Cleistochlamys kirkii chemical constituents: Antibacterial activity and synergistic effects against resistant Staphylococcus aureus strains

Ethnopharmacological relevance: Cleistochlamys kirkii (Benth) Oliv., (Annonaceae) is a medicinal plant traditionally used in Mozambique to treat infectious diseases.

Aims of the study: To find antibacterial lead compounds from C. kirkii and provide scientific validation for its use in traditional medicine.

Materials and methods: Through bioassay-guided fractionation, nine compounds (1-9), with different scaffolds, were isolated from the methanol extract of C. kirkii whose structures were identified by spectroscopic methods. Compounds 1-9 were evaluated for their in vitro antibacterial activity against a panel of eight Gram-positive, including five drug-resistant strains of Staphylococcus aureus, vancomycin-resistant Enterococcus faecalis, and two Gram-negative bacteria strains. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined. A chemosensitization assay, using the checkerboard method, was also performed in order to evaluate the type of interaction of compounds with antibiotics/compounds against two S. aureus resistant strains (ATCC 9144 and CIP 106760) and a susceptible strain (ATCC 6538).

Results: Dichamanetin (3), a rare C-benzylated flavanone, was very active against all the Gram-positive strains tested, displaying MIC values in the range of 1-7.5 μg/mL. The C-benzylated flavanones chamanetin (1), isochamanetin (2), and the α,β-unsaturated lactone (-)-cleistenolide (6) also showed relevant antibacterial activity against some of the Gram-positive strains assayed. Compounds 4, 5, and 7-9 have shown no significant activity at the concentration ranges tested. In the combination with antibiotics, polycarpol (8) (MIC 125 μg/mL) showed a strong synergistic effect against the methicillin-resistant S. aureus ATCC 9144. When combined with oxacillin (MIC 125 μg/mL), compound 8 reduced the MIC to 1.5 μg/mL (FICI=0.11). Similarly, it reduced the MIC of amoxicillin (MIC 250 μg/mL) to 7.5 μg/mL (FICI=0.18). Synergy was also obtained when this compound was combined with both β-lactam antibiotics (FICI=0.30) and with vancomycin (FICI=0.24) against vancomycin-intermediate S. aureus (VISA) CIP 106760. Remarkable, compound 8 was also able to reduce synergistically the MIC value of dichamanetin (3) (FICI=0.18) against this strain.

Conclusions: These results suggested that C. kirkii constituents may be valuable as a leads for restoring antibiotic activity against resistant S. aureus strains.