Anti-Salmonella Agents from the Brazilian Medicinal Plant Tanacetum balsamita and their Applications - Natural Antioxidants and Biocides from Wild Medicinal Plants - page 241

and is widely distributed in many plants
(Schauenstein, 1977). In our continuing
search for antimicrobial agents from edi-
ble plants, 2 E -hexenal was previously
characterized as an antimicrobial agent from
the volatile fraction of the cashew apple
(Muroi et al. , 1993), coriander and olive oil
(Kubo et al. , 1995a; Bisignano et al. , 2001).
The bactericidal effect of 2 E -hexenal was
confirmed by the time kill curve experiment
as shown in Fig. 16.2. Cultures of S. choler-
aesuis , with a cell density of 1 × 10 5 CFU/ml,
were exposed to two different concentra-
tions of 2 E -hexenal. The number of viable
cells was determined following different
periods of incubation with 2 E -hexenal. The
result verifies that minimum inhibitory
concentration (MIC) and minimum bacteri-
cidal concentration (MBC) are the same. In
contrast to 2 E -hexenal, hexanol did not
show any activity against S. choleraesuis up
to 1600 mg/ml, but hexanal and hexanoic
acid still exhibited some activity, albeit to a
lesser extent than 2 E -hexenal. Thus the con-
jugated double bond is not essential to elicit

the antibacterial activity but is associated
with increasing the activity.
The maximum antimicrobial activity of
2 E -alkenals is dependent on the balance of
the hydrophobic alkyl (tail) chain length
from the hydrophilic aldehyde group (head)
(Kubo et al. , 1995a, 2003a). It is well known
that the hydrophobicity of molecules is
often associated with biological action
(Hansch and Dunn, 1972). However, the
rationale for this observation, especially the
role of the hydrophobic portion, is still
poorly understood and widely debated. To
clarify this, 2 E -alkenals are a superior model
for structure and anti- Salmonella activity
relationship (SAR) study because these mol-
ecules possess the same hydrophilic por-
tion, the enal group, which explains the role
of the hydrophobic alkyl portion.
A homologous series of aliphatic
2 E -alkenals and their related analogues are
common in many plants and readily avail-
able. Therefore, a homologous series of
aliphatic 2 E -alkenals, as well as the corre-
sponding alkanals, from C5 to C13 were
tested for their antibacterial activity against
S. choleraesuis using a twofold broth dilu-
tion method for comparison. The results are
listed in Table 16.1. The range of the anti-
bacterial activity of the 2 E -alkenals tested
against S. choleraesuis is between 6.25
and 200 mg/ml, and the MICs and MBCs
are markedly the same. The antibacterial
activity against this foodborne bacterium is

8

7

6

5

4

Table 16.1. Antibacterial activity ( m g/ml)
of aliphatic aldehyde compounds against
S. choleraesuis subsp. choleraesuis ATCC 35640.

3

2

2 E -Alkenal Alkanal

1

Aldehydes
tested MIC MBC MIC MBC

0

0 4 8 12

Time (h)

16 20 24

C 5 200 200

- -

C 6 100 100 400 800

C 7 100 100 400 400

Fig. 16.2. Bactericidal effect of 2 E -hexenal against
S. choleraesuis subsp. choleraesuis ATCC 35640.
Exponentially growing cells of S. choleraesuis
were inoculated at 37°C in nutrient yeast glucose
(NYG) broth with 0 (), 50 ( ▼ ) or 100 ( • ) m g/ml of
2 E -hexenal. Viability was established by the
number of colonies formed on NYG plate after
incubation at 30°C for 24 h.

C 8 100 100 200 400

C 9 50 50 100 200

C 10 25 25 100 100

C 11 12.5 12.5 100 100

C 12 6.25 6.25 100 100

C 13 25 200 >800 >800

-, Not tested.