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.

Allicin (diallylthiosulfinate) is a defence molecule from garlic (Allium sativumL.) with broad antimicrobial activities in the low µM range against Gram-positive and -negative bacteria, including antibiotic resistant strains, and fungi. Allicin reacts with thiol groups and can inactivate essential enzymes. However, allicin is unstable at room temperature and antimicrobial activity is lost within minutes upon heating to >80 °C. Allicin’s antimicrobial activity is due to the thiosulfinate group, so we synthesized a series of allicin analogues and tested their antimicrobial properties and thermal stability. Dimethyl-, diethyl-, diallyl-, dipropyl- and dibenzyl-thiosulfinates were synthesized and testedin vitroagainst bacteria and the model fungusSaccharomyces cerevisiae, human and plant cells in culture andArabidopsisroot growth. The more volatile compounds showed significant antimicrobial properties via the gas phase. A chemogenetic screen with selected yeast mutants showed that the mode of action of the analogues was similar to that of allicin and that the glutathione pool and glutathione metabolism were of central importance for resistance against them. Thiosulfinates differed in their effectivity against specific organisms and some were thermally more stable than allicin. These analogues could be suitable for applications in medicine and agriculture either singly or in combination with other antimicrobials.

Garlic (Allium sativum) has potent antimicrobial activity due to allicin (diallylthiosulfinate) synthesized by enzyme catalysis in damaged garlic tissues. Allicin gives crushed garlic its characteristic odor and its volatility makes it potentially useful for combating lung infections. Allicin was synthesized (>98% pure) by oxidation of diallyl disulfide by H₂O₂ using formic acid as a catalyst and the growth inhibitory effect of allicin vapor and allicin in solution to clinical isolates of lung pathogenic bacteria from the genera Pseudomonas, Streptococcus, and Staphylococcus, including multi-drug resistant (MDR) strains, was demonstrated. Minimal inhibitory (MIC) and minimal bactericidal concentrations (MBC) were determined and compared to clinical antibiotics using standard European Committee on Antimicrobial Susceptibility Testing (EUCAST) procedures. The cytotoxicity of allicin to human lung and colon epithelial and murine fibroblast cells was tested in vitro and shown to be ameliorated by glutathione (GSH). Similarly, the sensitivity of rat precision-cut lung slices (PCLS) to allicin was decreased by raising the [GSH] to the approximate blood plasma level of 1 mM. Because allicin inhibited bacterial growth as a vapor, it could be used to combat bacterial lung infections via direct inhalation. Since there are no volatile antibiotics available to treat pulmonary infections, allicin, particularly at sublethal doses in combination with oral antibiotics, could make a valuable addition to currently available treatments.

SUMMARY: Downy mildew of Arabidopsis is not a hugely destructive disease of an important crop plant, neither is it of any economic importance. The most obvious symptom, the aerial conidiophores, might, at a glance to the casual observer, be mistaken for the trichomes normally present on the leaves. However, a huge research effort is being devoted to this humble pathosystem which became established as a laboratory model in the 1990s. Since then, enormous progress has been made in cloning and characterizing major genes for resistance ( RPP genes) and in defining many of their downstream signalling components, some of them RPP ‐gene specific. Resistance is generally associated with an oxidative burst and a salicylic acid dependent hypersensitive reaction type of programmed cell death. Biological and chemical induction of systemic acquired resistance (SAR) in Arabidopsis protecting against downy mildew were demonstrated early on, and investigations of mutants have contributed fundamentally to our understanding of host–pathogen interactions and the mechanisms of plant defence. This review will attempt to collate the wealth of information which has accrued with this pathosystem in the last decade and will attempt to predict future research directions by drawing attention to some still unanswered questions.  Taxonomy: Hyaloperonospora Constant. parasitica (Pers.:Fr) Fr. (formerly Peronospora parasitica ), Kingdom Chromista, Phylum Oomycota, Order Peronosporales, Family Peronosporaceae, Genus Hyaloperonospora , of which it is the type species. The taxonomy of the group of organisms causing downy mildew of brassicas has undergone a number of revisions since ) originally coined the genus Peronospora . All isolates pathogenic on brassicas were described initially as P. parasitica but ) classified isolates from different brassicaceous hosts distinctly and thus defined 52 new species based on conidial dimensions and host range. After much debate it was decided to revert to the aggregate species of P. parasitica for all brassica‐infecting downy mildews, whilst recognizing that these show some isolate‐specific differences (). The latest re‐examination of P. parasitica by ) has placed isolates of P. parasitica and five other downy mildew species in a clear new subgroup on the basis of their hyaline conidiospores, recurved conidiophore branch tips and ITS1, ITS2 and 5.8S rDNA sequence comparisons; meriting the coining of the new genus ‘ Hyaloperonospora Constant’. The class Oomycetes in the Kingdom Chromista (Straminipila) comprises fungus‐like organisms with heterokont zoospores (i.e. possessing two types of flagellae, whiplash and tinsel). The Oomycetes have non‐septate hyphae with cellulose‐based walls containing very little or no chitin. The latter is regarded as a major distinction separating the Oomycetes from the true fungi, and reports of the presence of chitin had generally been regarded as due to small amounts of contamination (). However, in view of recent studies by ) showing a chitin synthase gene in an Oomycete and demonstrating the presence of the polymer itself by an interaction with wheat germ agglutinin (WGA), it is perhaps safe to say that we have not seen the last taxonomic revision which will affect this group! The families within the Oomycetes show a clear evolutionary trend to a lesser absolute dependence on an aqueous environment and some members of the Peronosporales, e.g. H. parasitica , have no zoosporic stage in the life cycle.  Host range: Isolates infecting Arabidopsis thaliana have so far proven to be non‐pathogenic on other crucifers tested but exist in a clear gene‐for‐gene relationship with different host ecotypes.  Disease symptoms: Infections are first apparent to the naked eye as a carpet or ‘down’ of conidiophores covering the upper and lower surfaces of leaves and petioles. This symptom is characteristic of this group of diseases and lends it its name.  Useful websites: < ">http://ppathw3.cals.cornell.edu/PP644/ references.htm> (links to references on Oomycetes), < ">http://www.arabidopsis.org/> (TAIR, The Arabidopsis Information Resource).

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.