Allicin (diallylthiosulfinate) is a defence molecule from garlic (Allium sativum L.) with a broad range of biological activities. Allicin is produced upon tissue damage from the non-proteinogenic amino acid alliin (S-allylcysteine sulfoxide) in a reaction that is catalyzed by the enzyme alliinase. Current understanding of the allicin biosynthetic pathway will be presented in this review. Being a thiosulfinate, allicin is a reactive sulfur species (RSS) and undergoes a redox-reaction with thiol groups in glutathione and proteins that is thought to be essential for its biological activity. Allicin is physiologically active in microbial, plant and mammalian cells. In a dose-dependent manner allicin can inhibit the proliferation of both bacteria and fungi or kill cells outright, including antibiotic-resistant strains like methicillin-resistant Staphylococcus aureus (MRSA). Furthermore, in mammalian cell lines, including cancer cells, allicin induces cell-death and inhibits cell proliferation. In plants allicin inhibits seed germination and attenuates root-development. The majority of allicin's effects are believed to be mediated via redox-dependent mechanisms. In sub-lethal concentrations, allicin has a variety of health-promoting properties, for example cholesterol-and blood pressure-lowering effects that are advantageous for the cardio-vascular system. Clearly, allicin has wide-ranging and interesting applications in medicine and (green) agriculture, hence the detailed discussion of its enormous potential in this review. Taken together, allicin is a fascinating biologically active compound whose properties are a direct consequence of the molecule's chemistry.

The volatile organic sulfur compound allicin (diallyl thiosulfinate) is produced as a defense substance when garlic (Allium sativum) tissues are damaged, for example by the activities of pathogens or pests. Allicin gives crushed garlic its characteristic odor, is membrane permeable and readily taken up by exposed cells. It is a reactive thiol-trapping sulfur compound that S-thioallylates accessible cysteine residues in proteins and low molecular weight thiols including the cellular redox buffer glutathione (GSH) in eukaryotes and Gram-negative bacteria, as well as bacillithiol (BSH) in Gram-positive firmicutes. Allicin shows dose-dependent antimicrobial activity. At higher doses in eukaryotes allicin can induce apoptosis or necrosis, whereas lower, biocompatible amounts can modulate the activity of redox-sensitive proteins and affect cellular signaling. This review summarizes our current knowledge of how bacterial and eukaryotic cells are specifically affected by, and respond to, allicin.

The common foodstuff garlic produces the potent antibiotic defense substance allicin after tissue damage. Allicin is a redox toxin that oxidizes glutathione and cellular proteins and makes garlic a highly hostile environment for non-adapted microbes. Genomic clones from a highly allicin-resistant Pseudomonas fluorescens (PfAR-1), which was isolated from garlic, conferred allicin resistance to Pseudomonas syringae and even to Escherichia coli. Resistance-conferring genes had redox-related functions and were on core fragments from three similar genomic islands identified by sequencing and in silico analysis. Transposon mutagenesis and overexpression analyses revealed the contribution of individual candidate genes to allicin resistance. Taken together, our data define a multicomponent resistance mechanism against allicin in PfAR-1, achieved through horizontal gene transfer.

Allicin is a natural antibiotic produced by garlic as a defence against pathogens and pests. Due to the worldwide increase in antibiotic resistance, new antibiotics are desperately required. Allicin is such a candidate and is active against several multidrug-resistant (MDR) strains of human pathogens, including methicillin-resistant Staphylococcus aureus (MRSA). When administered orally, allicin is titrated out by glutathione in the cells and blood, and effective therapeutic concentrations are difficult to achieve at the site of an infection. However, in the case of lung infections, allicin can be delivered directly to pathogens via the pulmonary route. In this study, we designed and constructed an in vitro lung test rig, which allowed us to model accurately the exposure of lung air-passage surfaces to allicin and gentamicin, in order to examine the feasibility of combating lung infections by direct inhalation. A prototype test rig of lung bronchi with three bifurcations was constructed, which could be coated internally with a thin layer of bacteria-seeded agar medium. The deposition of antimicrobial aerosols on the modelled bronchial surfaces was followed in preliminary tests without the need for animal experiments. The differential sensitivity of the test bacteria to different antibiotics and the dose-dependency of inhibition was shown using the model. Furthermore, a synergistic effect of allicin vapour and ethanol in inhibiting bacterial growth was demonstrated. The modelling of the axial velocity air-flow distribution correlated with the regions indicating the inhibition of bacterial growth, demonstrating that the model has predictive value and can reduce the requirement for animal sacrifice in pre-clinical trials of novel antibiotics.

The antibiotic defense substance allicin (diallylthiosulfinate) is produced by garlic (Allium sativum L.) after tissue damage, giving garlic its characteristic odor. Allicin is a redox-toxin that oxidizes thiols in glutathione and cellular proteins. A highly allicin-resistant Pseudomonas fluorescens strain (PfAR-1) was isolated from garlic, and genomic clones were shotgun electroporated into an allicin-susceptible P. syringae strain (Ps4612). Recipients showing allicin-resistance had all inherited a group of genes from one of three similar genomic islands (GI), that had been identified in an in silico analysis of the PfAR-1 genome. A core fragment of 8-10 congruent genes with redox-related functions, present in each GI, was shown to confer allicin-specific resistance to P. syringae, and even to an unrelated E. coli strain. Transposon mutagenesis and overexpression analyses revealed the contribution of individual candidate genes to allicin-resistance. Moreover, PfAR-1 was unusual in having 3 glutathione reductase (glr) genes, two copies in two of the GIs, but outside of the core group, and one copy in the PfAR-1 genome. Glr activity was approximately 2-fold higher in PfAR-1 than in related susceptible Pf0-1, with only a single glr gene. Moreover, an E. coli Δglr mutant showed increased susceptibility to allicin, which was complemented by PfAR-1 glr1. Taken together, our data support a multi-component resistance mechanism against allicin, achieved through horizontal gene transfer during coevolution, and allowing exploitation of the garlic ecological niche. GI regions syntenic with PfAR-1 GIs are present in other plant-associated bacterial species, perhaps suggesting a wider role in adaptation to plants per se.

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