Allicin: chemistry and biological properties

Āyurveda's Contributions to Vegetarian Nutrition in Medicine

Āyurveda is one of the oldest codified traditional systems of medicine on a global scale. Grounded in Indian culture it has developed a sophisticated approach to healthy nutrition and nutritional therapy for the management of diseases within the framework of its whole systems paradigm. Medical interventions have been considered to be ineffective without the support of a balanced diet. Vegetarian nutrition plays a key role in customized preventive and therapeutic Āyurvedic strategies based on an individualized approach to healthcare, while broadly labeling Āyurveda as 'vegetarian medicine' would be an exaggeration. Revolt against animal slaughter, compassion through spiritual practices, and the Āyurvedic understanding of the nutritional properties of the plant kingdom led to the dominance of vegetarianism in India. Āyurveda elaborates on 8 crucial aspects to be considered with respect to a balanced diet. This paper illuminates the complex historic embedding of the Āyurvedic nutritional approach and gives a detailed text-based explanation on why and how vegetarian nutrition can be applied in both prevention and cure of diseases from the perspective of traditional Indian medicine.

Allicin Induces Thiol Stress in Bacteria through S-Allylmercapto Modification of Protein Cysteines

Allicin (diallyl thiosulfinate) from garlic is a highly potent natural antimicrobial substance. It inhibits growth of a variety of microorganisms, among them antibiotic-resistant strains. However, the precise mode of action of allicin is unknown. Here, we show that growth inhibition of Escherichia coli during allicin exposure coincides with a depletion of the glutathione pool and S-allylmercapto modification of proteins, resulting in overall decreased total sulfhydryl levels. This is accompanied by the induction of the oxidative and heat stress response. We identified and quantified the allicin-induced modification S-allylmercaptocysteine for a set of cytoplasmic proteins by using a combination of label-free mass spectrometry and differential isotope-coded affinity tag labeling of reduced and oxidized thiol residues. Activity of isocitrate lyase AceA, an S-allylmercapto-modified candidate protein, is largely inhibited by allicin treatment in vivo. Allicin-induced protein modifications trigger protein aggregation, which largely stabilizes RpoH and thereby induces the heat stress response. At sublethal concentrations, the heat stress response is crucial to overcome allicin stress. Our results indicate that the mode of action of allicin is a combination of a decrease of glutathione levels, unfolding stress, and inactivation of crucial metabolic enzymes through S-allylmercapto modification of cysteines.

Allicin inhibits oxidative stress-induced mitochondrial dysfunction and apoptosis by promoting PI3K/AKT and CREB/ERK signaling in osteoblast cells

Osteoporosis is a disease of the skeleton that is characterized by the loss of bone mass and degeneration of bone microstructure, resulting in an increased risk of fracture. Oxidative stress, which is known to promote oxidative damage to mitochondrial function and also cell apoptosis, has been recently indicated to be implicated in osteoporosis. However, there are few agents that counteract oxidative stress in osteoporosis. In the present study, the protective effects of allicin against the oxidative stress-induced mitochondrial dysfunction and apoptosis were investigated in murine osteoblast-like MC3T3-E1 cells. The results demonstrated that allicin counteracted the reduction of cell viability and induction of apoptosis caused by hydrogen peroxide (H₂O₂) exposure. The inhibition of apoptosis by allicin was confirmed by the inhibition of H₂O₂-induced cytochrome c release and caspase-3 activation. Moreover, the inhibition of apoptosis by allicin was identified to be associated with the counteraction of H₂O₂-induced mitochondrial dysfunction. In addition, allicin was demonstrated to be able to significantly ameliorate the repressed phosphoinositide 3-kinase (PI3K)/AKT and cyclic adenosine monophosphate response element-binding protein (CREB)/extracellular-signal-regulated kinase (ERK) signaling pathways by H₂O₂, which may also be associated with the anti-oxidative stress effects of allicin. In conclusion, allicin protects osteoblasts from H₂O₂-induced oxidative stress and apoptosis in MC3T3-E1 cells by improving mitochondrial function and the activation of PI3K/AKT and CREB/ERK signaling. The present study implies a promising role of allicin in oxidative stress-associated osteoporosis.

Allicin inhibits cell migration and invasion in human hepatocellular carcinoma cell line MHCC97H

AIM: To observe the effect of allicin on the proliferation, migration and invasion of human hepatocellular carcinoma cell line MHCC97H and to explore the underlying mechanisms.

METHODS: MTT assay and transwell assay were used to assess the proliferation, migration and invasiveness of MHCC97H treated with varying concentrations of allicin for 24 h. Real-time fluorescence quantitative PCR was performed to detect the mRNA expression of MMP-2, MMP-9 and CD24.

RESULTS: MTT assay demonstrated that the proliferation of MHCC97H cell was inhibited by different concentrations of allicin. Transwell assay demonstrated that compared with the negative control group, the number of penetrating cells in the experimental group was decreased significantly (P < 0.01). Real-time fluorescence quantitative PCR showed that allicin could down-regulate the mRNA levels of MMP-2, MMP-9, and CD24 in a dose-dependent manner in MHCC97H cells (P < 0.01).

CONCLUSION: Allicin inhibits the proliferation, migration and invasion of MHCC97H cells via mechanisms possibly related to down-regulation of the mRNA expression of MMP-2, MMP-9, and CD24.

Use of allicin as feed additive to enhance vaccination capacity of Clostridium perfringens toxoid in rabbits

The present study assessed the efficacy of Clostridium perfringens (C. perfringens) toxoid and/or allicin - as feed additive - in rabbits for preventing or minimizing the severity of infection with locally isolated strain of C. perfringens type A. Serum biochemical, immunological and pathological investigations were also done. One hundred rabbits of 6 weeks of age were divided into five equal groups (G1-G5). G1 were kept as normal control. G2 was allocated for C. perfringens type A infection. G3 was vaccinated with C. perfringens toxoid at zero time and then with a booster dose at the 3rd week of the experimental period. G4 was treated with allicin 20% added to the ration (200mg/kg ration) all over the experimental period. G5 was vaccinated with C. perfringens toxoid at the zero time then with a booster dose at the 3rd week of the experiment period, and treated with allicin 20% from the zero time till the end of the experiment. At the 4th week, G2, G3, G4 and G5 were challenged orally (5 ml) and subcutaneously (2 ml) with 24h cooked meat broth containing 1 × 10(7) colony-forming units/ml of C. perfringens type A strain. Blood and tissue samples were collected from all groups po st-vaccination then post-challenge for biochemical analysis, serum neutralization test and histopathological examinations. Results revealed that rabbits treated with both allicin and toxoid vaccine demonstrated high level of antitoxin titre post-challenge, improved liver and kidney functions, and reduced morbidity and mortality rates and the severity of histopathological changes associated with challenge of rabbits with C. perfringens type A strain. In conclusion, vaccination of rabbits with C. perfringens toxoid combined with allicin 20% gave better protection, enhanced immune response and had no adverse effects on the general health conditions against C. perfringens type A infection compared to rabbits vaccinated with C. perfringens toxoid only.

Inhibition of α-Glucosidase by Thiosulfinate as a Target for Glucose Modulation in Diabetic Rats

Postprandial hyperglycemia is a predisposing factor for vascular dysfunction and organ damage. α-glucosidase is a hydrolytic enzyme that increases the glucose absorption rate and subsequently elevates blood glucose levels. Garlic (Allium sativum L.) is a rich source of several phytonutrients, including thiosulfinate (THIO). The aim of this study was to evaluate the ability of THIO, a potent inhibitor of intestinal α-glucosidase, to reduce postprandial blood glucose. Male albino rats were randomly assigned to five different groups (n = 10/group). Group 1 served as the control group. Groups 2–5 were injected intraperitoneally with a single dose of streptozotocin (STZ) to induce diabetes. Group 2 comprised untreated diabetic rats. Groups 3 and 4 contained diabetic rats that were given THIO orally (20 mg/kg body weight/day and 40 mg/kg body weight/day, resp.). Group 5 was the positive control having diabetic rats treated orally with acarbose (10 mg/kg body weight/day; positive control). Diabetic rats treated with THIO displayed a significant blood glucose reduction (p < 0.001 and < 0.01 by analysis of variance, resp.) and a significant elevation in insulin compared with that of untreated rats. THIO is an effective noncompetitive intestinal α-glucosidase inhibitor that promotes hypoglycemic action (p < 0.001) in STZ-injected rats. THIO is a promising agent for the management of postprandial hyperglycemia.

Inhibition of streptococcal pyrogenic exotoxin B using allicin from garlic

Streptococcal pyrogenic exotoxin B (SpeB) is an important virulence factor of group A streptococci (GAS) and inactivation of SpeB results in the significantly decreased virulence of the bacterium. The protein is secreted as an inactive zymogen of 40 KDa (SpeBz) and undergoes proteolytic truncation to result in a 28 KDa mature active protease (SpeBm). In this study the effect of allicin on the proteolytic activity of SpeBm was evaluated using azocasein assay. Allicin neutralized the SpeBm proteolytic activity in a concentration dependent manner (IC50 = 15.71 ± 0.45 μg/ml). The loss of activity was completely reversed by subsequent treatment with a reducing agent, dithiothreitol (DTT; 10 mM final concentration), suggesting that allicin likely inhibits the SpeBm by forming a disulfide linkage with an active thiol group in its active site. This mechanism of action was further confirmed with the fact that DTT did not reverse the SpeBm activity in the presence of E-64, a cysteine protease-specific inhibitor, which works specially by forming a thioether linkage with free sulfhydryl groups in enzymes active site. The MIC of allicin against GAS was found to be 32 μg/ml. Exposure of GAS culture to allicin (25 μg/ml) inhibited maturation of SpeBz to the SpeBm. In conclusion, the results of this study suggest that allicin inhibits the maturation of SpeBz and proteolytic activity of SpeBm and could be a potential therapeutic agent for the treatment of GAS infections.

Characteristics and Health Benefits of Phytochemicals

In food science the term 'phytochemicals' includes a variety of plant ingredients with different structures that are capable of health-promoting effects. Phytonutrients are natural substances but are not called nutrients in the traditional sense, since they are synthesized by plants neither in energy metabolism nor in anabolic or catabolic metabolism, but only in specific cell types. They differ from primary plant compounds in that they are not essential to the plant. Phytonutrients perform important tasks in the secondary metabolism of plants as repellents to pests and sunlight as well as growth regulators. They occur only in low concentrations and usually have a pharmacological effect. Since antiquity, these effects have been used in naturopathy in the form of medicinal herbs, spices, teas, and foods. With the development of highly sensitive analytical methods, a variety of these substances could be identified. These phytochemicals may have health benefits or adverse health effects, depending on the dosage. In the past, these effects were studied in cell and tissue cultures as well as in animal models. Meanwhile there are numerous epidemiological data that point to the extensive health potential of phytochemicals in humans. A high dietary intake of phytochemicals with vegetables, fruits, nuts, legumes, and whole grain is associated with a reduced risk for cardiovascular and other diseases.

Diallyl Sulfide and Its Role in Chronic Diseases Prevention

Diallyl sulfide (C₆H₁₀S, DAS) is one of the novel natural organosulfur compounds, which is mostly obtained from the genus Allium plants. Numerous studies have revealed several unique properties of DAS in terms of its health-promoting effects. DAS has proved to be anticancer, antimicrobial, anti-angiogenic, and immunomodulatory like unique functions as demonstrated by the multiple investigations. Diallyl sulfide can also impede oxidative stress and chronic inflammation as suggested by the literature. Studies also explored that DAS could thwart the development of chronic diseases like cancer, neuronal, cardiovascular disease through modulating mechanistic pathways involved in pathogenesis. In this book chapter, we have attempted to give the comprehensive view on DAS about the physiochemical and biological properties, and its preventive role in chronic diseases with a mechanistic overview.

A Simple Synthesis of Alliin and allo-Alliin: X-ray Diffraction Analysis and Determination of Their Absolute Configurations

A simple method for the isolation of the bioactive compound alliin from garlic, as well as a method for the synthesis of diastereomerically pure alliin and allo-alliin on a preparative laboratory scale, was developed. The absolute configuration of the sulfur atom in alliin and allo-alliin was assigned on the basis of enzyme reactivity, optical rotatory dispersion, and circular dichroism analyses. A comparison of the results from these analyses, in combination with an X-ray diffraction study on a protected allo-alliin derivative, revealed S and R configurations of the sulfur atoms in alliin and allo-alliin, respectively. In addition, the same (1)H NMR spectrum was observed for synthetic and natural alliin. The absolute configuration of natural alliin was assigned for the first time on the basis of the NMR spectrum and X-ray coordinates.

Attenuation of oxidative stress, inflammation, and endothelial dysfunction in hypercholesterolemic rabbits by allicin

Allicin, the active substance of garlic, exerts a broad spectrum of pharmacological activities and is considered to have potential therapeutic applications. The present study was designed to investigate the possible beneficial effects of allicin against oxidative stress, inflammation, and endothelial dysfunction in hypercholesterolemic rabbits. Male New Zealand white rabbits were used in this study. Rabbits randomly received 1 of the following treatments: normal chow diet for 4 weeks, 1% high cholesterol diet (HCD), HCD plus allicin (10 mg/kg/day), or HCD plus atorvastatin (10 mg/kg/day). Blood samples were collected at the end of experimental diets for measurement of serum total cholesterol (TC), triglycerides (TGs), high-density lipoprotein cholesterol (HDL-C), C-reactive protein (CRP), malondialdehyde (MDA), reduced glutathione (GSH), and superoxide dismutase (SOD). In addition, the aorta was removed for measurement of vascular reactivity, histopathological changes, intima/media (I/M) ratio, and immunohistochemical staining of both tumor necrosis-alpha (TNF-α) and nuclear factor (NF)-κB. HCD induced significant increases in serum TC, TGs, low-density lipoprotein cholesterol (LDL-C), CRP, and MDA. Moreover, HCD caused significant decrease in serum GSH and SOD. In addition, aortic relaxation response to acetylcholine (ACh) was impaired. Immunohistochemical staining of aortic specimens from HCD-fed rabbits revealed high expression levels of both TNF-α and the oxidant-induced transcription factor, NF-κB. Allicin supplementation significantly decreased serum MDA and CRP, increased serum HDL-C, GSH, and SOD levels while nonsignificantly affecting HCD-induced elevations in serum TC and LDL-C. Additionally, allicin significantly protected against HCD-induced attenuation of rabbit aortic endothelium-dependent relaxation to ACh and elevation in I/M ratio. This effect was confirmed by histopathological examination of the aorta. Moreover, allicin has substantially beneficial effects on aortic expression of TNF-α and NF-κB compared with HCD-fed rabbits. In conclusion, these findings demonstrate that allicin may be useful in reducing oxidative stress, inflammation, vascular dysfunction, and the aortic pathology in hypercholesterolemic rabbits.

In vitro efficacy of allicin on chicken Eimeria tenella sporozoites

Chicken coccidiosis is a major parasitic disease caused by Eimeria spp. It is controlled and treated using chemical anticoccidial agents. Development of partial or complete resistance toward these anticoccidials is considered a major problem in poultry industry. Allicin is an organosulfur compound produced as a result of the reaction between alliin and alliinase after hacking of garlic. In this study, tenfold dilution from 180 mg/ml to 1.8 ng/ml of allicin in distilled water was tested against E. tenella in vitro. The percent of inhibition in allicin was from 99.9 to 71.53% using 180 mg/ml and 180 ng/ml, respectively. The percent of inhibition was 56.24% using 1.8 ng/ml. We used allicin as a treatment from plants against chicken coccidiosis; however, in vivo study should be performed to confirm these results.

Allicin as a possible adjunctive therapeutic drug for stage II oral submucous fibrosis: a preliminary clinical trial in a Chinese cohort

The objective of this study was to investigate the efficacy and safety of allicin in the treatment of stage II oral submucous fibrosis (OSF) in a Chinese patient cohort. A randomized clinical trial was performed. Triamcinolone acetonide (TA) or allicin was injected intralesionally weekly for 16 weeks. Improvements in mouth opening, burning sensation, and oral health-related quality of life were evaluated. Forty-eight subjects completed the study without obvious adverse reactions. At 40 weeks, the net gain in mouth opening was 2.27 ± 0.84 mm in the TA group and 5.16 ± 1.04 mm in the allicin group. Burning sensation improved by 2.79 ± 0.87 in the TA group and by 4.33 ± 1.04 in the allicin group. The OHIP-14 score improved by 4.67 ± 2.94 in the TA group and by 12.58 ± 9.82 in the allicin group. Allicin intralesional injections improved mouth opening, burning sensation, and oral health-related quality of life in these stage II OSF patients. Allicin appears to be a potential adjunctive therapeutic drug.

Mechanistic studies of a novel C-S lyase in ergothioneine biosynthesis: the involvement of a sulfenic acid intermediate

Ergothioneine is a histidine thio-derivative isolated in 1909. In ergothioneine biosynthesis, the combination of a mononuclear non-heme iron enzyme catalyzed oxidative C-S bond formation reaction and a PLP-mediated C-S lyase (EgtE) reaction results in a net sulfur transfer from cysteine to histidine side-chain. This demonstrates a new sulfur transfer strategy in the biosynthesis of sulfur-containing natural products. Due to difficulties associated with the overexpression of Mycobacterium smegmatis EgtE protein, the proposed EgtE functionality remained to be verified biochemically. In this study, we have successfully overexpressed and purified M. smegmatis EgtE enzyme and evaluated its activities under different in vitro conditions: C-S lyase reaction using either thioether or sulfoxide as a substrate in the presence or absence of reductants. Results from our biochemical characterizations support the assignment of sulfoxide 4 as the native EgtE substrate and the involvement of a sulfenic acid intermediate in the ergothioneine C-S lyase reaction.

Oxidation of Disulfides to Thiolsulfinates with Hydrogen Peroxide and a Cyclic Seleninate Ester Catalyst

Cyclic seleninate esters function as mimetics of the antioxidant selenoenzyme glutathione peroxidase. They catalyze the reduction of harmful peroxides with thiols, which are converted to disulfides in the process. The possibility that the seleninate esters could also catalyze the further oxidation of disulfides to thiolsulfinates and other overoxidation products under these conditions was investigated. This has ramifications in potential medicinal applications of seleninate esters because of the possibility of catalyzing the unwanted oxidation of disulfide-containing spectator peptides and proteins. A variety of aryl and alkyl disulfides underwent facile oxidation with hydrogen peroxide in the presence of catalytic benzo-1,2-oxaselenolane Se-oxide affording the corresponding thiolsulfinates as the principal products. Unsymmetrical disulfides typically afforded mixtures of regioisomers. Lipoic acid and N,N′-dibenzoylcystine dimethyl ester were oxidized readily under similar conditions. Although isolated yields of the product thiolsulfinates were generally modest, these experiments demonstrate that the method nevertheless has preparative value because of its mild conditions. The results also confirm the possibility that cyclic seleninate esters could catalyze the further undesired oxidation of disulfides in vivo.

Allicin Alleviates Dextran Sodium Sulfate- (DSS-) Induced Ulcerative Colitis in BALB/c Mice

The objective of this study is to evaluate the effect of allicin (10 mg/kg body weight, orally) in an experimental murine model of UC by administering 2.5% dextran sodium sulfate (DSS) in drinking water to BALB/c mice. DSS-induced mice presented reduced body weight, which was improved by allicin administration. We noted increases in CD68 expression, myeloperoxidase (MPO) activities, and Malonaldehyde (MDA) and mRNA levels of proinflammatory cytokines, such as tumor necrosis factor- (TNF-) α, interleukin- (IL-) 1β, IL-6, and IL-17, and decrease in the activities of enzymic antioxidants such as superoxide dismutase (SOD), Catalase (CAT), Glutathione reductase (GR), and Glutathione peroxidase (GPx) in DSS-induced mice. However, allicin treatment significantly decreased CD68, MPO, MDA, and proinflammatory cytokines and increased the enzymic antioxidants significantly (P < 0.05). In addition, allicin was capable of reducing the activation and nuclear accumulation of signal transducer and activator of transcription 3 (STAT3), thereby preventing degradation of the inhibitory protein IκB and inducing inhibition of the nuclear translocation of nuclear factor (NF)-κB-p65 in the colonic mucosa. These findings suggest that allicin exerts clinically useful anti-inflammatory effects mediated through the suppression of the NF-κB and IL-6/p-STAT3^Y705 pathways.

Effect of combined treatment with cyclophosphamidum and allicin on neuroblastoma-bearing mice

OBJECTIVE

To evaluate the efficacy of allicin combined with cyclophosphamide on neuroblastoma (NB)-bearing mice and explore the immunological mechanism in it.

METHODS

A total of 30 NB-bearing mice were equally randomized into model group, cyclophosphamide group and combined therapy group, 10 nudemice were set as normal saline (NS) group. Cyclophosphamide group and combined therapy group were weekly injected with 60 mg/kg cyclophosphamide for four weeks; besides, combined therapy group was given with allicin (10 mg/kg/d) by gastric perfusion for 4 weeks; model group and NS group were given with the same volume of NS. Serum VEGF content was detected by ELISA pre-treating (0 d) and on the 3rd d, 14th d and 28th d; on 29th d, all mice were sacrificed and the tumor, liver, spleen and thymic tissues were weighted. Tumors were made into paraffin section for detecting tumor cell apoptosis and proliferation by TUNEL and BrdU method, respectively. Survival curves were drawn by Kaplan-Meier method.

RESULTS

After treatment, both treatment groups relieved on viscera indexes, VEGF level, T cell subsets distribution and tumor growth and each index of combined therapy group was better than cyclophosphamide group (P<0.05 or 0.01); only combined therapy group could significantly increase the lifetime of NB-bearing mice (μ (2)=5.667, P=0.017).

CONCLUSIONS

Allicin can improve T cell subsets distribution and inhibit VEGF expression through its immunomodulatory activity, thereby improve the efficiency on NB in coordination with cyclophosphamide.

Analyzing the antibacterial effects of food ingredients: model experiments with allicin and garlic extracts on biofilm formation and viability of Staphylococcus epidermidis

To demonstrate different effects of garlic extracts and their main antibiotic substance allicin, as a template for investigations on the antibacterial activity of food ingredients. Staphylococcus epidermidis ATCC 12228 and the isogenic biofilm-forming strain ATCC 35984 were used to compare the activity of allicin against planktonic bacteria and bacterial biofilms. The minimal inhibitory concentration (MIC) and the minimum biofilm inhibitory concentration (MBIC) for pure allicin were identical and reached at a concentration of 12.5 μg/mL. MBICs for standardized garlic extracts were significantly lower, with 1.56 and 0.78 μg/mL allicin for garlic water and ethanol extract, respectively. Biofilm density was impaired significantly at a concentration of 0.78 μg/mL allicin. Viability staining followed by confocal laser scanning microscopy showed, however, a 100% bactericidal effect on biofilm-embedded bacteria at a concentration of 3.13 μg/mL allicin. qRT-PCR analysis provided no convincing evidence for specific effects of allicin on biofilm-associated genes. Extracts of fresh garlic are more potent inhibitors of Staphylococcus epidermidis biofilms than pure allicin, but allicin exerts a unique bactericidal effect on biofilm-embedded bacteria. The current experimental protocol has proven to be a valid approach to characterize the antimicrobial activity of traditional food ingredients.

Garlic revisited: antimicrobial activity of allicin-containing garlic extracts against Burkholderia cepacia complex

The antimicrobial activities of garlic and other plant alliums are primarily based on allicin, a thiosulphinate present in crushed garlic bulbs. We set out to determine if pure allicin and aqueous garlic extracts (AGE) exhibit antimicrobial properties against the Burkholderia cepacia complex (Bcc), the major bacterial phytopathogen for alliums and an intrinsically multiresistant and life-threatening human pathogen. We prepared an AGE from commercial garlic bulbs and used HPLC to quantify the amount of allicin therein using an aqueous allicin standard (AAS). Initially we determined the minimum inhibitory concentrations (MICs) of the AGE against 38 Bcc isolates; these MICs ranged from 0.5 to 3% (v/v). The antimicrobial activity of pure allicin (AAS) was confirmed by MIC and minimum bactericidal concentration (MBC) assays against a smaller panel of five Bcc isolates; these included three representative strains of the most clinically important species, B. cenocepacia. Time kill assays, in the presence of ten times MIC, showed that the bactericidal activity of AGE and AAS against B. cenocepacia C6433 correlated with the concentration of allicin. We also used protein mass spectrometry analysis to begin to investigate the possible molecular mechanisms of allicin with a recombinant form of a thiol-dependent peroxiredoxin (BCP, Prx) from B. cenocepacia. This revealed that AAS and AGE modifies an essential BCP catalytic cysteine residue and suggests a role for allicin as a general electrophilic reagent that targets protein thiols. To our knowledge, we report the first evidence that allicin and allicin-containing garlic extracts possess inhibitory and bactericidal activities against the Bcc. Present therapeutic options against these life-threatening pathogens are limited; thus, allicin-containing compounds merit investigation as adjuncts to existing antibiotics.

Special issue: redox active natural products and their interaction with cellular signalling pathways

During the last decade, research into natural products has experienced a certain renaissance. The urgent need for more and more effective antibiotics in medicine, the demand for ecologically friendly plant protectants in agriculture, “natural” cosmetics and the issue of a sustainable and healthy nutrition in an ageing society have fuelled research into Nature’s treasure chest of “green gold”. Here, redox active secondary metabolites from plants, fungi, bacteria and other (micro-)organisms often have been at the forefront of the most interesting developments. These agents provide powerful means to interfere with many, probably most cellular signaling pathways in humans, animals and lower organisms, and therefore can be used to protect, i.e., in form of antioxidants, and to frighten off or even kill, i.e., in form of repellants, antibiotics, fungicides and selective, often catalytic “sensor/effector” anticancer agents. Interestingly, whilst natural product research dates back many decades, in some cases even centuries, and compounds such as allicin and various flavonoids have been investigated thoroughly in the past, it has only recently become possible to investigate their precise interactions and mode(s) of action inside living cells. Here, fluorescent staining and labelling on the one side, and appropriate detection, either qualitatively under the microscope or quantitatively in flow cytometers and plate readers, on the other, enable researchers to obtain the various pieces of information necessary to construct a fairly complete puzzle of how such compounds act and interact in living cells. Complemented by the more traditional activity assays and Western Blots, and increasingly joined by techniques such as proteomics, chemogenetic screening and mRNA profiling, these cell based bioanalytical techniques form a powerful platform for “intracellular diagnostics”. In the case of redox active compounds, especially of Reactive Sulfur Species (RSS), such techniques have recently unraveled concepts such as the “cellular thiolstat”, yet considerably more research is required in order to gain a full understanding of why and how such compounds act—often selectively—in different organisms.