In vitro antimicrobial activity of less-utilized spice and herb extracts against selected food-borne bacteria

In this study we compared the antimicrobial activities of extracts from four under-utilized spices and herbs including goraka (Garcinia quaesita), galangal (Alpinia galanga), lemon iron bark (Eucalyptus staigerana) and mountain pepper (Tasmannia lanceolata) to the three common spices and herbs pepper (Piper nigrum), rosemary (Rosmarinus officinalis), and oregano (Oreganum vulgare). Different extraction solvents were used (water, ethanol and hexane) and extracts were tested against four food-borne bacteria (Escherichia coli, Salmonella Typhimurium, Listeria monocytogenes and Staphylococcus aureus) using agar disc diffusion and broth dilution assays. Solvent type greatly influenced the antimicrobial activity of the spice and herb extracts except for those of P. nigrum, which had little or no activity. In general the spice and herb extracts with antimicrobial activity were more effective against Gram-positive than Gram-negative bacteria. Extracts from the under-utilized herbs and spices had significant activity. In particular, A. galanga hexane and ethanol extracts and E. staigerana ethanol and water extracts had strong antimicrobial activity against S. aureus and/or L monocytogenes. Interestingly the minimal inhibitory concentrations determined using the broth dilution method and the diameter of inhibition zones using the disc diffusion assay were not strongly correlated (r(2) ranged from 0.10 to 0.70) in most extracts, suggesting that choosing just one method for antimicrobial testing may lead to indefinite conclusions. The total phenolic content of two extracts from each spice and herb was assayed to establish any relationship between antimicrobial activity and phenolic compound levels, however this was found to poorly correlated (r(2) <0.30). This study has demonstrated that simple extracts of novel under-utilized herbs and spices have potential antimicrobial activity against food-borne bacterial species.