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correlated with the hydrophobic alkyl (tail)
chain length from the hydrophilic aldehyde
group (head). Among the compounds tested,
2 E -dodecenal ( 2 ) was found to possess the
most potent activity against S. choleraesuis ,
followed by 2 E -undecenal ( 3 ). 2 E -Dodecenal
exhibited the activity with both MIC and
MBC of 6.25 mg/ml (34 mM), suggesting that
no residual bacteriostatic activity is
involved. Notably, this MBC value is slightly
more potent than that of gentamicin. It
appears that S. choleraesuis showed differ-
ent susceptibilities to aldehydes possessing
different chain lengths. This result is
broadly similar to those of the correspond-
ing alkanols against many microorganisms
(Kubo et al. , 1995b, 2003b), indicating at
least in part the similarity of their mode of
action. Because of their easy availability
andbroadantimicrobialactivity,2 E -alkenals,
2 E -hexenal and 2 E -dodecenal were further
studied in detail.
The bactericidal effect of 2 E -dodecenal
was confirmed by the time kill curve method
as shown in Fig. 16.3. Cultures of S. cholerae-
suis , with a cell density of 10 5 CFU/ml, were
exposed to three different concentrations
of 2 E -dodecenal. The number of viable
cells was determined following different
periods of incubation with 2 E -dodecenal.
The result verifies that the MIC and MBC
are the same. Notably, lethality occurred
quickly, within the first 1 h after the addi-
tion of 2 E -dodecenal. This rapid lethality
very likely indicates that antibacterial activ-
ity of 2 E -dodecenal against S. choleraesuis
is associated at least in part with physico-
chemical damage to the cells, such as the
disruption of the membrane.
The bactericidal effect of 2 E -hexenal
occurred slower than that of 2 E -dodecenal,
which needed 7 h. Such a slow cell death is
thought to proceed independently of the
membrane disruptive action. The result
obtained indicates that the mode of antibac-
terial action of 2 E -hexenal and 2 E -dodecenal
against S. choleraesuis differs to some
extent. The effects of 2 E -dodecenal and
2 E -hexenal against S. choleraesuis were fur-
ther tested by holding the viable cell number
in the presence of chloramphenicol. This
antibiotic is known to restrict cell division
by inhibiting protein synthesis. Figure 16.4
shows that the effect of chloramphenicol
against S. choleraesuis cells is bacteriostatic
for the first 3 h after the addition of the drug.
It should be noted that chloramphenicol is
known to be bacteriostatic for a wide range
of Gram-positive and Gram-negative bacte-
ria, but this antibiotic expressed a bacteri-
cidal effect against S. choleraesuis after 8 h
incubation. In the presence of chloramphen-
icol, 2 E -hexenal decreased viable cell
numbers slightly more quickly than in the
absence. 2 E -Dodecenal induced rapid
decrease in viability regardless of the pres-
ence of chloramphenicol. The inhibition of
cell division by chloramphenicol did not
influence the bactericidal effects of
2 E -hexenal and 2 E -dodecenal. The reduced
viability might not be due to interaction
with the biosynthesis of cell wall or plasma
membrane components. The synthesis of
macromolecules such as DNA, RNA and
proteins was not related to the reduction.
The observation of the rapid bactericidal
effect of 2 E -dodecenal very likely indicates
that the primary action of 2 E -dodecenal is
on the cell membrane.
8
7
6
5
4
3
2
1
0
04812
Time (h)
16
20
24
Fig. 16.3. Effect of 2 E -dodecenal on the growth of
S. choleraesuis subsp. choleraesuis ATCC 35640.
Exponentially growing cells were inoculated into
NYG broth and then cultured at 37°C. The arrow
indicates the time when drug was added.
2 E -Dodecenal 0 (), 1.56 ( ), 3.13 ( ) and 6.25
( ) m g/ml.
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