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heptanol inhibited only actively growing
and dividing
S. choleraesuis
cells. Hence,
there seem to be other factors involved to
explain the antibacterial action of short-
chain (C8) alkanols, but the rationale still
remains unknown.
Combining two or more antimicrobial
compounds seems to be superior to enhance
and/or broaden the total activity. In previ-
ous papers, a common phenylpropanoid,
anethole (
17
), was described to enhance the
antifungal activity of 2
E
-undecenal (
3
)
against
S. cerevisiae
(Kubo and Kubo, 1995).
Hence, dodecanol was combined with anet-
hole to see if the combination has any
enhancing activity against
S. choleraesuis
.
The combination of anethole synergistically
retarded the growth rate of this foodborne
bacterium to a large extent, but the combi-
nation was not bactericidal. Its bactericidal
action was only marginally synergistic. In
connection with this, anethole itself exhib-
ited antibacterial activity against
S. choler-
aesuis
with both MIC and MBC of 200 mg/ml
(1.35 mM). No differences in its MIC and
MBC were noted, suggesting that the activ-
ity is bactericidal. The bactericidal effect of
anethole against
S. choleraesuis
was con-
firmed by the time kill curve method (Kubo
et al
., 1995b; Fujita
et al
, 2007). The reason
for the residual bacteriostatic activity of the
combination against
S. choleraesuis
remains
unknown.
In our previous studies on structure-
antifungal activity relationships with a
series of primary alkanols, we reported that
the antifungal activity of medium chain
(C9-C12) alkanols against
S. cerevisiae
was
mediated primarily owing to their non-ionic
surface-active properties disrupting the
lipid-protein interface non-specifically,
and the maximum activity can be obtained
when balance between the hydrophilic and
hydrophobic portions becomes the most
appropriate (Kubo
et al
., 1995b, 2003b).
This surfactant concept can also be applica-
ble in part against
S. choleraesuis
, because
in the time kill experiment: (i) lethality
occurred notably quickly within the first 1 h
after the addition of one of the medium-
chain alkanols; (ii) bactericidal activity was
found at any growth stage; and (iii) dodecanol
rapidly killed
S. choleraesuis
cells in which
cell division was inhibited by chloram-
phenicol. Moreover, the antimicrobial activ-
ity of alkanols is non-specific and the
potency of the activity against
S. cholerae-
suis
was distinctly increased with each
additional CH
2
group, up to dodecanol. The
observed results support medium chain
alkanols' ability to function as non-ionic
surfactants.
As non-ionic surfactants, alkanols first
approach the binding site with the electron
negativity of the hydroxyl oxygen atom.
This hydrogen bond acceptor will affect the
hydrogen bonds that regulate the permea-
bility of the lipid bilayers. For example, the
hydroxyl group of cholesterol resides near
the membrane-water interface in the lipid
bilayers and is likely to be bonded with
the carbonyl group of phospholipids
(Brockerhoff, 1974; Chauhan
et al
., 1984;
Chiou
et al
., 1990). Alkanols may function
by disrupting and disorganizing the hydro-
gen bonds. Cholesterol is a major compo-
nent of the animal plasma membrane and
owes its membrane-closing properties to its
rigid longitudinal orientation in the mem-
brane. Cholesterol has profound influences
on membrane structure and function; there-
fore, if the hydrogen bond is broken, cell
function will be impaired.
Given the surfactant-like properties of
medium-chain (C9-C12) alkanols, it is pos-
sible to suggest that alkanols also act at the
lipid-protein interface of integral proteins,
such as ion channels and/or transport pro-
teins, denaturing their functioning confor-
mation in a similar manner as that described
against
S. cerevisiae
(Heidmann
et al
., 1983;
Nikaido, 1994). The common nature among
these alkanols should be considered in that
the electron negativity on the hydroxyl oxy-
gen atom forms an intermolecular hydrogen
bond with a nucleophilic group in the mem-
brane, thereby creating disorder in the fluid
bilayer of the membrane. The fluidity of the
cell membrane can be disturbed maximally
by hydrophobic compounds of particular
hydrophilic hydroxyl group. Thus, the
medium-chain alkanols disrupt the hydro-
gen bonding in the lipid-protein interface
in
S. choleraesuis
. The data obtained are
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