Development and Application of a Novel 'Green' Antibacterial Black Garlic (Allium sativum)-Based Nanogel in Epidermal Wound Healing§.

Research background: Black garlic has been reported to have several health-promoting properties compared to fresh, raw garlic. The enzyme alliinase, which converts alliin to allicin, is deactivated at moderately high temperature, thus stripping away the typical pungent odour of fresh garlic during fermentation and rendering black garlic devoid of the typical garlic-like smell. To date, the antimicrobial activity of alliin-rich extract obtained from black garlic powder has not been reported. The objectives of this study are to explore the antibacterial/antifungal activity of alliin-rich black garlic extract against Staphylococcus aureus, Escherichia coli and Candida albicans, and to formulate a topical drug, based on the efficacy of the extract, using non-toxic, green ingredients in the form of a nanogel with promising wound-healing property and safe for human use.

Experimental approach: Authenticated fresh garlic (Allium sativum) cloves were first fermented to yield black garlic. After fermentation, the brownish-black garlic cloves were peeled and ground into powder. The alliin-rich extract was then obtained by Soxhlet extraction. Nanogels were formulated using the alliin-rich extract and were subjected to a kinetic study of in vitro release. The antibacterial potency of the nanogels was also evaluated against Staphylococcus aureus (ATCC 29213) and Escherichia coli (ATCC 25922 and their multiple drug-resistant strains), followed by a skin irritation study on New Zealand albino rabbits.

Results and conclusions: Soxhlet extraction of pulverized black garlic cloves using distilled water yielded an alliin-rich extract (6.4 mg/100 g garlic), which also contained additional bioactive organosulfur compounds with no reported toxicity. The antimicrobial potency (in terms of its minimum inhibitory concentration (MIC)) of the extract was evaluated against potent skin pathogens and was found to be ~15 µg/mL. The nanogels formulated with the alliin-rich extract showed shear thinning rheology and admirable sensory properties when tested by a panel. The in vitro release kinetic study showed a burst release of alliin (75 % of its content) from either gel within 5 min. Following a skin irritation test performed on male New Zealand albino rabbits, no clinical signs of toxicity/mortality, redness or swelling were observed in the animals. The nanogels applied individually on the epidermal wounds prevented external infection and accelerated wound healing from day 2 onwards, achieving complete healing by day 7. Moreover, the gel containing 4 % extract did not leave a scar on the wounded area after complete healing on day 7, establishing it as a promising topical antibacterial nanogel with accelerated epidermal wound-healing property, compared to a commercial broad-spectrum topical gel, used as a positive control.

Novelty and scientific contribution: This study is the first to report on a newly developed 'green' nanogel containing antimicrobial bioactive compounds, namely, organosulfur compounds (diallyl disulfide, diallyl trisulfide, methyl-allyl-disulfide and methyl-allyl-trisulfide). The nanogel showed promising epidermal wound-healing properties and is therefore promising in clinical applications against common and potent human skin pathogens.