Indigo naturalis is effective in treating nail
psoriasis coexisting with microorganism
infections. This study examines the antimicrobial effects of
indigo naturalis prepared from Strobilanthes formosanus Moore. Eight bacterial and seven fungal strains were assayed using the
agar diffusion method to examine the effects of
indigo naturalis and its bioactive compounds. The bioactive compounds of
indigo naturalis were purified sequentially using GFC, TLC, and HPLC. Their structures were identified using mass spectrometry and NMR spectroscopy. UPLC-MS/MS was applied to compare the metabolome profiles of
indigo naturalis
ethyl-acetate (EA) extract and its source plant, Strobilanthes formosanus Moore. The results of in vitro antimicrobial assays showed that
indigo naturalis EA-extract significantly (≥1 mg/disc) inhibits Gram-positive bacteria (Staphylococcus aureus, S. epidermis and methicillin-resistant S. aureus (MRSA)) and mildly inhibits non-dermatophytic
onychomycosis pathogens (Aspergillus fumigates and Candida albicans), but has little effect on dermatophyes.
Isatin and
tryptanthrin were identified as the bioactive compounds of
indigo naturalis using S. aureus and S. epidermis as the bioassay model. Both bioactive ingredients had no effect on all tested fungi. In summary,
indigo naturalis prepared from Strobilanthes formosanus Moore exhibits antimicrobial effects on Staphylococcus and non-dermatophytic
onychomycosis pathogens.
Tryptanthrin and
isatin may be its major bioactive ingredients against Staphylococcus and the inhibitory effect on MRSA may be due to other unidentified ingredients.