Antimicrobial activity of naturally-occurring substances against select microorganisms

The emergence of antimicrobial resistance has launched a renewed interest in plants as alternative antimicrobial sources. This study investigated the potential antimicrobial activity of 86 naturally-occurring substances against microorganisms of medical importance, namely, Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans. A microtiter plate assay was used to determine the minimum inhibitory concentration (MIC) values of the test substances against the target microorganisms using the antibacterial ciprofloxacin and the antifungal amphotericin B, as positive controls. The results indicated inhibition for eight of the 86 substances tested following their testing in triplicate. The MICs determined ranged from 0.125 mg/mL to 1 mg/mL for pine bark extract, green coffee bean extract, acacia rigidula powder, malic acid, citric acid, gallic acid, morin and chlorogenic acid. Viable plate counts were determined and time-kill curves were constructed to confirm the positive results. Pine bark extract and green coffee bean extract killed S. aureus acacia rigidula powder killed P. aeruginosa malic acid and citric acid killed E. coli, S. aureus and P. aeruginosa while gallic acid killed E. coli and P. aeruginosa and inhibited S. aureus. C. albicans was not susceptible to any of the test substances. Thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC) were used to analyze the active chemical compounds found in green coffee bean extract. Four primary fractions were identified using TLC and subsequently fractionated using HPLC. The fractions were tested for antimicrobial activity using a microtiter plate assay and a MIC value of 1 mg/mL was determined for three of the four fractions against the susceptible S. aureus. Viable plate counts confirmed the inhibitory activity of two fractions against the bacterium. The results of this study will contribute to identifying the potential of naturally-occurring substances as antimicrobial agents.