3.2. Antibacterial activity of catechin against E. coli
3.2.1. Growth assay in liquid medium
E. coli were incubated for 24 h in LB liquid culture medium
containing a range of concentrations of catechin (0e5 mg/mL). As
shown in Fig. 4, the OD600 values ranged from 0.9 for control to 0.8
for the medium supplemented with 5.0 mg/mL of catechin. This
observation suggesting a minor growth inhibitory effect was fully
consistent with CFU/mL values ranging from 6 108 (control with
no catechin) to 4 108 when bacteria were grown in presence of
5.0 mg/mL catechin. Both the OD600 values and the CFU/mL values
showed no significant differences (p > 0.05) compared to the corresponding
control.
3.2.2. Inhibition halo test for catechin
Bacteria were cultured for 24 h on LB agar plates containing a
series of wells loaded with different amounts of catechin. As shown
in Fig. 5, the diameters of the growth inhibition halos around the
wells increased gradually with the increasing content of catechin in
the range from 0 for the control condition to 14 mm for the presence
of 15 mg of catechin. Growth inhibition halos were significantly
higher than control (p < 0.05) with amounts of catechin
equal or greater than 7.5 mg.
3.3. Effect of combining gallic acid and catechin on E. coli growth
E. coli was grown on LB agar plates containing a series of wells
loaded with a fixed amount of catechin (7.5 mg/well) plus
increasing doses of gallic acid from 0 to 1.0 mg/well (Fig. 6). After
24 h of incubation at 37 C the diameters of the growth inhibition
halos were measured. A dose of 7.5 mg of only catechin produced
inhibition halos around 5.0 mm in diameter. Likewise, when inhibition
halos were examined in relation to the presence of a range of
doses of gallic acid, no inhibitionwas observed with doses less than
1.0 mg of gallic acid. On the other hand, when both polyphenols were deposited together in single wells, the growth inhibitory effect was similar to the one produced by the effective dose of catechin
alone (p > 0.05).