Antimicrobial Effects of Indigo Naturalis

Antimicrobial Effects of Indigo Naturalis

Indigo naturalis ( Qing Dai in Chinese) is a dark-blue powder originating from the leaves and branches of various indigo-producing plants such as Baphicacanthus cusia (Nees) Bremek, Indigofera tinctoria L., Isatis indigotica Fort, Polygonum tinctoria Ait, and Strobilanthes formosanus Moore.

Indigo naturalis has been used as a textile dye, paint pigment and medicine in China for many centuries. In traditional Chinese medicine (TCM), indigo naturalis has been used commonly for treating various infectious and inflammatory skin diseases. Modern research has revealed that indigo naturalis is useful for antipyretic, anti-inflammatory, antiviral, antimicrobial, anticancer and detoxifying purposes.

Taiwan clinical centres used indigo naturalis to treat more than 10,000 patients with psoriasis during the past 11 years. Clinical trials showed that topical application of indigo naturalis significantly improved skin psoriasis. In the past 5 years, indigo naturalis extract in oil (Lindioil) has been used to treat nail psoriasis and a preliminary non-controlled trial found such a treatment to be effective. In their previous observations, topical indigo naturalis can ameliorate psoriatic nails coexisting with fungus and bacterial infection, such as onychomycosis and chronic paronychia. Nail psoriasis is a notoriously difficult disease to treat and has the following characteristics: nail dystrophy, thickening, loss of luster, raising, changes in color, and friability. Dystrophic nails lose their natural protective barrier and are, therefore predisposed to fungal infections. People with nail psoriasis have a higher incidence of onychomycosis, which is the most common nail disease worldwide, constituting almost half of all onychopathies. The causative pathogens in onychomycosis include dermatophytes, Candida species, and non-dermatophytic molds. Dermatophyes are the most common pathogens that infect the nail apparatus. Among them, Trichophyton rubrum is the most common, and Epidermophyton floccosum, Trichophyton violaceum, Microsporum gypseum and Trichophyton tonsurans are less common. On the other hand, onychomycosis caused by Candida spp. and non-dermatophytic molds (such as Aspergillus spp.) represents a higher incidence in the tropics and subtropics with a hot and humid climate. Paronychia is an inflammation of the structure surrounding the nails. Whether acute or chronic, paronychia is often associated with onycholysis and/or nail plate dystrophy which results in a breakdown of the protective barrier between the nail and the nail fold. The introduction of organisms into the moist nail crevice results in bacterial or fungal (yeast or mold) colonization of the area. Paronychia results from multi-microbial infections, with the presence of both aerobic and anaerobic bacteria in about three-fourths of the cases. Typically Staphylococcus aureus, Streptococcus spp.

Klebsiella pneumoniae, Pseudomonas aeruginosa, and Candida albicans are the predominant pathogens. S. aureus and C. albicans cause soft tissue infection of the nail fold, which often induces painful acute inflammation in chronic paronychia. Pseudomonas may colonize the dystrophic nail plate, causing a greenish discoloration. Although clinical observations have indicated that indigo naturalis can ameliorate psoriatic nail coexisting with onychomycosis and paronychia, it is still unknown if the topical therapy directly inhibits the growth of pathogens on the nails and/or prevents the invasion of pathogens by restoring the damaged nail structure. The aim of this study is to investigate whether indigo naturalis possesses antimicrobial effects on the pathogens of skin and nail infections. The active compounds of indigo naturalis were purified and identified. In addition, a metabolomic approach was applied to elucidate the origin of its bioactive compounds.