Biology Reference
In-Depth Information
In addition to the surfactant concept,
2
E
-alkenals may enter into the cells by pas-
sive diffusion across the plasma membrane.
It also probably permeates in part through
pores derived from membrane damage. If
this is so, the a,b-unsaturated aldehyde
group should not be overlooked because this
group is chemically highly reactive and read-
ily reacts with biologically important nucle-
ophilic groups, such as sulfhydryl, amino or
hydroxyl. Once inside the cells, 2
E
-alkenals
may react with various intercellular compo-
nents, for example, sulfhydryl groups in pro-
teinsandlowermolecularweightcompounds.
It is known that a,b-unsaturated aldehyde
group reacts with sulfhydryl groups mainly
by 1,4-additions under physiological condi-
tions (Schauenstein
et al
., 1977). 2
E
-Alkenals
were reported to cause depletion of cytoplas-
mic and mitochondrial glutathione, which
functions in eliminating reactive oxygen spe-
cies (Machida
et al
., 1998). Sulfhydryl groups
in proteins and lower molecular weight com-
pounds such as glutathione are known to
play an important role in the living cell.
Bacteria protect themselves against hydro-
gen peroxide in various ways (Brul and
Coote, 1999), and some of the most ubiqui-
tous systems include glutathione. Taking
these factors altogether, 2
E
-alkenals first act
as a surfactant and then inhibit various cel-
lular functions non-specifically, and thus
2
E
-alkenals do not act by a single defined
process but have multiple functions, depend-
ing on their alkyl chain length.
Subsequently, hexanal (C6) (
4
) was also
found to exhibit the antibacterial activity
against
S. choleraesuis
with MIC and MBC of
400 and 800 mg/ml, respectively. It seems
that the antibacterial activity against
S. chol-
eraesuis
should not be specific to 2
E
-alkenals
because the conjugated double bond is not
essential in eliciting activity, but is involved
with increasing the activity. This prompted
us to test the corresponding alkanals for their
antibacterial activity against
S. choleraesuis
for comparison. The results are listed in
Table 16.1. The activity of alkanals is weaker
than those of the corresponding 2
E
-alkenals.
Similar to 2
E
-alkenals, their MIC and MBC
values are approximately the same and the
activity also increased in general with
increasing carbon chain length up to dodeca-
nal (C12) (
5
). It should be noted, however,
that there is a slight difference between
2
E
-alkenals and alkanals. For example, decanal
(C10) (
6
), undecanal (C11) (
7
) and dodecanal
are the most effective, but their MIC and
MBC values against
S. choleraesuis
are all
the same. The increase in the activity as car-
bon-chain length increases is not distinct in
the case of alkanals as compared to those of
2
E
-alkenals. The bactericidal effect of hex-
anal and dodecanal were also confirmed by the
time kill curve method (data not illustrated).
In contrast to 2
E
-hexenal, hexanol (
8
) did not
show any activity against
S. choleraesuis
up
to 1600 mg/ml as listed in Table 16.2. Both
(a)
9
8
7
6
5
4
3
2
1
0
0
246810
12
(b)
9
8
7
6
5
4
3
2
1
0
0
246
Time (h)
8 0 2
Fig. 16.4.
Effect of (a) 2
E
-hexenal and
(b) 2
E
-dodecenal in the presence of
chloramphenicol against
S. choleraesuis
subsp.
choleraesuis
ATCC 35640. Exponentially growing
cells were inoculated into NYG broth and then
cultured at 37°C. Chloramphenicol 0 (); and 6.25
(
)
m
g/ml was added to the culture after 1 h
cultivation. 2
E
-Hexenal (100
m
g/ml) or
(2
E
)-dodecenal (6.25
m
g/ml) and chloramphenicol
(6.25
m
g/ml) were added at 1 (
■
), 2 (
▼
) and 3 (
•
) h.
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