In Vitro Assessment of Antimicrobial Activity and Synergistic Effects of Ethanolic Extracts from Six Medicinal Plants

This in vitro experimental study examines and evaluates the antimicrobial and synergistic effects of the ethanolic extract of six plants: Biancaea sappan (L.) Tod, Bauhinia malabarica Roxb, Carthamus tinctorius L., Derris scandens (Roxb.) Benth, Hibiscus sabdariffa L., and Piper nigrum L. against common microbial species representing gram-positive, gram-negative bacteria, and fungi, consisting of Staphylococcus aureus, Escherichia coli, Staphylococcus epidermidis, Pseudomonas aeruginosa, and Candida albicans. The plants were extracted using 90% ethanol. According to the standard method of agar diffusion assay, the micro-dilution method for minimal inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined. This study found that among the six plants, only B. sappan and B. malabarica exhibited moderate inhibitory effects against S. aureus and S. epidermidis. B. sappan had MIC values of 250 µg/ mL and 125 µg/ mL, respectively, and B. malabarica showed MIC values of 62.50 µg/ mL and 31.25 µg/ mL, respectively. The synergistic effects of a combination of B. sappan and B. malabarica extracts at a ratio of 25:75 were analyzed, and it was found that the combination inhibited S. aureus and S. epidermidis with MIC values of 250 µg/ mL and 125 µg/ mL, respectively. The fractional inhibitory concentration index (FICI) and the fractional bactericidal concentration index (FBCI) indicated antagonistic or synergistic effects of the combination, with FICI and FBCI values of 2.5–5.0 for both B. sappan and B. malabarica extracts in the 25:75 mixture. In conclusion, single plant ethanolic extracts of B. sappan and B. malabarica possess potent antimicrobial activity to varying degrees. However, the antimicrobial potency of the 25:75 ratio mixture of these extracts was shown to decrease against the same organisms, with in vitro antimicrobial activity and antagonistic effects observed only against the tested gram-positive bacteria.