Comparing the Antimicrobial Effects of Common Antibiotics and Natural Antimicrobial Products on Bacillus subtilis (Gram-

Overview

As antibiotic resistance increases, interest in natural antimicrobial alternatives continues to grow. This study evaluated the antibacterial effectiveness of oregano oil, tea tree oil, honey, and three common antibiotics (Amoxicillin, Cefazolin, and Bactrim (Sulfamethoxazole and Trimethoprim)) against Escherichia coli K-12 and Bacillus subtilis. It was hypothesized that the common antibiotics would produce larger zones of inhibition than the natural antimicrobial products and that Bacillus subtilis would demonstrate greater overall susceptibility due to common antibiotics generally impacting gram positive bacteria more effectively than gram-negative bacteria. Using the Kirby–Bauer disk diffusion method, the chosen antimicrobials were applied to inoculated agar plates in multiple trials, and zones of inhibition were measured at 24 and 48 hours of incubation. With Bacillus subtilis, oregano oil produced substantially larger mean zones of inhibition (approximately 74 mm at both time points) than any of the other antimicrobials tested, while honey and tea tree oil showed no measurable inhibition. In contrast, results for E. coli were more comparable across treatments, with oregano oil producing a 36.5 mm mean inhibition zone at 24 hours and certain antibiotics demonstrating slightly greater consistency and marginally higher inhibition at 48 hours. Overall, these findings indicate that antibiotics exhibited stable performance across trials, while oregano oil demonstrated notably stronger antibacterial activity in diffusion-based assays under the experimental conditions tested. The observed variability in this experiment warrants further investigation, such as examining different concentrations of natural antimicrobial oils under similar conditions.

Competition history

• CSEF 2026 Microbiology (Senior Division) · Entry S-16-07

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