Garlic Counteracts Virulent Bacteria

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Garlic has long been recognised as a malodorous but important medical plant with a wide range of health supporting properties. Until recently when Turmeric overtook it, it had the greatest number of research papers in PubMed for a food derived health supporting ingredient. In relation to its content of small molecule chemistry, garlic not only has antibacterial properties; it has antiviral, antifungal and antiprotozoal properties as well, and it has beneficial effects on the cardiovascular and immune systems.[1]

However, despite its long-time favourite amongst natural health care professionals it remains something of a mystery in the manner in which it impart its help, in particular in its capacity for resolving stubborn bacterial infections.

Garlic contains a substance that is particularly effective in encounters with even the hardiest bacterial strains. This substance is called ajoene,[2] a sulphur-containing compound with potential as an anti-pathogenic drug.  Ajoene is the major component in a cocktail of quorum sensing inhibitory sulphur-containing compounds, which is produced when garlic is crushed. It is important to note that QS inhibitors do not kill target bacteria, but they weaken their biofilms to the exposure of conventional antibiotics and the immune system.

Across the world aggressive multi-resistant infections constitute an increasing health problem. The underlying biology behind their resistance is thought to be the ability of bacteria to form biofilms, which consist of structured and aggregated (often surface-attached) communities of bacteria.[3]

Bacteria are also developing resistance at an alarming pace, so new pharmaceuticals and natural agents that can combat this threat are in great demand, but few are being developed and a real threat exists as to whether any will be in the near future.

The majority of antimicrobials target a limited number of basal life processes in bacteria, such as DNA and RNA replication, protein biosynthesis and cell wall synthesis. These were the ‘low-hanging fruits’ for traditional methods of antibiotic discovery. Although antibiotics have saved millions of lives, they strongly promote development of resistant bacterial variants by their mode of action.

Ajoene is a potent natural chemical compound that neutralises resistant bacteria by paralysing their communication system. Ajoene specifically prevents the bacteria from secreting the toxin rhamnolipid[4] a compound that shields the microbe from normally antimicrobial (phagocytic) white blood cells. White blood cells are indispensable because they play a crucial role in the immune defence system, not only warding off infection, but also killing bacteria.

A tough sheath of biofilm

When bacteria clump together in what is known as biofilm – where they surround themselves with a tough film of organic materials and they become resistant to antibiotics. Researchers have been devoting much of their attention to Pseudomonas aeruginosa bacteria, which cause infections in patients with chronic leg ulcers, for example, and in the lungs of patients suffering from cystic fibrosis.

Ajoene supplementation (Garlic concentrates) supports and improves treatment with conventional antibiotics and therefore may extend the efficacy and lifespan of the currently available medications, whilst also conferring direct local benefits. Whilst individually there appears to be significantly limited direct effect – one alters biofilm aggregation and the other kills the bacteria – when taken together the effects are greatly enhanced.[5]

Combination treatment with ajoene and antibiotics kills more than 90 per cent of the normally virulent biofilm.[6],[7]

From a technical perspective, the ajoene blocks the communication system – known as Quorum Sensing – in the bacteria, which is used for purposes including creating infection.


It is unlikely that any natural food source has biologically relevant amounts of quorum sensing inhibitors to cure already-established infections. For example, to produce the eradication effect obtained previously with garlic extract on infected mice would for an adult human require the daily intake of 50 bulbs of garlic for several days. This may result in garlic concentrates that contain ajoene to be used rather than raw garlic when infections are present. Other foods apart from garlic that show promise as a preventative strategy include beansprouts, chamomile, carrots, lemongrass, chili habanero, yellow peppers and beeswax all of which show quorum sensing inhibition activities.

What is particularly interesting is that the combination of quorum sensing inhibitors and other antimicrobials (in this way accessing a number of antimicrobial targets) may produce synergistic effects that would reduce the otherwise higher concentrations that are required of each of the pure compounds. This is known to be true for certain antibiotics, but should also be relevant for natural antimicrobials as well – indicating that garlic supplementation at the same time as antimicrobial treatment will induce a greater effect in bacterial infections.


[1] Block E. Garlic and Other Alliums: The Lore and the Science Publisher. Royal Society of Chemistry, Cambridge, UK (2010).

[2] Jakobsen TH, van Gennip M, Phipps RK, Shanmugham MS, Christensen LD, Alhede M, Skindersoe ME, Rasmussen TB, Friedrich K, Uthe F, Jensen PØ, Moser C, Nielsen KF, Eberl L, Larsen TO, Tanner D, Høiby N, Bjarnsholt T, Givskov M. Ajoene, a sulfur-rich molecule from garlic, inhibits genes controlled by quorum sensing. Antimicrob Agents Chemother. 2012 May;56(5):2314-25. View Abstract

[3] Microbial biofilms. Costerton JW, Lewandowski Z, Caldwell DE, Korber DR, Lappin-Scott HM. Annu Rev Microbiol. 1995;49:711-45. Review. View Abstract

[4] Chrzanowski Ł, Ławniczak Ł, Czaczyk K. Why do microorganisms produce rhamnolipids? World J Microbiol Biotechnol. 2012 Feb;28(2):401-19. View Full Article

[5] Givskov M. Future Microbiol. Beyond nutrition: health-promoting foods by quorum-sensing inhibition. 2012 Sep;7(9):1025-8. doi: 10.2217/fmb.12.84. View Full Article

[6] Bhardwaj AK, Vinothkumar K, Rajpara N. Bacterial quorum sensing inhibitors: attractive alternatives for control of infectious pathogens showing multiple drug resistance. Recent Pat Antiinfect Drug Discov. 2013 Apr;8(1):68-83. View Abstract

[7] Christensen LD, van Gennip M, Jakobsen TH et al. Synergistic antibacterial efficacy of early combination treatment with tobramycin and quorum-sensing inhibitors against Pseudomonas aeruginosa in an intraperitoneal foreign-body infection mouse model. J. Antimicrob View Full Article

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