Graduation Year

2025

Document Type

Campus Only Senior Thesis

Reader 1

Pete Chandrangsu

Reader 2

Aditi Vyas

Abstract

Antibiotic resistance that is obtained by microbes is a rapidly-developing threat to the health of people worldwide in the modern era of antibiotic drug usage. Linked with selective pressure–factors that allow bacteria to survive and reproduce in their environment–the advancing progression of antimicrobial resistance must be hindered in the interest of global health. Employing traditional herbal medicines is a worthwhile approach to treating bacterial infections. Ajwain has been implemented in various regions around the globe–from India to Iran–to combat bacterial infections. Thymol and carvacrol–both terpenes and phenolic compounds in ajwain– are recognized for their antimicrobial abilities to inhibit the growth of bacteria such as Gram-positive Staphylococcus aureus. While the bactericidal properties of thymol against S. aureus have been observed, its ability to hinder the growth of other bacteria such as Staphylococcus epidermidis and Bacillus subtilis–both Gram-positive–must be further investigated as well their susceptibility to selective pressures. This study focuses on exploring the efficacy of thymol as an antimicrobial agent in combating the growth of S. epidermidis and B. subtilis through bacterial evolution experiments, which contributes to our understanding of how bacteria reproduces, evolves, and adapts to its environment. Through liquid dilution testing, the MIC (minimum inhibitory concentration) of thymol was observed to be between 0.125 and 0.25 mg/mL against S. epidermidis. While evolving S. epidermidis was also found to be difficult through the use of the agar growth assay, growth was visible in the presence of 0.25 and 0.35 mg/mL of thymol, which was not observed through the liquid dilution testing. Nucleotide BLAST analysis demonstrated that the original samples of S. epidermidis growth in the presence of 0.25 and 0.35 mg/mL of thymol was contaminated with B. subtilis. Moreover, Kirby-Bauer assays demonstrated that these B. subtilis samples displayed greater zone of inhibitions in comparison to the wild-type B. subtilis, indicating that these samples have not evolved to resist the antimicrobial properties of thymol. One-way ANOVA testing indicated that there is no statistically significant difference–(p-value ≈ 0.134)–between the varied zone of inhibitions of the three samples. Finally, these collective results demonstrated that further research must be conducted in investigating the mechanisms by which S. epidermidis and B. subtilis evolve resistance to thymol, considering how the experimental results observed in this study are relatively inconsistent.

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