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Not Just For Vampires!

Those who read my small intestinal bacterial overgrowth (SIBO) series know that I credit garlic, along with probiotics and prebiotics, for helping me cure myself of it. I described my experience in the last post of that series.

I’ve since received many emails from people about their experiences using this herb. Some have noted no changes whatsoever, while others have seen dramatic improvements in their gastrointestinal conditions.

As with any drug, both synthetic or herbal, your mileage may vary. That said, I still think raw garlic should be part of a healthy diet along with probiotic-rich foods.

Today I want to focus on some compounds in garlic that have some pretty powerful medicinal effects. In order to keep this post from morphing into a book, I plan on just covering those properties that relate to oral and gastrointestinal health.

Keep in mind that everything that follows relies on allicin formation, the active ingredient in garlic. Allicin and its numerous sulfur breakdown products include diallyl sufide, diallyl disulfide, diallyl penta sulfide and diallyl trisulfide to name a few. Actually, there are over 400 different compounds found in garlic, with about thirty having medicinal properties

An enzyme called allinase is activated when raw garlic is pressed, cut, diced or chewed. This enzyme interacts with alliin, the main sulfur-containing and odorless component found in garlic.

Allinase interacts with alliin forming allicin. It’s allicin that gives garlic its distinctive smell and taste that, depending on your preferences, can either make you feel very happy or gross you out.

Garlic imported into the United States undergoes irradiation by the U.S. Department of Agriculture. You can consider irradiation cold pasteurization in that it kills pathogens in foods by inactivating their DNA.

How this affects allinase is still an open question. I couldn’t find any research that specifically spoke to this issue. The closest I could find was a paper from the European Food Safety Authority.

This paper looked at four different enzymes (none of them allinase) and found that irradiation decreased enzymatic activity in all of them. I suspect this is also true for allinase. Therefore, I stand by my recommendation to buy organic garlic or garlic that is domestically grown.

Bacterial Pathogens

Garlic has been used for centuries against bacterial infections. It has been shown effective against Mycobacterium tuberculosis (the bacteria that causes tuberculosis), E. coli, Salmonella, Staphylococcus, Streptococus, Klebsiella, Proteus, Bacillus, Clostridium and Helicobacter pylory. (1)

It has also been proven effective against Campylobacter jejuni (C. jejuni). (2) C. jejuni is considered the most prevalent cause of food-borne illness in the world as it readily survives on food for extended periods of time. Signs of C. jejuni infection includes abdominal cramps, fever, diarrhea and bloody urine.

Let me interject here that food poisoning can easily cause SIBO, especially in those who have a preexisting case of gut dysbiosis. One of the many roles of beneficial gut flora is to support an environment inhospitable to pathogens.

One way commensal, lactic-acid-forming small intestinal bacteria do this is by producing bacteriocidals that kill or inhibit pathogen growth. Lactobacillus acidolphilus, for example, inhibits Salmonella infection by producing antimicrobial compounds that prevents its attachment to the gut wall. (3) L. acidolphilus also prevents Escherichia coli and Yersinia pseudotuberculosis from doing the same. (4)

When small intestinal dysbiosis is present, this defense mechanism is no longer operative, meaning that any pathogen that happens to breach the gastric barrier now has an opportunity to infect the gut wall. Once this happens, the formation of what are known as biofilms can result.

Biofilms are a community of pathogens. Many of the bacteria in these communities are of different types and capable of communicating with each other using signaling molecules in a process known as quorum sensing.

This community can be quite impervious to antibiotics because it emits a type of slime that protects its members from both your immune system and any antibiotics you may throw at it. Also, once in a biofilm, bacterial DNA can be switched on making these bacteria more virulent and able to evade the best antibiotics and immune defenses.

Because of this, biofilms are as much as 1,000 times more resistant to antibiotics than individual or planktonic microbes. (5) Effectively neutralizing these biofilms to treat infections is now the focus of intense interest in medical and pharmaceutical research.

What researchers have discovered is that garlic, specifically its diallyl sulfide components, can disrupt these biofilms. A May 2012 paper details an experiment where C. jejuni biofilms were subjected not only to diallyl sulfide, but also to the antibiotics ciproflaxacin and erythromycin. (6)

What these researchers found was that diallyl sulfide killed C. jujuni biofilms at concentrations that were 100-times less than either ciproflaxacin or erythromycin! These scientists note:

“Recent studies indicate that plants, specifically Allium spp., contain antimicrobial agents such as diallyl sulphide [diallyl sufide] that are highly effective against major foodborne pathogens. We hypothesized that diallyl sulphide might be more effective in inactivating bacterial biofilms than erythromycin or ciprofloxacin based on its ability to freely penetrate the phospholipid bilayers of bacterial cell walls. The objectives of this study were to compare systematically the effectiveness of diallyl sulphide with antibiotics commonly used to treat campylobacteriosis. The novelty of this study is that researchers have not examined the antimicrobial activity of diallyl sulphide against any type of bacterial biofilms, including C. jejuni. Moreover, we used biophysical and biochemical techniques to investigate the differences in antimicrobial mechanisms of diallyl sulphide and antibiotics against C. jejuni biofilms. We show for the first time that the anti-microbial activity of diallyl sulphide against C. jejuni planktonic cells and biofilms is much greater than that of selective antibiotics. In addition, we are the first to use vibrational spectroscopy to validate that C. jejuni planktonic cells have a different interaction mode with antibiotics compared with their sessile cell counterparts after biofilm EPS has been destroyed. Planktonic cells and sessile cells have a similar mode of susceptibility to diallyl sulphide, and it was much easier to destroy biofilm EPS with diallyl sulphide than with antibiotics.”

As mentioned, bacteria living in biofilms communicate with each other by quorum sensing. This ability to talk to each other is for one reason and one reason only, to ensure the survival of the community. Therefore, anything that inhibits this communication makes the biofilm more susceptible to destruction.

Another breakdown product of garlic is ajoene, which has been shown to prevent this communication. Ajoene has antimicrobial properties against a number of pathogens: Echerichia coli, Klebsiella pneumoniae and Xanthomonas maltophilia. However, ajoene by itself is ineffective against the biofilm-forming pathogen Pseudomonas aeruginosa (P. aeruginosa).

An interesting experiment that took note of ajoene’s ineffectiveness against P. aeruginosa biofilms tested whether it would be effective when paired with the antibioitic tobramycin. (7) Tobramycin was chosen because it too is ineffective against this particular biofilm.

However, when both antimicrobials were used, P. aeruginosa biofilms were eradicated. The researchers were able to prove that ajoene was inhibiting quorum sensing thus allowing tobramycin to kill off the now vulnerable pathogens. Pretty cool if you ask me and exciting news given the continuing growth of antibiotic-resistant bacterial strains.

Garlic is also effective against all tested oral pathogens. (8) Not only does it kill Porhpyromonas gingivalis, the oral pathogen responsible for chronic periodontal disease, it is highly effective against Streptococcus mutans, the major pathogen responsible for dental plaque and cavities. The presence of oral pathogens is a known risk factor for gastrointestinal and cardiovascular diseases as swallowing these pathogens can invade the gastrointestinal tract and enter systemic circulation when gastric-barrier function is compromised.


Garlic doesn’t just work against bacteria. It’s also able to kill the fungus Candida albicans.

I’ve stated in the past that I’m not entirely happy with the descriptive term, small intestinal bacterial overgrowth. Yes, the condition is characterized by an overgrowth of bacteria, typically gram-negative, in this part of the digestive tract.

And certainly in many cases this is all that’s happening. Nevertheless, in those who have had recurring courses of antibiotics, SIBO is likely to be accompanied by an overgrowth of Candida.

That was true for me and I suspect it’s the case for many. I believe that a number of people who fail to reap any lasting benefit from using antibiotics or other herbals to treat their SIBO may have an underlying Candida overgrowth that has gone unresolved.

Let me state for the record that there is no possibility of ever eradicating Candida. It’s been with you since you were born, and it will accompany you to the grave. It’s a normal part of your gut ecology, and as long as it’s kept in check by beneficial gut bacteria it’s totally harmless. And it’s entirely possible that future research may uncover that it has beneficial effects when part of a healthy gut flora, much as we’re learning about H. pylori. However, as with any organism an overgrowth can cause some intractable problems.

Garlic’s diallyl disulfide has been shown to deplete glutathione in Candida albicans. This is important because just as this powerful antioxidant protects our cells against free-radical damage, it does the same for Candida. It’s this property of Candida that thwarts your immune system’s ability to control it once it gets out of hand.

Diallyl disulfide, however, short-circuits this defense mechanism. It also inhibits mitochondrial respiration in this fungus, which also serves to kill it. (9)

If Candida is a persistent problem for you, I would strongly urge you to make use of garlic’s demonstrated ability to keep it under control. And used in a diluted wash, garlic is also effective against fungal skin outbreaks.


Garlic has proven quite lethal to parasites. It is able to kill Entamoeba histolytica and trypanosomes. (10) (11)

Garlic compounds have also been shown to kill Giardia intestinalis. Giardia is the most commonly diagnosed cause of diarrhea in the United States and worldwide. The cysts that transmit it can survive in cold water for several months and are readily transmitted by animals such as voles, muskrats and birds.

If ingested, these cysts are activated by stomach acid and can rapidly colonize the mid-section (jejunum) of the small intestine. This in turn can lead to malabsorption of food, nausea, diarrhea, bloating, cramps and weight loss. In the very young, the elderly and in those with compromised immune function, a Giardia infestation can be deadly.

Treatment involves anti-parasitic medication. Unfortunately, resistant strains of Giardia are emerging.

Garlic compounds rapidly diffuse throughout the biological membranes of this parasite making them fragile. These compounds also stimulate the production of nitric oxide synthase (NOS), and this has been shown to be highly toxic to Giardia.

Of the garlic compounds studied, the following were found to have the most effect against Giardia: allyl alcohol, allyl mercaptan, diallyl disulfide, dimethyl disulfide and methyl propyl sulfide. (12)

Colon Cancer Cells

Colon cancer is one of the most pervasive cancers worldwide. Tumors originate from polyps that grow on the inner wall of the colon.

Butyrate, the short-chain fatty acid produced by the fermentation of soluble fibers by bifidobacteria, has demonstrated effects in preventing colon-cancer cell proliferation and activating pathways that lead to cancer cell death or apoptosis. (13) Anyone out there dealing with colon cancer needs to increase their populations of bifidobacteria by consuming prebiotics.

Butyrate is not alone in its cancer-fighting effects, however. Diallyl disulfide has also been shown to cause death of HT-29 colon cancer cells. Combining butyrate and diallyl disulfide produces a pronounced synergistic effect. (14)

At equal concentrations, diallyl disulfide and butyrate reduces colon-cancer cell number by 3.6% and 6.8% respectively. Combined, the reduction rate is 12.6%.


In one of my earlier posts I called garlic one of my favorite prebiotics. Now you know why.

The medicinal properties of garlic are truly amazing. This post has only scratched the surface of what this herb is capable of. There’s a good reason it’s been used as a medicine for thousands of years and why it was nicknamed Russian penicillin.

Nonetheless, I want to offer a few caveats.

1) Many of these studies were conducted in labs, so how relevant they are in humans remains to be seen. That said most drugs are first developed this way as well, although they undergo further clinical trials to test for efficacy and safety.

2) Like any herbal medication, there is no controlling for the amount of active ingredient in whatever garlic preparation you take. I’ve had different reactions from one bulb to the next. Many factors can account for this variability: the soil the garlic was grown in, what fertilizers were used, pesticide use, time of year grown, how long since harvest, whether it was irradiated, etc. One huge advantage pharmaceuticals have over any herbal remedy is that the amount of active ingredient is a known variable.

3) Raw garlic is a prebiotic. As such it will be partly fermented by friendly gut flora in the colon into short-chain fatty acids, which is fantastic. But a necessary byproduct of any fermentation is gas production.

If bloating is already an issue for you, garlic can exacerbate it if you go overboard with it. The problem is defining what is “overboard”. For one person, ingesting two cloves of raw garlic may cause them no difficulty whatsoever, whereas for someone else it may cause a lot of bloating. It can be difficult controlling for this so caution must be exercised when taking it.

Finally, some people just can’t tolerate any garlic, raw or cooked. Either it makes them nauseous, or they just can’t handle the taste or smell.

And before sending any emails asking me to comment on this or that particular brand of garlic supplement, I want to say upfront that I have no opinion on any of them. What is advertised on the box or bottle may or may not reflect what’s actually in the pill you’re taking.

In the meantime, I’ll just stick to the real deal.

In an age of microbes and parasites increasingly resistant to the most powerful medications we have, I predict that garlic and its many compounds will be taking center stage in the world’s pharmacopeia.


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