Optimization of antibacterial activity of ethanolic extracts of Eucalyptus tereticornis and Nigella sativa: Response surface methodology

Abstract

The screening of plants for medicinal purposes represents an effort to discover newer, safer, and possibly more effective drugs. Design of the present study was made aiming to the optimization of the antibacterial activity of ethanolic extracts of Eucalyptus tereticornis (leaves) and Nigella sativa (seeds) against bacteria belongings to both Gram-positive (Bacillus subtilis and Staphylococcus aureus) and Gram-negative (Escherichia coli) spectrum by using response surface methodology. 20 g powder of each E. tereticornis (leaf) and N. sativa (seeds) were mixed with 200ml of ethanol at room temperature, and then it was centrifuged at 4000 rpm for 10 min to separate the supernatants, and allowed to dry in order to obtain ethanol free extracts. A fresh bacterial culture of 100µl of test microorganism was inoculated onto media and spread homogeneously. The antimicrobial activity of ethanolic extracts showed that all the concentrations tested were effective against the test microorganisms. The diameters of zones of inhibition exhibited by S. aureus PCSIR-83 were in the range of 0-28mm, E. coli PCSIR-102 (0-28mm) and B. subtilis PCSIR-05 (15-26mm). The combination of N. sativa (15mg/µl) and E. tereticornis (20mg/µl) were found most effective at pH 9.0 and temperature 35°C. Our results clearly indicate that Gram positive bacteria showed more sensitivity than Gram-negative bacteria.