Garlic's secret weapon
Eager to answer the essential "how-come" question, Mirelman and colleagues looked at Entamoeba and found that allicin can defang two common enzymes Entamoeba needs to enter and damage cells. Enzymes are the chemicals that do life's heavy lifting; they initiate, for example, the chemical changes that convert starch to sugar, or carbohydrate to fat.)

The mouth-twisting enzymes in question -- cystiene proteinase and alcohol dehydrogenase -- are called thiol enzymes. Mirelman suspects that allicin inhibits other pathogens by interfering with their thiol enzymes.

Given the growing problem of antibiotic resistant bacteria, such a broad-spectrum antibiotic could be extremely handy if, as Mirelman suspects, pathogens indeed had a hard time evolving a way to overcome allicin.

One bright bulb...
Evolution, curiously enough, may explain why garlic makes allicin in the first place: If you're a lowly bulb growing in the dirt, the ability to defeat soil fungi, bacteria and parasites would obviously help in your endless struggle for survival.

Mirelman's team also solved one of life's minor mysteries: why garlic is odorless until it's crushed. Garlic contains no allicin, but it does have separate compartments that hold a precursor molecule and an enzyme that converts this "raw material" into allicin.

When mechanical damage -- by a garlic press or a soil organism -- breaks these compartments, the chemicals combine and the resulting chemical reaction makes allicin.

The mechanism -- which reminds us of the two-part canisters used in nerve gas warheads -- is elegant and effective, Mirelman says. "If you assume a fungus is attacking a garlic clove, it will cause the generation of allicin in a very specific area of the attack. Not all of the clove will be involved."

And that's just as well, since allicin could harm the very cells in the garlic it's supposedly protecting. Although high concentrations of allicin are toxic to mammalian cells, a protective chemical in mammals can repair some of allicin's damage.

There's further evidence of the system's self-regulation. Most enzymes are present in tiny quantities, but garlic contains equal amounts of precursor and enzyme. Mirelman explains that the enzyme is "suicidal. Allicin kills the enzyme, and even though raw material for more allicin remains available, the result is a very localized, short-lived reaction that works like a shotgun against any intruder."

And now for the bad news
So far, we've spoken of a potential antibiotic whose only drawback seems to be the bad breath it causes. But there are some drawbacks. First, allicin affects the two ameba enzymes only when fresh -- it breaks down and evaporates quickly, and cooking nukes its antibiotic qualities.

Second, since the active chemical is also the one with the odor, allicin therapy could drive away fastidious friends.

Third, nobody knows if allicin can actually be developed into a safe, broad-spectrum antibiotic until it's proven in clinical trials. Unfortunately drug companies don't like spending megabucks on chemicals like allicin that can't be patented because they were discovered too long ago.

Still, we're not talking about something obscure. So long as you've a garlic press, allicin is available "over the counter" in the supermarket.

In fact, you might find it near onions, which also may make medicinal chemicals when injured.