222
R. Tesser et al.
8.3.3 Activity of Essential Oils
Although many literature reviews have been focused, in the past, on the antimicrobial
properties of essential oils and their components [34], lack of extensive information
is still present about the details of the action mechanism [120, 121]. Very likely,
the antibacterial/antifungal activity of EOs cannot be attributed to a single specific
mechanism due to a large number of different categories of chemical compounds the
are present (see Chap. 2); on the contrary, it is a common opinion that several targets
are located into the cells that can be considered as sites for EOs action [35, 201].
The number of chemical substances present in EOs is very high and frequently, the
most abundant of them have shown a more or less pronounced antibacterial property.
Moreover, as reported by many authors [1, 44, 66, 71, 120, 121], the antimicrobial or
antifungal activity of essential oils can be correlated or compared with those of the
individual components of the oils themselves. On the other hand, different authors
examined various essential oil components and their combinations to find possible
synergistic effects [58, 115, 116]. An interesting review, for example, among these
property–structure relationships studies, tried to correlate the structure and molecular
properties of 60 terpenoids with their antimicrobial activity [79].
Numerous literature studies have been devoted to the activity of EOs and focused
on the determination of which compounds are responsible for the major antioxidant or
antimicrobial effect. For example, thymol, carvacrol and eugenol are the main components responsible for the antioxidant activity of basil and thyme oils [123]. Even if
the EOs activity mechanisms are not fully understood and clearly described, it seems
that the antimicrobial activity is essentially due to their hydrophilicity. This characteristic enables them to be partitioned in the lipids of the bacterial cells membrane and
mitochondria, disturbing the structures and rendering them more permeable [109,
200]. Terpenes, that in many cases are the major compounds of EOs, can disrupt and
penetrate the lipidic structure of the bacteria cell membrane, leading to the denaturing
of proteins and the destruction of the cell membrane [217]. Lambert et al. [120, 121]
reported that essential oils containing a high percentage of phenolic compounds, such
as eugenol, carvacrol and thymol, present stronger antibacterial properties against
foods pathogens. In other cases, the action mechanism of EOs components (mainly
carvacrol) against microorganisms is the channel formation through the membrane
[219] by pushing apart the fatty acid chains of the phospholipids, allowing ions to
leave the cytoplasm. Another study [210], reported that eugenol can induce cell wall
deterioration and a high degree of cell lysis. In conclusion, not all the components
of essential oils exhibit the same mechanism of action. From the point of view of
the antibacterial/antifungal activity of EOs, it seems to be ascertained that phenolic
components are the main responsible for the antibacterial properties [42].
As discussed before in paragraph 2, as the chemical composition of EOs depends
on different factors, such as origin country and harvesting period, this type of variation could be enough large to justify the variability in the sensitivity of microorganisms and fungi to these substances. Moreover, it is a common opinion that the
overall effectiveness in antimicrobial activity of EOs cannot be attributed only to
R. Tesser et al.
8.3.3 Activity of Essential Oils
Although many literature reviews have been focused, in the past, on the antimicrobial
properties of essential oils and their components [34], lack of extensive information
is still present about the details of the action mechanism [120, 121]. Very likely,
the antibacterial/antifungal activity of EOs cannot be attributed to a single specific
mechanism due to a large number of different categories of chemical compounds the
are present (see Chap. 2); on the contrary, it is a common opinion that several targets
are located into the cells that can be considered as sites for EOs action [35, 201].
The number of chemical substances present in EOs is very high and frequently, the
most abundant of them have shown a more or less pronounced antibacterial property.
Moreover, as reported by many authors [1, 44, 66, 71, 120, 121], the antimicrobial or
antifungal activity of essential oils can be correlated or compared with those of the
individual components of the oils themselves. On the other hand, different authors
examined various essential oil components and their combinations to find possible
synergistic effects [58, 115, 116]. An interesting review, for example, among these
property–structure relationships studies, tried to correlate the structure and molecular
properties of 60 terpenoids with their antimicrobial activity [79].
Numerous literature studies have been devoted to the activity of EOs and focused
on the determination of which compounds are responsible for the major antioxidant or
antimicrobial effect. For example, thymol, carvacrol and eugenol are the main components responsible for the antioxidant activity of basil and thyme oils [123]. Even if
the EOs activity mechanisms are not fully understood and clearly described, it seems
that the antimicrobial activity is essentially due to their hydrophilicity. This characteristic enables them to be partitioned in the lipids of the bacterial cells membrane and
mitochondria, disturbing the structures and rendering them more permeable [109,
200]. Terpenes, that in many cases are the major compounds of EOs, can disrupt and
penetrate the lipidic structure of the bacteria cell membrane, leading to the denaturing
of proteins and the destruction of the cell membrane [217]. Lambert et al. [120, 121]
reported that essential oils containing a high percentage of phenolic compounds, such
as eugenol, carvacrol and thymol, present stronger antibacterial properties against
foods pathogens. In other cases, the action mechanism of EOs components (mainly
carvacrol) against microorganisms is the channel formation through the membrane
[219] by pushing apart the fatty acid chains of the phospholipids, allowing ions to
leave the cytoplasm. Another study [210], reported that eugenol can induce cell wall
deterioration and a high degree of cell lysis. In conclusion, not all the components
of essential oils exhibit the same mechanism of action. From the point of view of
the antibacterial/antifungal activity of EOs, it seems to be ascertained that phenolic
components are the main responsible for the antibacterial properties [42].
As discussed before in paragraph 2, as the chemical composition of EOs depends
on different factors, such as origin country and harvesting period, this type of variation could be enough large to justify the variability in the sensitivity of microorganisms and fungi to these substances. Moreover, it is a common opinion that the
overall effectiveness in antimicrobial activity of EOs cannot be attributed only to