Abstract

In vitro antibacterial activities of five extracts from dietary medicinal plants were investigated by agar-well diffusion method (AWD), minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) against 13 foodborne pathogenic bacteria reference strains; four Gram positive bacteria including Staphylococcus aureus (NCINB 50080), Bacillus cereus (NCINB 50014), Citrobacter freundi (ATCC 8090) and Listeria innocua (ATCC 33090) as well as nine Gram negative bacterial reference strains including Escherichia coli (ATCC 11775), E. coli O157 (ATCC 700728), Salmonella typhimurium (ATCC 13311), Shigella boydii (ATCC 9207), Shigella sonnei (ATCC 25931), Shigella flexneri (ATCC 12022), Pseudomonas aeruginosa (NCINB 50067), Klebsiella pneumoniae (NCTC 9633) and Proteus mirabilis (ATCC 14153). Four ethanolic extracts underwent acetone wash then analyzed for their principal components using gas chromatography-mass spectrometry (GC-MS), Oleamide was the predominant compound in onion, garlic, wheat germ and Nigella sativa which have great antibacterial effect. The tested acetone extracts exhibit variable antibacterial activity against foodborne pathogens which differ according to the compounds clarified in the GC-MS analysis. Garlic extract showed the best antibacterial activities, GC-MS analysis showed the presence of five compounds including; tetrasulfide, monosilane, oleamide, stearoylamide and vitamin E.  Testing for the presence of 91 pesticides in the tested extracts using GC-MS analysis proved complete absence of pesticides which indicate that the antibacterial activities showed was due to the active components in the tested extracts and not due to the pesticides contaminants. Antimicrobial activities of plant extracts revealed that garlic has greatest inhibitory effect against S. aureus NCINB 50080 followed by S. Typhimurium ATCC 13311 with zone of inhibition 28 mm, 30 mm for AWDT and and 2.61 µg/ml for MIC, respectively. The best hindrance abilities was shown with garlic extracts with mean zone of inhibition (23.46 mm) followed by onion (18.15 mm), wheat germ  extract (17.38 mm), mint (17.15 mm) then Nigella sativa (15.69 mm). Results of MIC and MBC confirm the antibacterial activities of the tested extracts.

Key words: Antibacterial activity, agar well diffusion test (AWDT) minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), seed, bulb, ethanolic extracts.