Systematic Approach to In-Depth Understanding of Photoelectrocatalytic Bacterial Inactivation Mechanisms by Tracking the Decomposed Building Blocks

Abstract

A systematic approach was developed to understand, in-depth, the mechanisms involved during the inactivation of bacterial cells using photoelectrocatalytic (PEC) processes with Escherichia coli K-12 as the model microorganism. The bacterial cells were found to be inactivated and decomposed primarily due to attack from photogenerated H 2 O 2 . Extracellular reactive oxygen species (ROSs), such as H 2 O 2 , may penetrate into the bacterial cell and cause dramatically elevated intracellular ROSs levels, which would overwhelm the antioxidative capacity of bacterial protective enzymes such as superoxide dismutase and catalase. The activities of these two enzymes were found to decrease due to the ROSs attacks during PEC inactivation. Bacterial cell wall damage was then observed, including loss of cell membrane integrity and increased permeability, followed by the decomposition of cell envelope (demonstrated by scanning electronic microscope images). One of the bacterial building blocks, protein, was found to be oxidatively damaged due to the ROSs attacks, as well. Leakage of cytoplasm and biomolecules (bacterial building blocks such as proteins and nucleic acids) were evident during prolonged PEC inactivation process. The leaked cytoplasmic substances and cell debris could be further degraded and, ultimately, mineralized with prolonged PEC treatment.

Differences in photoelectrocatalytic inactivation processes between E. coli and its isogenic single gene knockoff mutants: Destruction of membrane framework or associated proteins?

“…A PEC reactor (50 mL volume) was used [20] to perform inactivation experiments, with a TiO 2 nanotube array photoanode [21]. Detailed experimental conditions are described below; materials included a counter electrode, platinum foil; reference electrode, saturated Ag/AgCl; bias potential of the anode, 1 V versus Ag/AgCl; light source, LED lamp with maximum emission at 365 nm and the light intensity adjusted to 27 mW cm −2 .…”

“…PEC treatment was applied to 50 mL of the bacterial suspension prepared above; surviving bacterial cells were monitored using a colony counting method [21]. The bacterial viability was also tested using a Live/Dead Backlight Bacterial Viability Kit (L13152, Invitrogen, USA), following the manufacturer's instructions.…”

“…Further tests were therefore done to confirm these membrane integrity changes. K + is an important ion in cytoplasm, and the destruction of bacterial membrane should induce the leakage of K + and cytoplasm [11,21]. As Fig.…”

“…This suggests that PEC inactivation may not cause the elevated lipid peroxidation level, or that the cell membrane lipid was not damaged by lipid peroxidation. However, these results can be explained by the fact that the dominant bactericidal RSs was in-situ generated H 2 O 2 rather than • OH [21]. Lipid peroxidation is generally induced by • OH [15].…”

Photoelectrocatalytic Oxidation of Cu-cyanides and Cu-EDTA at TiO2 nanotube electrode

“…Recently, photoelectrocatalytic (PEC) behavior of semiconductor photocatalyst under a positive potential and light irradiation has been paid more attention in the environmental field [10][11][12][13]. PEC oxidation in which a small positive potential was applied across the photocatalytic anode can enhance the separation efficiency of photogenerated holes and electrons and has proven to be more efficient for the pollutants oxidation than the individual photocatalytic process and [14][15][16][17].…”