Preliminary Phytoconstituent Analysis and In Vitro Evaluation of Antibacterial and Antioxidant Activities of Verbascum sinaiticum Benth Leaf Extracts.

Abstract

Verbascum sinaiticum Benth. commonly known as "Qetetina" in Ethiopia, the family Scrophulariaceae has considerable importance in traditional medicine worldwide because of its antioxidant and anti-inflammatory activities. V. sinaiticum is one of the commonly used traditional medicinal plants in Ethiopia for treating wounds, cancer, and animal trypanosomiasis. Hence, the present study was undertaken to screen the phytochemical constituents and evaluate the antibacterial and antioxidant activity of V. sinaiticum chloroform and methanol leaf extracts against two Gram-positive bacteria (Staphylococcus aureus and Streptococcus pneumoniae) and two Gram-negative bacterial strains (Klebsiella pneumoniae and Escherichia coli) by the agar disc well diffusion method. The study revealed the presence of phenols, flavonoids, steroids, saponins, terpenoids, alkaloids, and tannins. Thus, the present investigation was undertaken to estimate total phenols and flavonoids in chloroform and methanol extracts of V. sinaiticum leaves. In vitro antioxidant activity was also evaluated in the methanol and chloroform extracts using DPPH and FRAP assays. The DPPH assay showed that standard IC₅₀ for ascorbic acid (AA) was 23.79 μg/mL, whereas for methanol extract the IC₅₀ value was 29.88 μg/mL and 86.38 μg/mL for chloroform extract, while the FRAP assay showed standard IC₅₀ value for AA as 46.2 μg/mL, 80.3 µg/mL for methanol extract and 201.2 µg/mL for chloroform extract with p values < 0.05, which indicates that methanol extract exhibits higher antioxidant activity than chloroform crude extract. The methanolic extracts of V. sinaiticum leaves showed moderate amounts of total phenolic and flavonoid contents, 7.81 ± 0.14 mg GAE/g (milligrams of gallic acid equivalent) and 14.20 ± 0.98 mg QE/g of dried crude, respectively. Using the agar well diffusion method, the two extracts were evaluated against Gram-positive (S. aureus and S. pneumonia) and Gram-negative (Salmonella typhi and E. coli) bacterial strains. The leaf extract displayed a greater impact on both Gram-positive bacterial strains and Gram-negative bacterial strains. 21.7 ± 0.6 against E. coli at concentrations of 100 and 50 μg/mL and 19.7 ± 0.6 against S. typhi compared to positive control ciprofloxacin 23.6 ± 0.4 and 28.6 ± 0.6, respectively. The minimum inhibitory concentration assay was determined using the concentrations of 100 mg/mL, 50 mg/mL, 25 mg/mL, and 12.5 mg/mL by the agar well diffusion method. All extracts had shown a significant antibacterial activity (p value < 0.05) within the average inhibition zone ranging from 10.2 ± 1.3 to 21.7 ± 0.6. V. sinaiticum leaves showed strong antibacterial activities against S. aureus, S. pneumonia, E. coli, and S. typhi. Therefore, it is evident from this study that the highest therapeutic efficacy possessing majority of secondary metabolite classes of compounds in the leaf extract of V. sinaiticum, which can be quantified for application in the pharmaceutical industry. Especially, flavonoids and tannins were strongly detected. Finally, the authors recommend future isolation and structural characterization of specific bioactive compounds, as well as in vivo toxicity and biological studies.