Antibacterial screening, synergy studies and phenolic content of seven South African medicinal plants against drug-sensitive and -resistant microbial strains

Extracts from Bolusanthus speciosus, Cucumis myriocarpus, Ekebergia capensis, Protea caffra, Prunus africana, Searsia lancea and Solanum panduriforme were screened for antigonococcal activity using microdilution and agar disk-diffusion techniques. In addition, combinations of the different plant extracts, as well as plant extracts with four antibiotics (ampicillin, cefotaxime, chloramphenicol and penicillin) were also evaluated for antibacterial synergistic interactions against multidrug-resistant (MDR) Gram-negative bacterial strains (Escherichia coli and Klebsiella pneumoniae) using the checkerboard titration method and the time-kill assay. Phytochemical analysis for phenolics were also conducted using aqueous (50%) methanol plant extracts. B. speciosus methanol and dichloromethane bark extracts, as well as dichloromethane leaf extracts of P. africana and S. lancea demonstrated moderate antigonococcal properties when screened using the microdilution assay, with minimum inhibition concentration values ranging from 313 to 625 μg/ml. The checkerboard assay detected antibacterial synergistic interactions in combinations of chloramphenicol with each of B. specious methanol leaf extracts, P. africana methanol and dichloromethane leaf extracts against MDR E. coli (fractional inhibitory concentration index ≤ 0.5). However, the time-kill assay did not detect any significant synergistic interactions in any of these three aforementioned combinations. Total phenolic content in the medicinal plant extracts investigated ranged from 2.38 to 62.73 mg GAE/g dry matter. Variations in the quantity of flavonoids and condensed tannins between these plant extracts were also observed. Overall, the current findings indicated that both drug-sensitive and -resistant bacterial strains could potentially be managed using efficacious medicinal plant extracts, used either in mono- or combination therapies.