Garlic

GARLIC

Garlic (Allium sativum) has been used as a culinary ingredient for thousands of years, but it has also been shown to possess a range of pharmacological activities leading to uses in cardiovascular disease and cancer. Its use in these conditions is outside the remit of this chapter; however, of interest here is the fact that it has also been reported to have antimicrobial activity against a broad spectrum of bacterial pathogens.

The chemistry of garlic is highly complex and like other plant extracts the material is made up of a large number of compounds. The main sulphur-containing components of intact garlic are γ-glutamyl-S-allyl cysteines and S-allyl-l-cysteine sulphoxides (of which the main one is known as alliin). When garlic is crushed or damaged, the enzyme alliinase, which is normally contained within vacuoles, comes into contact with the cytosolic cysteine sulphoxides and converts them to thiosulphinates such as allicin. This process is thought to represent a protective effect for the plant because assault by predators (worms, bacteria, etc.) would result in a localized high concentration of these toxic products. Allicin was the component that was first shown to be responsible for the marked antimicrobial activity of fresh garlic, and on the basis of this a number of garlic-containing products were marketed for their antibacterial effects. However, allicin was subsequently found to be highly unstable and to degrade rapidly within a matter of days to a variety of sulphides in both aqueous and alcoholic solutions. In the context of the plant’s protective strategy this makes sense, because having generated a toxic metabolite to counter an insult it is in the plant’s best interest to neutralize that toxin as rapidly as possible after the invader has been repelled.

Orally administered allicin is degraded in the stomach acid and is not absorbed from the gut, thus demonstrating that it cannot be responsible for any of the reported in vivo antibacterial effects. Attention has thus switched to other components, particularly the allicin breakdown products including diallyl sulphide and diallyl disulphide. Studies in mice have shown that oral dosing of these agents can reduce MRSA viability in blood, liver, kidney and spleen. They also provide immunological protective properties. However, the number of studies conducted is small and the methodologies variable. More comprehensive clinical trials are required together with a rigorous study of the role the individual components play in the overall biological activity.

Garlic suffers from many of the issues raised above for TTO in that the composition of the product varies markedly depending on the source of the raw material, how it is processed to extract the active ingredients, how it is formulated and how it is stored. Inconsistencies reported for the activity of garlic preparations are primarily due to a lack of standardization of the product. As things currently stand, garlic is unlikely to contribute significantly to the resolution of the issue of diminishing antibiotic availability.