Collection of plant specimens:
The plants i.e. Eclipta prostrata, Azadirachta indica, Aloe vera and Catharanthus roseus had been collected from their natural habitat, from different locations of University of Barishal, Bangladesh. The plant specimens were identified and authenticated in the Department of Botany, University of Barishal, Bangladesh using standard literature (Ahmed et al., 2008-2009). Then the plant specimens had been subjected to ex-situ conservation at the net house of mentioned University for future uses.
Preparation of extraction:
Collected medicinal plants leaves were washed with tap water, ethanol and distilled water; and dried in the sunshine and at oven (50?C temperature). Then the dried samples were ground by mortar and pestle (Meghla et al., 2016; Ramchandran et al., 2012). The conical flasks were sterilized in the oven for 1 hour. 5 g of powder from each of the samples was poured into the conical flask. Following this, 50 mL of ethanol (Merck, Germany) and 50 mL of methanol (Merck, Germany) was added to each flask separately and gently mixed it perfectly. Then solutions were shaken for 3 days at room temperature at orbital shaker (Model No-JSOS-300). The preparation of each solution was repeated and kept for 3 days at static conditions at room temperature. The extracts were filtered, centrifuged and evaporated (Meghla et al., 2016; Ramchandran et al., 2012) and were kept for further uses.
Collection of bacterial strain:
Isolated pathogenic bacterial strains of Escherichia coli and Staphylococcus aureus were collected form Popular Diagnostic Centre and Hospital, Sylhet. The collected strains were incubated in an incubator (Model No-Binder RL-53) and inoculated on sterile, analytical nutrient agar medium and stored at 4°C until further use.
Study of antibacterial activities:
The antibacterial activity of medicinal plants was studied by the disk diffusion method (Suhag et al., 2014; Bell et al., 2009). At first nutrient broth medium was prepared by beef extract (Sigma-Aldrich, India), yeast extract (Sigma-Aldrich, India), peptone (Sigma-Aldrich, India) and NaCl (Sigma-Aldrich, India) in distilled water and autoclaved for 15 minutes. Then a single colony of Escherichia coli and Staphylococcus aureus bacteria was added in 20 mL broth separately and incubated at 37°C for 24 hours. After that, a stock solution of 50 mg mL−1 was made by dissolving ethanol extract of medicinal plants in distilled water. 10 μL of stock solution (500 μg sample) was socked in each sterilized filter paper (Whatman, 0.45 micro pore) disk of uniform diameter (5 mm) and thickness (1 mm). Then 25 μL of activated bacterial solution was placed onto the surface of an agar plate (agar plate was prepared by 35 gL-1 of bacteriological nutrient agar) with the help of micropipette with sterilized tips and spread evenly over the surface by using a sterilized bent glass rod.
Isolation of DNA:
DNA was isolated from human cheek cells by general salt extraction process (by using sodium chloride and detergent solution), purified by chloroform and precipitated in ice-cooled ethanol (Garbieri et al., 2017). Purity of DNA was evaluated by a UV-Vis spectrophotometer (Lamda-365).
DNA binding studies:
The qualitative viscosity experiments were carried out by using Ostwald viscometer at room temperature. The relative viscosity of solutions was measured at a fixed concentration of DNA (25 μM) and different concentrations of plant extracts (50-500 ppm) with respect to pure DNA solution (25 μM) in water (Ramachandran et al., 2012 and Prasad et al., 2011).