ABSTRACT
Aim: To assess inhibitory effect of extracts, alone and in combination, from Carissa spinarum Linn (C. spinarum L.) and Carica papaya Linn (C. papaya L.) on Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli). The combined extracts used were C. papaya L leaves petroleum ether extract/C. spinarum L root methanolic extract (CPLP/CSRM), C. spinarum L leaves petroleum ether extract/C. papaya L seed ethanolic extract (CSLP/CPSE), C. spinarum L root ethanolic extract/C. papaya L leaves ethanolic extract (CSRE/CPLE), C. papaya L root ethanolic extract/C. spinarum L bark ethanolic extract (CPRE/CSBE) and C. papaya L leaves methanolic extract/C. spinarum L leaves methanolic extract (CPLM/CSLM). Study Design: In vitro antibacterial assay. Place and Duration of Study: Samples were collected from Samunge village at Loliondo in Ngorongoro district located in northern Tanzania. Antimicrobial bioassay was carried out at the Department of Molecular Biology and Biotechnology, University of Dar-es- Salaam, between March 2013 and June 2013. Methodology: The broth micro dilution method was used to determine minimum inhibition concentration (MIC). Fractional inhibitory concentrations were calculated from MICs of individual and combined extracts to determine interactions. Results: Plant extracts demonstrated MICs ranging from 312 to 5000 μg/ml. The combination of plant extracts against S. aureus resulted into antibacterial activity of CPSE, CPRE, CPLM, CSLM and CPLP extracts to increase by 4-, 2-, 4-, 4-, and 2-fold, respectively. Activity of CSLP, CPLM and CSLM increased by 2-fold against E. coli. Synergy was demonstrated by CPLM/CSLM against S. aureus. Some combinations were additive including CPRE/CSBE, CPLP/CSRM and CSLP/CPSE against S. aureus and CSLP/CPSE, CPRE/CSBE, CPLM/CSLM against E. coli. Nevertheless, antagonism was demonstrated by CSRE/CPLE, CPLP/CSRM against E. coli and CSLP/CPSE and CSRE/CPLE against S. aureus. Conclusion: This study revealed the importance of using plant-based antibacterial agents in combined therapy to increase efficacy. Extracts of C. spinarum L and C. papaya L could be a source of antibacterial agents when utilized in combination therapy for patients with severe E. coli and staphylococcal infections. These predictors, however, need to be validated to improve their quality.