In Vitro Evaluation of the Antibacterial Activity of Crassocephalum crepidioides (Karaangan) Leaves Traditionally Utilized by the Talaandig Tribe of Sungco, Lantapan, Bukidnon Against Selected ATCC Reference Strains (Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa)

Antimicrobial resistance (AMR) continues to limit the effectiveness of conventional antibacterial therapy and has increased interest in medicinal plants as potential sources of antimicrobial compounds. This study evaluated the in vitro antibacterial activity of Crassocephalum crepidioides (Karaangan) leaf extract against selected American Type Culture Collection (ATCC) reference strains of Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa. A laboratory-based in vitro experimental design was employed using the Kirby–Bauer disk diffusion method. Leaf extracts prepared at 25%, 50%, and 75% working concentrations were tested, with ciprofloxacin (5 µg) serving as the positive control. Antibacterial activity was assessed based on the measured zones of inhibition (ZOI), and the results were analyzed using descriptive statistics, one-way analysis of variance (ANOVA), and Tukey’s post hoc test. The extract produced limited inhibition under the tested conditions. Mean ZOI values ranged from 5.70 to 7.37 mm for S. aureus, 4.33 to 7.90 mm for E. coli, and 7.33 to 8.43 mm for P. aeruginosa. Ciprofloxacin produced substantially larger inhibition zones, ranging from 28.33 to 36.10 mm. ANOVA showed no significant difference among extract concentrations against S. aureus, F(2,6) = 0.49, p = .633, or P. aeruginosa, F(2,6) = 1.17, p = .373. A significant difference was observed for E. coli, F(2,6) = 9.38, p = .014. Tukey’s test showed significant differences between the 25% and 50% concentrations and between the 25% and 75% concentrations, whereas the 50% and 75% concentrations did not significantly differ. Qualitative phytochemical screening using thin-layer chromatography detected flavonoids but did not detect alkaloids among the selected compounds tested. The findings indicate limited antibacterial activity of the crude leaf extract under the conditions examined. The results do not establish clinical efficacy, antibiotic equivalence, or a causal relationship between flavonoids and antibacterial activity. Further investigation using solvent-matched controls, standardized independent replication, minimum inhibitory concentration and minimum bactericidal concentration assays, quantitative phytochemical analysis, compound isolation, and toxicity evaluation is warranted.