The antimicrobial activities of selected local medicinal herbs against Streptococcus pneumoniae
Kamuzu University of Health Sciences
Streptococcus pneumoniae (S. pnuemoniae) is a major cause of invasive pneumococcal infections. Current treatment involves antibiotic therapy and vaccination. The emergence of antibiotic resistance and vaccine serotype replacement negatively affects the control of pneumococcal infections, necessitating the need for other treatment alternatives. We investigated the antimicrobial activity of local herbs against pneumococcal serotypes 1 and 6A. Dry powdered leaves/stem material (50 g) from 6 plants; Annona senegalensis (A. senegalensis), Bidens pilosa (B. pilosa), Dichrostachys cinerea (D. cinerea), Erythrina abyssinica (E. abyssinica), Lippia javanica (L. javanica) and Trichodesma zeylenicum (T. zeylenicum)) were extracted using 200 mL of distilled water or methanol. Disc diffusion method was employed for antimicrobial activity testing. The minimum inhibitory concentrations (MIC’s) and minimum bactericidal concentrations (MBC’s) were determined using agar dilution method. Column chromatography was employed for fractionation of crude extracts. Penicillin and DMSO (1 % in DPBS) were used as positive and negative control, respectively. Water extraction yielded 5.1 % (10.2 g/200 g) of the crude extract material versus 12.1 % (24.3 g/200 g) from methanol. The Bp (1.0 mg/mL) and Ea (1.0 mg/mL) water extracts showed highest activity against serotype 1 (zone of inhibition (ZI) ~9.0 mm each), while Bp and Dc (ZI ~11.0 mm) exhibited the highest activity against serotype 6A. The methanol extracts of B. pilosa (1.0 mg/mL) and D. cinerea (1.0 mg/mL), showed the highest activity against serotype 1 (ZI ~15.6 ± 0.57, MIC<100 𝜇g/mL, MBC ~ 2 mg/mL and ZI ~13.6 ± 0.57 mm, 100 ≤ MIC <512 𝜇g/mL, MBC ~ 4 mg/mL respectively) and serotype 6A (ZI ~24.3 ± 0.00 mm, MIC <100 𝜇g/mL, MBC ~2 mg/mL and ZI ~15 ± 0.00 mm, 100 ≤ MIC<512 𝜇g/mL, MBC ~ 4 mg/mL respectively). The ZI for both serotypes 1 and 6A were 0.00 mm for DMSO (1 %) and 25 ± 0.00 mm for penicillin (25 μg). Column fractionation vi was conducted on methanol extracts of B. pilosa and D. cinerea. Of the 9 fractions from B. pilosa, only one fraction showed activity against serotypes 1 and 6A (ZI ~10 mm). Of 11 fractions from D. cinerea, only one showed activity against pneumococcal serotypes 1 (ZI ~8 mm) and 6A (ZI ~7 mm). Phytochemical analysis of methanol extracts showed higher content of total polyphenols, flavonoids, 2, 2-Diphenyl-1-Picrylhydrazyl (DDPH), Ferric Reducing Antioxidant Power (FRAP) and total alkaloids compared to water extracts. The Brine Shrimp’s lethality analysis for highly active plant (Bp and Dc) showed no and mild toxicity respectively. We recommend further exploration of these two plants as potential antimicrobials against pneumococcal infections.