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across the plasma membrane and/or through
porin channels (Schulz, 1996), and long-
chain alkanals that are dissolved in the
medium are incorporated into the plasma
membrane lipid bilayers (Franks and Lieb,
1986). The amount of alkanals entering into
the cytosol or lipid bilayer is dependent
on the length of the alkyl chain. Once inside
the cytosol or lipid bilayer, alkanals may
react with biologically important substances,
because aldehydes are chemically reactive
compounds. Tridecanal is inactive because
it may not attain high enough concentra-
tions in the lipid bilayer. It appears that the
bactericidal action of alkanals is similar to
those described for alkanols in many aspects
but differs to some extent.
Alcohols are among the most versatile of
all organic compounds, and free and esteri-
fied alcohols are known to occur widely in
nature. Safety is a primary consideration for
chemical preservatives, especially concern-
ing their use in food products, which may be
utilized in unregulated quantities on a regu-
lar basis. The phytochemicals characterized
as anti-
Salmonella
agents from edible plants
should be superior to non-natural preserva-
tives. In addition, alcohols have another
superior property as antimicrobial agents
compared with benzoic acid (
18
), a common
commercial antimicrobial agent. As a weak
acid antimicrobial agent, benzoic acid's
activity is pH dependent and increases as the
pH of the substrate decreases (Sofos, 1983).
At higher pH values (>7) benzoic acid did
not show any antibacterial activity against
S. choleraesuis
up to 1600 mg/ml, owing to a
higher degree of dissociated molecules. In
contrast, the alcohols are not affected by pH.
This would seem to be of greater overall
value than other pH-sensitive antimicrobi-
als, because many foods have near neutral
pH values. Moreover, alcohols were previ-
ously reported to inhibit soybean lipoxygen-
ase-1 (Kuninori
et al
., 1992).
homologous series of acyclic 2
E
-alkenals,
we reported the antifungal activity of
amphipathic medium chain (C9-C12)
2
E
-alkenals against
S. cerevisiae
. This was
largely due to their non-ionic surface-active
properties, and the maximum activity can
be obtained when balance between
hydrophilic and hydrophobic portions
becomes the most appropriate, possibly
similar to that described for acyclic alkanols
(Kubo
et al
., 1995b, 2003). In other words,
the antifungal activity of 2
E
-alkenals against
S. cerevisiae
is in part due to biophysical
processes. This concept can be extended to
the antibacterial activity of the same
medium chain 2
E
-alkenals against
S. chol-
eraesuis,
because in the time kill experi-
ment: (i) lethality occurred notably quickly,
within the first 1 h after the addition of
2
E
-dodecenal; (ii) bactericidal activity was
found at any growth stage; and (iii)
2
E
-dodecenal rapidly killed
S. choleraesuis
cells in which cell division was inhibited
by chloramphenicol. Moreover, the antimi-
crobial activity of 2
E
-alkenals is non-
specific (Kubo
et al
., 1995a; Bisignano
et al
., 2001; Nakamura and Hatanaka, 2002;
Lanciotti
et al
., 2003) and the potency of the
activity against
S. choleraesuis
was dis-
tinctly increased with each additional CH
2
group, up to 2
E
-dodecenal. The results
observed support the ability of medium-
chain 2
E
-alkenals to function at least in part
as non-ionic surfactants. Similar relation-
ships between the activity of antimicrobial
compounds and their lipophilicity were
previously observed with some membrane-
active antimicrobial agents, such as alco-
hols and quaternary ammonium compounds
(Lien
et al
., 1968; Hamilton, 1971; Hansch
and Clayton 1973; Daoud
et al
. 1983; Kubo
et al
., 1993). For example, the antimicrobial
activity of a series of alkyldimethylbenzyl-
ammonium chlorides was a parabolic func-
tion of their lipophilicity and maximized
with alkyl chain lengths between C12 and
C16 (Daoud
et al
., 1983). The surfactant
concept of the same series of aldehydes was
well described (Lanciotti
et al
., 2003).
Moreover, the leakage of carboxyfluor-
escein (CF) in liposomes of phosphatidylcho-
line (PC) following exposure to 2
E
-alkenals
16.4
Conclusion
In our previous studies on structure-
antifungal activity relationships with a
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