Date of Graduation

12-2019

Document Type

Thesis

Degree Name

Master of Science in Physics (MS)

Degree Level

Graduate

Department

Physics

Advisor

Yong Wang

Committee Member

William Lin Oliver

Second Committee Member

Pradeep Kumar

Third Committee Member

Woodrow Shew

Keywords

antibiotic-resistance, e coli, flagella, hidden markov model, motility, silver

Abstract

In recent decades, the number of antibiotic-resistant bacterial infections has grown to become a serious global threat. This rise can be attributed to the widespread misuse of antibiotics and the lack of newly developed drugs to fight resistant organisms. Novel bactericidal substances have, therefore, garnered significant research interest. Silver, due to its powerful antimicrobial effects, is one such substance. Silver is typically most effective in cationic form; however, advancements in nanotechnology have paved the way for the controlled fabrication of nano-silver. Silver nanoparticles have been shown to have increased antibacterial potency for a variety of reasons, including the release of silver ions into aqueous media. Nonetheless, the entire antimicrobial mechanism of silver nanoparticles has not been completely elucidated. One such unexplored interaction is with bacterial motility. Motility allows bacterial cells to navigate their environment and steer themselves in favorable directions. Furthermore, motility has been shown to play an important role in virulence and biofilm-formation.

In this research, I investigated the interactions between silver ions and motility of Escherichia coli. By performing and collecting data from a series of phase-contrast microscopy experiments, I was able to show, through quantitative modeling and results, that silver ions cause a decrease in swimming velocity, an increase in tumbling frequency, and an increase in tumbling dwell time, all while not killing the cell. The experiments I performed included free-swimming experiments, in which bacteria swam in the 2-D focal plane, and tethering assay experiments, where a cell was trapped to a glass coverslip by a single flagellum. By modeling the rotational velocity of the tethered cells using hidden Markov models, I was able to show that silver ions cause a significant change to the tumble-to-run probabilities of treated cells. These results are of great importance for furthering the understanding of silver as a bactericide.

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