Phytochemical screening, UPLC analysis, evaluation of synergistic antioxidant and antibacterial efficacy of three medicinal plants used in Kinshasa, D.R. Congo

Plant extracts are used worldwide due to their biologically active compounds, which support food preservation and help combat various diseases through their antimicrobial and antioxidant properties. In the capital city of the Democratic Republic of the Congo, an ethnobotanical survey revealed the use of Dysphania ambrosioides (L.) Mosyakin & Clemants, Ocimum gratissimum L. and Tetradenia riparia (Hochst.) Codd, often in combination, for treating oral microbial diseases. While these plants have been widely studied individually; their combined potential has not been investigated. The present research aims to explore the phytochemical composition, the synergistic antimicrobial, and antioxidant potential of different extracts from these three mentioned plants. Phytochemical composition of the decocted and percolated extracts from the three plants was determined using qualitative analysis and the ultra-performance liquid chromatography quadrupole time of flight tandem mass spectrometry (UPLC-QTOF-MS). Antimicrobial activity was assessed using the broth dilution method, while antioxidant activity was evaluated using the DPPH method. For the antimicrobial studies, the decocted and percolated extracts were tested against Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa. Qualitative phytochemical analysis revealed the presence of alkaloids, saponins, flavonoids, iridoids, and anthraquinones in all the plant extracts. The methanolic extract of T. riparia exhibited the highest phenolic content (299.146 ± 0.143 mg GAE/g extract), while O. gratissimum had the highest flavonoid content (138.256 ± 0.277 mg QE/g extract). UPLC analysis identified several metabolites in the plant extracts including rosmarinic acid, cirsimaritin, xanthomicrol and kaempferol derivatives. Rosmarinic acid was consistently identified across all the plant extract combinations, while other flavonoids such as apigenin 7-glycosides, kaempferitin and luteolin 7-O-glucoside, were detected in specific plant extract combinations. The decocted plant extracts exhibited higher antioxidant activity than the percolated extracts, with O. gratissimum showing the highest antioxidant activity (11.744 ± 0.584 µg/mL), followed by T. riparia (12.916 ± 0.972 µg/mL). The extract combinations from O. gratissimum and T. riparia demonstrated synergistic antioxidant activity (CI = 0.57). Amongst all extracts, the highest antibacterial activity was observed in the decocted extracts of O. gratissimum and T. riparia against S. aureus (MIC = 500 µg/mL), with their combination showing additive antibacterial activity (FICI = 1). The aim of this study was primarily to evaluate the combinatory potential of these three plants as reliable sources of antimicrobials and antioxidants for the treatment of various microbial diseases in the future. The study provides evidence of the synergistic antioxidant and antibacterial potential of O. gratissimum and T. riparia. These results suggest that these plant extracts are promising sources of natural antimicrobial and antioxidant agents, with potential application in the pharmaceutical and food industries for combating several infectious diseases.