Garlic: A Survey Into Its Antibacterial Properties

On my latest Antibiotic Resistance I got a question by @learnsthgnew about the potential of several plants such as garlic and onions in disease prevention. I really like garlic and I cook with it a lot, so I decided to take a closer look at garlic and it´s potential health benefits. Rather than just telling you about what I found, I will walk you through the process of finding and analyzing research. I am going into this without any expectations. This will be an interesting topic. So here we go.

Step 1: Google Scholar

Yes, a Google search engine. Whatever you may think, this engine is very helpful for finding research articles. As you can see in this screenshot, I typed in "garlic health". It is a very general search term, but I am just starting my initial research. This is a promising start. The first three results are review articles about the health benefits of garlic and a substance called Allicin.

The first article I am checking out is called Allicin and Other Functional Active Components in Garlic: Health Benefits and Bioavailability. Sadly this is neither a free article, nor can I access it through my university. However, I do have access to the abstract, which is a summary of the article. The abstract mentions antimicrobial activity, anticancer activity, antioxidant activity among others. It also refers to allicin again. Sadly I cannot read the full article and the research postest, so this is merely a starting point.
The next article I am going to look at is called Garlic: Health benefits and actions, another review article I sadly cannot access. However, the abstract sounds very exciting again. It states that garlic has been used as a remedy for infection, colds, diabetes, heart disease. Furthermore, it mentions that garlic has been investigated for clinical use to lower blood pressure and cholesterol. After reading these two abstracts, I think I am on to something. The next article Antioxidant Health Effects of Aged Garlic Extract, is a hit. This is a free article, so I not only have access to the article, but also the works cited.
At this point, I need to make a few decisions. There are many effects garlic seems to have and I guess I could spend weeks researching. Due to time constraints I decided to focus on the antibacterial effects of garlic (as was asked by @learnsthgnew).

Garlic And Antibacterial Effects

After reading a few abstracts once compound arises as the potential antibacterial component of garlic - allicin. I am now reading Alliumsativum (garlic) — A natural antibiotic. Again, I can only access the abstract to this study, and it is from 1983 (for health related articles prefer sources from the last 4-5 years), however, it gives us a clue. It states that garlic has been found to have broad-spectrum antimicrobial activity against clinically relevant bacteria. I found another article titled Allicin, a naturally occurring antibiotic from garlic, specifically inhibits acetyl-CoA synthetase. It is from the 90s but it identifies a specific target of allicin, acetyl-CoA synthetase, an enzyme found in plants, fungy, bacteria and also mammals. This study found that allicin inhibts acetyl-CoA in bacteria, plants and mamals. While this may seem great at first, it may be problematic that allicin is not selective for only bacteria (which means an increased risk for side-effects). However, lets keep looking for sources before drawing conclusions.

A study - Activities of Garlic Oil, Garlic Powder, and Their Diallyl Constituents against Helicobacter pylori - has found a dose-dependent effect of several garlic derivatives on heliobacter pliori diseases. It seems that there may be some promise of garlic to fight diseases such as acid reflux and stomach ulcers.
Here is a list of bacterial strains that garlic inhibits according to Protection against Helicobacter pylori and Other Bacterial Infections by Garlic.

Aerobacter, Aeromonas, Bacillus, Citrella, Citrobacter, Clostridium, enterobacter, Escherichia, Klebsiella, Lactobacillus, Leuconostoc, Micrococcus, Mycobacterium, Proteus, Providencia, Pseudomonas, Salmonella, Serratia, Shigella, Staphylococcus, Streptococcus and Vibrio

I think at this points I feel comfortable to say yes, garlic has antibacterial effects. Now, what do we do with this information. I found an interesting study called Antibacterial Potential of Garlic-Derived Allicin and Its Cancellation by Sulfhydryl Compounds. Sadly, I am confined to the abstract once more. The abstract states that garlic extract was more potent against staphylococcus than allicin. When comparing allicin to clinically used antibiotics, it become evident that allicin is not nearly as potent. I wish I could access the whole study because I think this may have some data that would be very useful for this post. Well, maybe I can find another study. I could not sadly. However, I found more studies that were confirming the antibacterial effect of garlic. For example Staphylococcal growth and enterotoxins (A—D) and thermonuclease synthesis in the presence of dehydrated garlic shows that dehydrated garlic inhibited the growth of Staphylococcus aureus as well as the production of several enterotoxins.

Therapeutic values of onion (Allium cepa L.) and garlic (Allium sativum L.) confirms therapeutic effects of garlic oils. This study also warns that such product should be consumed in limited quantities, as they were found to be inhibiting thiol group enzymes. However, there are also studies that lowered the expectations. Allylsulfide constituents of garlic volatile oil as antimicrobial agents demonstrates that garlic is more effective against certain fungi, rather than bacteria. However, this study focused on only two strains of bacteria, and did find inhibitory effects of garlic on one of the species at high doses.
The mode of action of allicin is not understood yet. This study - The mode of action of allicin: trapping of radicals and interaction with thiol containing proteins - indicates that the main way that allicin acts is not through antioxidant activity, but rather by interaction with SH containing enzymes by a thiol-disulfide exchange reactions.
I was hoping to find some more specific posts so I went onto NCBI and searched for allicin antibiotic, which showed me that there are efforts to derive antibiotics that mimic allicin action. Allicin-inspired thiolated fluoroquinolones as antibacterials against ESKAPE pathogens and Allicin-inspired pyridyl disulfides as antimicrobial agents for multidrug-resistant Staphylococcus aureus present such antibiotics. I will not follow this trail because I want to focus on garlic and its properties, rather than mimicing antibiotics. However, if you are interested, check out the two linked studies and search out "allicin antibiotics" on NCBI. I moved on to a more general search on NCBI, entering "garlic antibiotics" as my search term.

The first study I am looking it is called Antimicrobial and antioxidant properties of various Greek garlic genotypes, which shows that the effects of garlic are more pronounced in certain types of garlic. While this study focused only on Greece, this may give us an idea that some garlic is better than other. While Phytochemical and Biological Activities of Pseudocalymma elegans: A False Garlic does not talk about garlic, it examines the effects of substances responsible for the garlic like smell in Pseudocalymma elegans. Some of which, like diallyl trisulfide, are constituents in garlic or very similar. Diallyl Polysulfides from Allium sativum as Immunomodulators, Hepatoprotectors, and Antimycobacterial Agents showed that compounds found in garlic have a potentially liver-protecting effect, while having antibacterial action.

Summary

Well this was a lot of information. So let me try and summarize this. I found a lot of studies that showed antibacterial effects of garlic. Sadly, I could not find any clinical studies because this sounds very promising. I am honestly wondering why garlic is not used more often. Possible reasons I can imagine is people disliking garlic, flatulence and maybe allergies. But considering the possible effects of garlic, these seem like rather small sacrifices. An encouraging thing at this point: there is a lot of research done about garlic and its constituents. This is likely due to the fact that we are faced with a growing number of antibiotic resistant bacterial strains, and garlic has been shown to be effective a few resistant strains. While this survey has found that garlic has numerous health benefits, I would not recommend eating three gloves of garlic a day now. Rather than that, incorporate a little more garlic into your diet. When you make scrambled eggs for example, add a little garlic shortly before you serve the eggs - just as an example. It is important to not heat the garlic too much, because that would destroy many of the beneficial components. I hope this posts helps to answer your question @learnsthgnew. Certainly, I did learn something new.
I hope I was further able to walk you through the process of finding sources appropriate for a scientific argument (or, as in this case, a science-based and informed opinion). Please feel encouraged to check out the studies I cited, as well as, conducting a search on your own using Google Scholar and NCBI.

Smallprint

Thank you very much for reading today´s post. Leave an upvote if you liked it. If there is anything that you want to add or contribute please do so in the comments. Maybe you like some of my other posts so check out my blog.

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Every picture is CC0, which means public domain and that it can be used by anybody without restrictions and without attribution.
While I do put a lot of effort into researching and writing my posts, the day only has so many hours and I have other commitments. There is a reason why I publish on Steemit and not in Nature. Please understand this post as an educated opinion and treat it as such. After all, my username is @lesshorrible and not "nothingbuttruth".

Again, thank you for reading and upvoting!

As always,
Cheers @lesshorrible!