Antibacterial activity of Andrographis paniculata and Piper betle and their Interactive Effects with Amoxicillin against selected Respiratory Pathogens

This study was undertaken to determine the antibacterial activity and interactive effects of the methanol, ethanol and aqueous extract of Andrographis paniculata and Piper betle leaves with amoxicillin against selected clinical isolates of respiratory pathogens: Escherichia coli USTCMS 1030, Pseudomonas aeruginosa USTCMS 10013, and Staphylococcus aureus USTCMS 1097. Antibacterial activity of the plant extracts using disk diffusion showed that the methanol extract of P. betle exhibited inhibitory activity against all the test organisms, whereas the methanol and ethanol extracts of A. paniculata exhibited antibacterial activity to S. aureus USTCMS 1097 only. The antimicrobial properties of each plant extract were further evaluated using broth microdilution. Results showed that the ethanol extract of P. betle had the most potent antibacterial activity against all test bacteria with minimum inhibitory concentrations of 6.5 mg/mL, 3.25 mg/mL, and 0.2 mg/mL for E. coli USTCMS 1030, P. aeruginosa USTCMS 10013 and S. aureus USTCMS 1097, respectively. However, resazurin showed an inhibitory activity against S. aureus USTCMS 1097 in usual concentrations used in the assay, which is a novel finding since it is typically used as an indicator. Based on disk diffusion, the methanol and aqueous extracts of P. betle showed promising synergistic effect with the antibiotic amoxicillin. This was confirmed by checkerboard assay wherein the aqueous extract of P. betle showed an additive effect to amoxicillin against E. coli USTCMS 1030 (FICI = 0.66), while the methanol extract of P. betle exhibited true synergism with amoxicillin against P. aeruginosa USTCMS 10013 (FICI = 0.33). This synergism between the ethanol extract of P. betle and amoxicillin was significant since the activity of amoxicillin increased by 128-fold. This combination has potential in treating diseases associated with amoxicillin- resistant P. aeruginosa.