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.
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.
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