Antimicrobial activities of extracts and isolates from Senna didymobotrya flowers.

Abstract

The emerging trends in pathogenic microorganisms becoming resistant to available antibiotics coupled with the evolution of new strains of pathogens causing diseases, is a major concern to the global health community and this requires urgent remedy. Prolonged treatment with conventional antimicrobial drugs has led to their toxicities among patients and therefore, for effective treatment of diseases caused by pathogenic microbes, search for new pharmaceuticals or some potential sources of novel drugs are necessary. Medicinal plants commonly used by various communities could be potential sources for bioprospection to help in the fight against this menace. This study focused on evaluating the antimicrobial activity of Senna didymobotrya flowers (Caesalpiniaceae), a plant widely used in Kenyan folklore medicine to manage bacterial and fungal infections. In parts of Kisumu, the flowers of the plant are used to manage diarrhoea and itching of the skin. Although the flowers are traditionally used by herbalists to manage microbial infections, very little work has been done to evaluate the efficacy of the extracts against pathogenic microbes. In addition, compounds responsible for the antimicrobial activity in the flowers have not been isolated and characterized. The objectives of this study were to evaluate the extracts and isolated compounds from flowers for antifungal and antibacterial activities. The dry and pulverized flowers of the plant were sequentially extracted with n-hexane, ethyl acetate and methanol. The methanol extract significantly (P≤0.05) inhibited M. gypseum (MIC 3.9µg/ml), T. mentagrophyte (MIC 7.8µg/ml), K. pneumoniae and C. albicans (MIC value 31.3 µg/ml each). This was followed by ethylacetate extract which gave moderate results against M. gypseum (MIC 15.6µg/ml), S. faecalis, B. anthracais, C. albicans and T. mentagrophyte (MIC value 62.5 µg/ml each). n-Hexane extract afforded MIC values ranging between 125 µg/ml to over 250 µg/ml. Fractionation of the various extracts led to the isolation of twelve compounds whose structures were determined using physical and spectroscopic methods as well as comparison with literature data. The compounds were: chrysophanol (7), physcion (9), emodin (10), quercetin (253), β-sitosterol (302), stigmasterol (303), lupeol (323), betulinic acid (324), oleanolic acid (335), 4’, 5-dihydroxystilbene-3-O-glucoside (336), 5-hydroxyflavone (337) and 7-hydroxyflavone (338). All the isolated compounds were screened for antimicrobial activities.The most active metabolite was emodin which gave a MIC in the range 15.6-250 µg/ml against all the microorganisms tested except E. coli (MIC >250 µg/ml). 7-Hydroxyflavone also afforded good results by displaying MIC values in the range 31.3-250 µg/ml against all microorganisms that were tested. The least active metabolites were β-sitosterol and stigmasterol which were observed to show mild activities against S. aureus (MIC 250 µg/ml). Activities of the three extracts and isolated compounds against the tested pathogenic microbes were significantly (P≤0.05) lower compared to the activities exhibited by kanamycin and fluconazole which were used as standards against bacteria and fungi, respectively. This study has authenticated the traditional use of S.didymobotrya flowers in the management of bacterial and fungal infections. It has also identified the individual compounds responsible for the activities. Compounds that exhibited high antimicrobial activities such as emodin, and 7-hydroxyflavone may be developed into formulations to be used as antimicrobials.