Single-Cell Level Measurements of Photocatalytic Damage to Escherichia coli Cells
Authors: Vidwans Niraj Ashutosh , Rhee Kathy Y. , Lele Pushkar P. , Vaddiraju Sreeram 
Published: November 2022

The increase in water demand due to the increasing population and the addition of emerging contaminants like drug-resistant bacteria, pharmaceuticals, and treatment-resistant chemicals such as PFAS are straining the current water treatment infrastructure. This straining infrastructure could be complemented by photocatalysis and other Advanced oxidation processes (AOP) [1] [2]. Photocatalytic water treatment methods, specifically, have generated a lot of interest due to their ability in eliminating pollutants like dyes and other toxic organic pollutants as well as eliminating pathogens such as bacteria, fungi, and viruses while minimizing the use of material resources. The elimination of bacteria by photocatalysis has been reported widely, with many reports focusing on the kinetics of photocatalytic disinfection, as well as some aspects of process scale-up. However, the mechanism for photocatalytic damage to the bacterial cells has thus far proven difficult to study [3] [4].
It is critical to determine the mechanisms of photocatalytic disinfection for the development of better methods of catalyst discovery and reactor design. The mechanisms can be determined if quantitative data on how single bacterial cells interact with catalysts is available. However, experimental measurements at a single-cell level are relatively few and assays for evaluating photocatalyst performance in real-time have not been developed to our knowledge. In this work, we addressed these shortcomings with a combination of light microscopy and particle-tracking algorithms. Using nanowire-shaped titanium dioxide (TiO2) catalyst particles, which maintain a high surface area while remaining visible using the established techniques for light microscopy, we investigated the interactions between catalyst particles and bacteria. We quantified cell motility during the course of photocatalytic disinfection as a loss in motility is expected to faithfully track damage to the membrane [5]. We further correlated the kinetics of antibacterial action of the photocatalytic treatment in E. coli with real-time changes in membrane integrity. Our investigations are anticipated to reveal the mechanisms by which photocatalytic treatments cause cell damage. Such single-cell approaches are expected to help build libraries of photocatalysts and optimize the use of antibacterial agents against superbugs, with a range of future applications in water-disinfection reactors to hospitals.
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