Combinatorial effects of selected Philippine medicinal plants with highest antibacterial activity

Abstract

Antibiotic resistance is an ongoing problem which reduces efficiency of administered drugs. Recent studies have sought another approach in treating bacterial infections by establishing the antibacterial potential of natural herbs. An emerging trend in preventing the development of resistant bacteria is the combination of effective antibacterial agents. This study evaluated the antibacterial activity of methanolic plant extracts of akapulko (Cassia alata), ampalaya (Momordica charantia), bawang (Allium sativum), bayabas (Psidium guajava), lagundi (Vitex negundo), niyog-niyogan (Quisqualis indica), sambong (Blumea balsamifera), tsaang gubat (Carmona retusa), ulasimang bato (Peperomia pellucida) and yerba buena (Mentha cordifolia) against Bacillus subtilis, Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus using disk diffusion and resazurin reduction assay. In the disk diffusion assay, antibacterial activity was indicated by inhibition zones surrounding the extract. At a high concentration (200 mg/ml), the highest antibacterial potential was observed for Q. indica with mean zones of inhibition and standard deviation of 26.67±0.58 mm, 29.33±2.31 mm, 29.33±1.5 mm and 30.33±0.58 mm for E. coli, P. aeruginosa, S. aureus and B. subtilis, respectively. The resazurin reduction assay determined the minimum concentration of the plant extract that could inhibit visual growth. The results showed that at low concentrations, the top three performing extracts based on visual minimum inhibitory concentration (MIC) were C. alata for E. coli, M. cordifolia for P. aeruginosa and M. charantia for B. subtilis. The MIC values of the aforementioned were 0.313 mg/ml for all. M. cordifolia and M. charantia were similar in terms of antibacterial potential against S. aureus based on the visual MIC of 0.313 mg/ml. The top performing extracts in the resazurin reduction assay were chosen for the checkerboard titration assay to determine their combinatorial effects. The combinations included M. charantia + C. alata, M. charantia + M. cordifolia and C. alata + M. cordifolia. Specifically, the combination of M. charantia + C. alata was found to be antagonistic when tested against S. aureus and acting autonomously on B. subtilis, E.coli and P. aeruginosa. Furthermore, all other combinations were found to be autonomous and yielded amplifications in their antibacterial activity. This was shown by a lower concentration needed to inhibit bacterial growth when combined together. Amplifications of inhibition strength of M. charantia were observed when combined with C. alata against B. subtilis, E.coli and P. aeruginosa while that of C. alata was only amplified in the combination against B. subtilis. Interestingly, combinations of M. charantia and M. cordifolia against all four organisms resulted in amplifications of the inhibition strength of both plant extracts. In addition, C. alata and M. cordifolia also had their antibacterial activity enhanced in the combination when tested against S. aureus and P. aeruginosa. Lastly, the antibacterial potential of C. alata was found to be enhanced upon combination with M. cordifolia against B. subtilis and E. coli. Thus, the amplifications observed suggest that these combinations may be more effective against multi-drug resistant bacteria than individual treatments of the selected plant extracts however, further studies must still be performed before these can be administered as fully-working drugs.