Plastotoma rotundifolium aerial tissue extract has antibacterial activities

• First report of the phytochemical composition of Platostoma rotundifolium.
• A new and four known pentacyclic tripenoids were isolated for the first time from this species.
• Ursolic and corosolic acids showed significant antibacterial activities against MRSA and Escherichia coli.

Despite efforts in recent years, infectious diseases remain a worldwide major public health problem. Several infectious agents have become resistant to conventional antibiotics and there is thus urgent need to discover new antimicrobial drugs to overcome resistances. Platostoma rotundifolium (Briq.) A. J. Paton, an African plant, is mainly used to treat microbial infections in traditional Burundian medicine. From the ethyl acetate extract of the aerial parts, five pentacyclic triterpenoid acids were isolated and characterized. Based on spectral analysis, these compounds were elucidated to be 2α, 3α, 19β-trihydroxyurs-12-en-28-oic acid (that was named jeremic acid) (1), 3β-hydroxyurs-12-en-28-oic acid (ursolic acid) (2), 2α, 3β-dihydroxyurs-12-en-28-oic acid (corosolic acid) (3), 2α, 3β, 19α-trihydroxyurs-2-en-28-oic acid (tormentic acid) (4) and 19-hydroxy-2-hydroxymethyl norursa-2, 12-dien-28-oic acid (hyptadienic acid) (5). Ursolic (MIC = 17.5–68 μM) and corosolic (MIC = 17–68 μM) acids showed significant antibacterial activities against both the Gram-positive Staphylococcus aureus (methicillin-susceptible and -resistant strains) and the Gram-negative Escherichia coli, which may substantiate the use of P. rotundifolium in traditional Burundian medicine. Such hydroxylated pentacyclic triterpenoid acids could point to new antimicrobial strategies that may help overcoming the antimicrobial resistances actually observed throughout the world.

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