Inhibitory activity of Thai condiments on pandemic strain of Vibrio parahaemolyticus

Antibacterial activity of 13 condiments used in Thai cooking was investigated with a pandemic strain of Vibrio parahaemolyticus. Using a disk diffusion technique, freshly squeezed extracts from galangal, garlic and lemon, at a concentration of 10 μl/disk produced a clear zone of 13.6±0.5, 11.6±0.5 and 8.6±1.2 mm, respectively. The inhibitory activity of these 3 condiments on pandemic strains was not significantly different from that on non-pandemic strains of V. parahaemolyticus. Because of its popularity in seafood cooking, galangal was subjected to further investigation. Only a chloroform extract of galangal inhibited growth of V. parahaemolyticus producing a clear zone of 9.5±0.5, 12.0±0 and 13.5±0.5 mm diameter at concentrations of 25, 50 and 100 μg/disk, respectively. One active component is identified as 1′-acetoxychavicol acetate. The activity of galangal was not reduced at pH 3 or in the presence of 0.15% bile salt but was reduced by freeze and spray drying. Heating a fresh preparation of galangal to 100 °C but not 50 °C for 30 min also reduced growth inhibition. Therefore, using fresh galangal in cooking was recommended. The MIC and MBC of a freshly squeezed preparation of galangal were 1:16 and 1:16, respectively. This is the first report of an inhibitory activity of a Thai medicinal plant, galangal that is used in Thai cooking, on the pandemic strain of V. parahaemolyticus.