2.6. Antibacterial activity

The agar disc diffusion method was used to check turmeric extracts' antibacterial activity as described in Gupta et al. (2015) with minor modifications. In this study, antibacterial patterns were studied for different extracts of C. domestica (rhizome) viz methanol extract (oleoresin) and aqueous extract (essential oil) for comparing three turmeric varieties. The standard strains Staphylococcus aureus ATCC 25923, Salmonella typhimurium ATCC 25953, and Escherichia coli ATCC 25922 used as test organism were obtained from collections of Ethiopian Biodiversity Institute, Addis Ababa. Turmeric extracts were dissolved in dimethyl sulfoxide (DMSO) (Sigma-Aldrich USA) to get 100 mg/mL concentration. Sterile filter paper discs that have 8 mm diameter were impregnated with 50 μl of diluted extract solution and then incubated for 15 min to ensure proper diffusion of the extract. The serial dilution was then surface spread onto nutrient agar plates with 0.1 ml of bacterial culture standardized to 0.5 McFarland standards (1.5 × 10⁸ CFU ml⁻¹) and incubated at 37 °C for 24 h. The inhibition zones' diameter was measured, and the mean value of three independent experiments (n = 3) was used in the statistical analysis. The control discs contained DMSO only, and the standard antibiotic gentamycin (10 μg/disc) was included in the assay as a positive control.