Semester

Summer

Date of Graduation

2007

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Civil and Environmental Engineering

Committee Chair

Lian-Shin Lin

Abstract

Introduction of engineered nanomaterials into water systems has potential of affecting microbial viability and resistance to commonly used disinfectants. We studied how photocatalytically induced oxidative stress by engineered nanoparticles can affect bacterial viability and resistance to UVC at 254 nm. Flow cytometry analysis, fecal viability analysis and respiratory measurements showed that oxidative stress induced by engineered TiO2 nanoparticles with concentrations in ppb range can cause sublethal effects and lowered oxygen utilization rate. Respiratory measurements showed that degree of reduction in oxygen utilization depended on exposure time to oxidative stress but was not sensitive to nanoparticle concentrations examined in the study.;Fecal coliforms in mixed culture displayed enhanced resistance to UVC irradiation as a result of the oxidative treatment. Degree of resistance enhancement increased with nanoparticle concentration and exposure time. The higher survival percentages were attributed to two possible reasons. First, the oxidative treatment caused a bacterial community shift due to diverse bacterial sensitivity to oxidative damages among different species. Second, the oxidative stress induced bacterial defense mechanisms such as induction of ROS-counteracting enzymes and those for DNA repair so that the bacterial species became more resistant to the oxidative damages caused by UVC irradiation.;To further clarify the causes of the enhanced resistance with the mixed culture, we conducted a similar experiment on an Escherichia coli strain (DH10B). The pure culture exhibited a similar enhancement in resistance to UVC irradiation as the mixed culture. The data corroborate that induction of microbial activities of counteracting the oxidative stress and its damages is a cause for the observation concerning bacterial resistance.;To elucidate possible bacterial activities counteracting the oxidative stress and its damages, we measured intracellular ROS level of the mixed culture with and without the oxidative treatment. Fluorimetric measurements showed that oxidative stress caused by low-level engineered TiO2 nanoparticles could trigger bacterial defense mechanisms that counteracted intracellular reactive oxygen species. The data suggest that induction of bacterial activities that counteracting intracellular ROS is a cause for the enhanced resistance observed.

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