[Preliminary study of antimicrobial activities on medicinal herbs of Thai food's ingredients]

To investigate the potential for use of medicinal herbs as natural antimicrobial additives for foods, the following fifteen medicinal herbs were examined for antimicrobial activity: coriander (Coriandrum sativum Linn), shallot (Allium ascalonicum L.), ginger (Zingiber offcinale Roscoe), galangal (Alpinia galanga Swartz), lemon grass (Cymbopogon citratus Stapf.), kaffir lime leaves and peels (Citrus hystrix DC), lime peels (Citrus aurantiifolia Swing), bird chili (Capsicum frutescens L.), curcuma (Curcuma longa L.), sweet basil (Ocimum basilicum L.), holy basil (Ocimum sanctum Linn), bitter cucumber (Momordica charantia L.), devil’s fig (Solanum torvum Sw.) and indian malberry (Morinda citriforia L.). Fresh extracts and oil extracts of all medicinal herbs in this study were tested against Bacillus cereus, Salmonella typhi and Staphylococcus aureus by using Agar Diffusion Method. Six of them including Zingiber officinale, Allium ascalonicum, Alpinia galanga, Citrus hystrix, and Citrus aurantiforia were shown high activities in the first screening. A second screening for antimicrobial activities of six potential medicinal herbs were performed in duplication. All of six herbs exhibited antimicrobial activities against Bacillus cereus and Staphylococcus aureus, and all except Z. officinale exhibited significant activity against Salmonella typhi. Antimicrobial activities of oil extracts were higher than fresh extracts. C. hystrix, A. ascalonicum, and C. aurantiifolium were selected as potential antimicrobial food additives, and the identification of active compounds and suitable purification method in these medicinal plants have been suggested to further studies. INTRODUCTION Traditional Thai food has a very distinctive character because of the special combination of herb and spices in Thai food preparation. These herb and spice ingredients are more than just good taste but they make Thai food more healthful. Recently, the demand for medicinal herb products has begun to grow and gain popularity. Natural antimicrobial agent development from Thai food ingredients may respond to the national drug policy and “Thai Herbs for Health” promotion. The fifteen medicinal herbs of Thai food’s ingredient were selected for antimicrobial screening are: coriander, shallot, ginger, galangal, lemon grass, kaffir lime leaves, kaffir lime peels, lime peels, bird chili, curcuma, sweet basil, holy basil, bitter cucumber, devil’s fig and indian malberry (Thiengburanathum, 1996; Liangmaneethep, 1997). The objective of this work was to investigate medicinal herbs of Thai food ingredients as natural antimicrobial agents. Proc. WOCMAP III, Vol. 1: Bioprospecting & Ethnopharmacology Eds. J. Bernath, E. Nemeth, L.E. Craker and Z.E. Gardner Acta Hort 675, ISHS 2005 112 MATERIALS AND METHODS Fifteen selected medicinal herbs samples for Thai food’s ingredients were collected from a local market in Bangkok. They were coriander (Coriandrum sativum Linn), shallot (Allium ascalonicum L.), ginger (Zingiber offcinale Roscoe), galangal (Alpinia galanga Swartz), lemon grass (Cymbopogon citratus Stapf.) kaffir lime leaves and peels (Citrus hystrix DC), lime peels (Citrus aurantiifolia Swing), bird chili (Capsicum frutescens L.), curcuma (Curcuma longa L.), sweet basil (Ocimum basilicum L.), holy basil (Ocimum sanctum Linn), bitter cucumber (Momordica charantia L.), devil’s fig (Solanum torvum Sw.) and indian mulberry (Morinda citrifolia L.). Each sample was prepared into fresh extracts and oil extract. Twenty grams of each chopped herbs were prepared for each extracts. Fresh extracts were prepared by using a manual extractor and squeezing. The oil extracts were extracted with alcohol by using Soxtec, Tecator model 1043. The preliminary study of antimicrobial activity on fresh and oil extracts of all herbs was performed by using agar diffusion method both in disk diffusion test and cup plate agar diffusion test (Isenberg, 1998; Mohon and Manuselis, 1995; Victor, 1991). The sensitivity of both extracts were tested against Bacillus cereus ATCC25923, Salmonella typhi ATCC19430 and Staphylococcus aureus ATCC25923 (TISTR culture collection, 1995). The sensitivity test was performed by using an antibiotic drug as a control. The antimicrobial activity was measured by the inhibition zones produced. All experiments were duplicated. RESULTS AND DISCUSSION This investigation of antimicrobial activity was performed on fifteen selected medicinal herbs. Six of them including ginger (Zingiber offcinale Roscoe), galangal (Alpinia galanga Swartz), kaffir lime leaves and kaffir lime peels (Citrus hystrix DC) and lime peels (Citrus aurantiifolia Swing) showed high antimicrobial activity in the first screening step (Table 1). The second screening step in the preliminary study for antimicrobial activity was the Disk Diffusion Test on Agar Diffusion Method. This test was used in order to determine antimicrobial activity of fresh extracts and oil extracts. The diameter of the clear zone indicated the inhibition activity. All of the plants tested showed antimicrobial effect against Bacillus cereus in both fresh and oil extracts (Table 2). For antimicrobial activity against Salmonella typhi, all plants except ginger showed high activity in oil extracts but ginger, galangal and kaffir lime leaves showed no activity in fresh extracts (Table 3). All except ginger in fresh extracts showed antimicrobial activity against Staphylococcus aureus (Table 4). The confirmatory step was performed using the Cup Plate Agar Diffusion Method and gave the same results as in the second step (Table 5). It was also shown that the oil extracts exhibited higher antimicrobial activity than fresh extracts. Kaffir lime peels, shallot and lime peels showed high potential on antimicrobial activity against Bacillus cereus, Salmonella typhi and Staphylococcus aureus. The results from this study indicated the possibility of using medicinal herbs from Thai food ingredients as natural antimicrobial agents. Appropriate extracting conditions and identification of active compounds through several purification methods should be examined in future studies. CONCLUSIONS Six of the fifteen selected medicinal herbs used as Thai food ingredients, ginger, galangal, kaffir lime leaves, kaffir lime peels and lime peels showed antimicrobial activity against Bacillus cereus, Salmonella typhi and Staphylococcus aureus on the first screening step. In the second and confirmatory steps, kaffir lime peels, shallot and lime peels showed high sensitivity for all studied microbials. The oil extracts exhibited higher antimicrobial activity than fresh extracts. The potential of kaffir lime peels, shallot and lime peels as natural antimicrobial agents have been confirmed. Further studies on active compound identification and suitable purification of these medicinal plants are suggested.