Onions And Garlic

Probably the most well characterized antimicrobial system in plants is that found in the juice and vapors of onions (Allium cepa) and garlic (Allium sativum). Growth and toxin production of many microorganisms have been shown to be inhibited by onion and garlic, including the bacteria Bacillus cereus, Clostridium botulinum type A, E. coli, Lactobacillus plantarum, Salmonella, Shigella, and Staphylococcus aureus, and the fungi Aspergillus flavus,A. parasiticus, Candida albicans, Cryptococcus,Rhodotorula, Saccharomyces, Torulopsis, and Trichosporon (18). The major antimicrobial component from garlic is allicin (dial-lyl thiosulfinate; thio-2-propene-l-sulfinic acid-5-allyl ester) (18), which is formed by the action of the enzyme al-linase on the substrate alliin (S-(2-propenyl)-L-cysteine sulfoxide). The reaction only occurs when cells of the garlic are disrupted, releasing the enzyme to act on the substrate. A similar reaction occurs in onion except the substrate is S-(l-propenyl)-L-cysteine sulfoxide and one of the major products is thiopropanal-S-oxide. The products apparently responsible for antimicrobial activity are also responsible for the flavor of onions and garlic. In addition to antimicrobial sulfur compounds, onions contain the phenolic compounds protocatechuic acid and catechol, which could contribute to their antimicrobial activity (90). The mechanism of action of allicin is most likely inhibition of sulfhydryl-containing enzymes (18).

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