Investigation of the effect of four organic acids in Radix Isatidis on E-coli growth by microcalorimetry

The inhibitory effects of four organic acids (OAs) in Radix Isatidis, a traditional Chinese medicinal herb, on Escherichia coli (E. coli) growth were investigated by microcalorimetry. The power-time curves of E. coli growth with and without OAs were acquired, meanwhile the extent and duration of inhibitory effects on the metabolism were evaluated by growth rate constants (k(1), k(2)), half inhibitory ratio (IC50), maximum heat output (P-max) and peak time (t(p)). The values of k(1) and k(2) of E. coli growth in the presence of the four OAs decreased with the increasing concentrations of OAs. Moreover, P-max was reduced and the value of tp increased with increasing concentrations of the four drugs. The sequence of anti-microbial activity of the four OAs was: syringic acid >2-amino-benzoic acid> salicylic acid>benzoic acid. IC50 of the four OAs was respectively 56 mu g/mL for syringic acid, 75 mu g/mL for 2-amino-benzoic acid, 86 mu g/mL for salicylic acid and 224 mu g/mL for benzoic acid. The existence of the functional groups on phenyl ring improves the anti-microbial activity compared to benzoic acid. The functional groups methoxyl at C(3) and C(5) improve anti-microbial activity more strongly than the other functional groups, and the functional group amino at C(2) improve anti-microbial activity more strongly than hydroxyl at C(2) on phenyl ring.