Screening for natural antimicrobials from Thai Medicinal plants

Twenty six Thai edible plants that are traditionally used for prevention or treatment some infective problems were screened for antimicrobial activity against 4 pathogens. Sixteen were sensitive against at least one strain studied, whereas five gave strongest inhibition zones against all four strains and two gave medium positive results against all strains. Six plants were selected for further study, including Sandoricum nervosum Car. (Zingiberaceae), Psidium guajava L. (Myrtaceae), Piper betle L. (Piperaceae), Garcinia atroviridis Griff. (Guttiferae), Garcinia mangostana L. (Guttiferae) and Punica granatum L. (Punicaceae). After confirmation by disk susceptibility test with 8 pathogens (4 resistant strains of Gram-negative bacteria and fungi were also added), all extracts were studied and compared according to extraction techniques and solvents applied. The minimum inhibitory concentration (MIC) of individual extracts was also determined. It was concluded that in most cases, extraction with water and ethyl acetate (5 times) was the best, but the worst in term of % yield of the extract. A comparison between methanol and ethylacetate reflux extraction was varying, depending on individual plant, but ethylacetate was more preferable for P. betle, even though all extracts from P. betle were active against all 8 strains studied, of which, its sensitivities were as follows: L. monocytogenes ≥ S. aureus≥ C. albicans> B. cereus ≥ S. typhimurium ≥ A. niger > E. coli > P. aeruginosa at MICs <0.10, <0.10, <0.10, 0.10, 0.10, 0.10, 0.68 and 1.82mg/ml respectively. However, extracts from methanol reflux in all cases gave highest % yields. The P. betle from water and ethylacetate extract as separated by thin layer chromotography (TLC) yielded two substances of Rf 0.57 and 0.35 which could be chavibetol and allylpyrocatechol (APC).