properties against a wide range of bacterial strain such as *Listeria monocytogenes*, *L. innocua*, *Salmonella typhimurium*, *Escherichia coli*, *Shigella dysenteria*, *Bacillus cereus*, *Staphylococcus aureus*, and *Salmonella typhimurium* [42]. Direct inhibition correlation due to presence of thymol and carvacrol in the essential oils of thyme and oregano can inhibit some pathogenic bacterial strains such as *E. coli*, *Salmonella enteritidis*, *Salmonella choleraesuis*, and *Salmonella typhimurium* [43]. The same correlation was also confirmed for oils rich in carvacrol alone. Eugenol and carvacrol showed an inhibitory effect against the growth of four strains of *Escherichia coli* O157:H7 and *Listeria monocytogenes* [44]. The carvacrol showed strong antibacterial activity due to presence of phenolic hydroxyl group. Some essential

5.**Methylene dioxy compounds**: e.g., apiole, myristicine and safrole

**158**

of vital cell contents [52]. Impairment of bacterial enzyme systems may also be a potential mechanism of action. Essential oils show bactericidal activity against oral and dental pathogenic microorganisms and can be incorporated into rinses or mouth washes for pre-procedural infection control, general improvement of oral health, inter-dental hygiene and to control oral malodor [53].
