Selection of Thai medicinal plants for growth inhibition of Listeria monocytogenes

Listeria monocytogenes is a foodborne pathogen and causes listeriosis. The isolation and detection of L. monocytogenes were carried out from 17 kinds of food comprising of 170 samples. They were beef, pork, chicken, swine intestine, chicken liver, sausage, fermented pork, pork ball, cooked milk, pasteurized milk, cheese, lettuce, morning glory, coleslaw, cuttlefish, sea mussel and fish ball. All samples were colleted from various shops in Bangkok and Nonthaburi areas. Forty-eight samples were analysed to be contaminated by Listeria spp. Among these only 10 samples were identified as L. monocytogenes. The main sources of L. monocytogenes in this study were raw meat and poultry i.e., beef, pork, chicken, swine intestine and chicken liver. Crude ethanolic extracts from 39 Thai medicinal plants were screened for growth inhibition of isolated strains (i.e., P2-1, 4,11) and type strains (i.e., IFRPD2068 and KUS1) of L. monocytogenes by the well assay technique. Crude extracts of galingale (Boesenbergia pandurata Holtt.), jackfruit heartwood (Artocarpus heterophyllus Lam.), mace (Myristica fragrans Houtt.), leadwort (Plumbago indica Linn.), licorice (Glycyrrhiza glabra Linn.), sappan (Caesalpinia sappan Linn.) and indian senna (Cassia angustifolia Vahl.) revealed listerial growth inhibition. The minimum inhibitory (MIC) concentration of jackfruit heartwood for L. monocytogenes in broth was 500 micro g/ml, whereas mace and licorice were 2,000 micro g/ml. The extracts from galingale, leadwort and sappan could not completely inhibit listerial growth even at 5,000 micro g/ml. The effect of pH and temperature on antilisterial activity of plant extracts were explored. The result showed that the antilisterial activity of plant extracts decreased as the pH of medium increased. At low temperature (4 deg C), the plant extracts have less activity than high temperatures (35 and 45 deg C). The antilisterial activity of crude extracts from jackfruit heartwood was investigated. The ethyl acetate and methanolic extracts of jackfruit heartwood were active against L. monocytogenes while the hexane and dichloromethane had no activity against this tested organism. Crude methanolic extract from jackfruit heartwood was further chemically studied by quick column chromatography. The majority of the antilisterial activity was found in the fraction eluted by 100 percent ethyl acetate, however the activity of this fraction was comparatively lower tha the crude methanolic extract.