Influence of Sequential Solvent Extraction on the Antibacterial Activity of Phyla lanceolata Leaf Extracts

Natural products constitute an important source of bioactive compounds with potential applications in the development of new antimicrobial agents. Phyla lanceolata, a medicinal plant traditionally used in Mexican folk medicine, has been reported to contain secondary metabolites with potential biological activity. In this study, the antibacterial activity of sequential solvent extracts obtained from P. lanceolata was evaluated against Escherichia coli. Leaf material was subjected to sequential extraction using solvents of increasing polarity (n-hexane, ethyl acetate, and methanol). The resulting crude extracts were tested at concentrations of 200, 100, 50, 25, and 12.5 µg/µL using the agar disk diffusion method on Mueller-Hinton agar. Doxycycline and ceftriaxone were included as positive controls. None of the extracts produced inhibition zones against E. coli at any of the tested concentrations, whereas the antibiotic controls consistently exhibited clear inhibition halos ranging from 22 × 24 mm to 41 × 42 mm, confirming the validity and sensitivity of the bioassay. These findings indicate that the evaluated extracts lack antibacterial activity against this Gram-negative bacterium under the experimental conditions employed. The absence of activity may be attributed to the intrinsic resistance conferred by the outer membrane of E. coli, which restricts the penetration of many plant-derived metabolites, or to insufficient concentrations of active constituents in the crude extracts. Although no antibacterial effect was observed, the results contribute valuable information regarding the biological evaluation of P. lanceolata and support future studies involving phytochemical characterization, bioactivity-guided fractionation, evaluation against Gram-positive bacteria, and assessment of additional biological activities.