Anti-microbial Activities of Selected Ghanaian Medicinal Plants and Four Structurally Similar Anti-protozoan Compounds against Susceptible and Multi-drug Resistant Bacteria

Antibacterial resistance is one of the fast rising health concerns globally. WHO emphasized the need for development of new drugs to combat antimicrobial resistance. Our group previously found several anti-protozoan compounds: ML-2-3, Molucidin and ML-F52 from a Ghanaian medicinal plant Morinda lucida and oregonin from a Japanese medicinal plant Alnus japonica, which share a similar aromatic ring structure. In this study, we investigated the antimicrobial activities of our compounds and some selected Ghanaian medicinal plants` extracts (n= 92) against five (5) Gram-negative (Escherichia coli (ATCC 25922), Pseudomonas aeruginosa (ATCC 27853), Klebsiella pneumoniae (ATCC 33495), Shigella flexneri (ATCC 12022), Proteus mirabilis (ATTC 35659)), two (2) Gram-positive bacteria, (Staphylococcus epidermidis (ATCC 12228) and Staphylococcus aureus (ATCC 29213)) and 28 Methicillin Resistant Staphylococcus aureus (MRSA) strains isolated from carriage and clinical infection in Ghana, in an in vitro colorimetric based assay. IC50, Minimum inhibition concentration (MIC) and Minimum bactericidal concentration (MBC) were determined with ampicillin and ciprofloxacin as reference antibiotics. Oregonin had activity against both Gram-positives and negatives, while the remaining three compounds had activity only against Gram-positive bacteria. 12 out of 92 plant extracts tested showed significant activity against the standard bacteria strains. Oregonin was the most active compound against all 28 isolates of MRSA with a least MIC of 100 µM and a least MBC of 400 µM; 19 isolates had IC50 < 100 µM.