Bioassay guided fractionation of rhizome extract of Rheum emodi wall as bio-availability enhancer of antibiotics against bacterial and fungal pathogens.

Rheum emodi Wall., is an important medicinal plant extensively used in Ayurvedic and Unani systems of traditional medicine. It is known to possess antioxidant, antibacterial, antifungal, anticancer, wound healing and immune enhancing activities.The aim of the current study was to investigate the antimicrobial activity and synergistic potential of different solvent fractions and phytocompounds of Rheum emodi rhizome against bacterial and fungal pathogens.The antimicrobial and synergistic potential of the crude methanolic extract, different solvent fractions (n-hexane, chloroform, ethyl acetate, and residual aqueous) and isolated phytocompounds of the rhizome of Rheum emodi were assayed by broth microdilution method. The bioactive phytocompounds were isolated through silica TLC and quantified using HPTLC and HPLC. The bioactive phytocompounds were identified by LC-MS analysis.

Phytochemical analysis of the sub-fractions showed that the TPC (417.94 ± 1.2 mg g^-1 GAE) and TFC (187.40 ± 0.5 mg g^-1 RE) were highest in residual aqueous extracts. The chloroform sub-fraction possessed the highest antimicrobial activity against bacterial (Escherichia coli, Staphylococcus aureus and Klebsiella pneumoniae) and fungal strains (Candida albicans MTCC 277 and ATCC 90028). The MIC of chloroform sub-fraction against S. aureus, K. pneumoniae, E. coli, C. albicans was 1.95, 3.91, 15.62 and 62.5 μg ml^-1, respectively. TLC and LC-MS analysis of chloroform sub-fraction identified phytocompounds namely emodin D4 (m/z 274.262), rhein13c6 (m/z 290.176), chrysophanol dimethyl ether (m/z 282.291), and resveratrol (m/z 340.456). Quantification of emodin content showed that the chloroform sub-fraction (101.4543 μg mg^-1, 194.8037 μg mg^-1 measured through HPTLC and HPLC, respectively), and its TLC fraction (II) (75.18 μg mg^-1, 232.384 μg mg^-1 measured through HPTLC and HPLC, respectively) are rich in emodin. Furthermore, chloroform sub-fraction, its TLC fractions and emodin showed profound synergistic activity in combination with antibacterial and antifungal antibiotics and lowered the dosage of antibiotics by 4-257 folds.

The bioassay guided fractionation of R. emodi rhizome methanolic extract identified phytocompounds (emodin, rhein13c6, chrysophanol dimethyl ether and resveratrol) that act as bioavailability enhancers of antibacterial and antifungal antibiotics, hence revealing their potential in treating multidrug resistance.