Using Cell Motility and Particle Tracking to Deduce Mechanisms and Kinetics Underlying Photocatalytic Water Disinfection in Real Time

Photocatalysis is a promising water disinfection method that has the potential to complement traditional approaches to water remediation. Large-scale implementation of photocatalysis for water remediation requires a two-fold improvement to the current state-of-the-art: understanding of the mechanisms underlying photocatalyst-pathogen interaction and pathogen inactivation during photocatalysis, and a process for rapid catalyst discovery that provides for evaluating photocatalyst efficacies in a reliable manner. Toward this goal, we will discuss the use of novel tools developed in our laboratory to track the viability loss of bacterial cells inactivated by photocatalytic treatment in real time. Using optical microscopy and particle-tracking algorithms, we observed that, when exposed to titanium dioxide (TiO2) nanowire-assisted photocatalytic stressors, the change in the motility of Escherichia coli (E. coli) cells tracks viability loss precisely. Furthermore, using phase and fluorescence optical microscopy, real-time observations of the interactions between the cells and the photocatalyst nanowires were also performed. Our findings suggest that these interactions occur through collisions between the nanowires and bacteria. Based on these observations, we developed a phenomenological model explaining the pseudo-first-order kinetics of E. coli inactivation. Through the use of fluorescence-based microscopy, we observed that the motility loss (and hence viability loss) was due to the dissipation of the proton motive force that powers motility, caused by cell membrane integrity loss. Our experiments show a good match between the kinetics of motility loss and viability loss for various operating conditions, showing that the methods presented here are versatile in applicability. Overall, our results indicate that these in situ methods offer significant time-savings for characterizing viability loss of pathogens over the traditional ex-situ methods. As indicated above, these methods will also help accelerate the evaluation of novel antibacterial agents, including, but not limited to novel photocatalysts, against emerging treatment-resistant pathogens, for applications ranging from water disinfection to equipment decontamination in healthcare facilities.