Evaluation of E. coli Inhibition by Plain and Polymer-Coated Silver Nanoparticles

Date of Award

Spring 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biological Sciences

First Advisor

Shreekumar R. Pillai

Second Advisor

Vida A. Dennis

Third Advisor

Shree R. Singh

Abstract

Escherichia coli ( E. coli ) causes various ailments such as enteritis, foodborne illnesses, and urinary tract infections which are of concern in the public health field due to antibiotic resistance. These organisms are more difficult to treat because they are becoming resistant to conventional antibiotics. Silver nanoparticles (AgNP) are known for their biocompatibility and antibacterial activity and may prove to be an alternative method of treatment, especially as wound dressings. AgNP are known for their broad-spectrum antibacterial activity and may prove beneficial to inhibit the growth of various bacteria. The efficacy of AgNP can be enhanced by adding a coating such as polyvinyl pyrrolidone (PVP) or through the conjugation of a polymer or antibiotic. The coating and conjugation reduce cellular damage while inhibiting the bacteria. However, the type of polymer coating may determine the antibacterial efficacy of the nanoparticles. In order to test this hypothesis, we investigated the antibacterial activity of two polymer coated AgNP either containing 10% Ag, or 99% Ag. The coated and functionalized nanoparticles were evaluated by several methods including atomic force microscopy (AFM), minimum inhibitory concentration (MIC) and bacterial growth curve assays. AFM was used to characterize the nanoparticles, and their antibacterial efficacy was compared by the MIC and bacterial growth curve assays, followed by molecular studies using scanning electron microscopy (SEM) and quantitative real-time polymerase chain reaction (qRT- PCR). AgNP inhibited the growth of E. coli only at 0.621 mg/mL, which was double the concentration required for both polymers coated nanoparticles (0.312 mg/mL). Similarly, bacterial growth was impeded as early as 8 h at 0.156 mg/mL concentration of both polymers coated nanoparticles compared to double that at 0.312 mg/mL for plain AgNP. SEM data showed that nanoparticles damaged the cell membrane, resulting in cell lysis, expulsion of cellular contents, and completed disintegration of some cells. The expression of genes associated with the tri-carboxylic acid (TCA) cycle ( aceF and frdB ) and amino acid metabolism ( gadB , metL , and argC ) were substantially downregulated in E. coli treated with polymer coated nanoparticles. The reduction in the silver ions (Ag + ) concentration of polymer coated AgNP did not affect their antibacterial efficacy against E. coli .

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