In Vitro Antibacterial and Antioxidant Activities of Extracts of Turnera diffusa Willd. Ex Schult and Its Polyphenol Profile

Turnera diffusa, belonging to the Turneraceae family, is used by communities in the Tehuacán-Cuicatlán Valley to treat gastrointestinal and respiratory illnesses. The objective of this study was to evaluate antibacterial and antioxidant effects of T. diffusa. The plant was collected in Santa María Ixcatlán, within Tehuacán-Cuicatlán Valley (Puebla, Mexico). The antibacterial activity of hexane, acetone, and methanol extracts was evaluated using diffusion and agar dilution methods. Microbial survival curves were generated for susceptible microorganisms. The antioxidant activity was evaluated using the DPPH and ABTS radical scavenging assays and the FRAP ferric reduction assay. The chemical composition was determined using colorimetric reactions, and polyphenol profile was analyzed by reverse-phase HPLC. The acetone extract inhibited the growth of 4 Gram-negative and 3 Gram-positive bacterial strains. Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, and S. epidermidis were most susceptible strains (MIC = 2.0 mg/mL). Microbial death curves showed a bactericidal effect of acetone extract on E. coli and S. aureus after two hours of exposure to the extract (4.0 mg/mL). The methanolic extract exhibited highest antioxidant capacity against DPPH and ABTS radicals (IC50 = 45.66 and 116.79 µg/mL, respectively) and the highest ferric reduction capacity (57.08%). This extract also presented highest concentration of total phenols (6.052%), suggesting that this group of secondary metabolites is responsible for effect. The polyphenol profile of these extracts consisted primarily of phenylpropanoids. The results confirm the antibacterial and antioxidant effects, which supports medicinal use of T. diffusa in treatment of infectious diseases and oxidative stress.