In-vitro antimicrobial activity of aqueous and methanol extracts of Tetracarpidium conophorum on some susceptible and multiresistant bacteria

Abstract

There is an increasing interest in the investigation of different plant extracts as potential sources of new antimicrobial agents in view of numerous multiresistant Gram negative bacteria and methicillin resistant Staphylococcus aureus (MRSA). We investigated in vitro antimicrobial activity of aqueous and methanol extracts of Tetracarpidium conophorum (African walnut) on some selected bacteria. A total of 70 non-duplicative bacterial strains comprising Gram positive and Gram negative bacteria were obtained from different hospitals. They were investigated for susceptibility to a panel of conventional antibiotics, minimum inhibitory concentrations (MICs) of aqueous and methanol extracts; leaf, stem bark, cooked kernel and uncooked kernel for each isolate was determined using punch-hole agar diffusion method. The MICs showed a high degree of resistance by Gram negative bacteria, with MIC90 values (MIC for 90% of the organisms) of ≥ 256 µg/ml for all antibiotics. The MICs of S. aureus to methicillin showed MIC50 and MIC90 to be 8 and >256 µg/ml, respectively while they were found to be susceptible to vancomycin with MIC90 of 2 µg/ml. All the bacterial isolates tested were resistant to the aqueous and methanol extracts of walnut (T. conophorum) leaf, stem bark, cooked kernel and uncooked kernel with no zone of inhibition (0 mm diameter). This however suggests either complete lack of antimicrobial potential, or the bacteria have developed resistance to walnut which was hitherto sensitive. Key words: walnut, bacteria, antibiotics, susceptibility, resistance