Photodynamic inactivation of bacterial pathogens – exploring the powers of light

This alternative format thesis presents two manuscripts investigating the use of photodynamic inactivation (PDI) as an innovative non-antibiotic approach to inactivate bacterial pathogens. The first published paper investigates the effectiveness of an immobilised copper-based photosensitiser as a novel photo-antimicrobial to kill pathogenic bacteria using visible light. Photosensitisers which produce reactive oxygen species under light excitation have emerged as an efficient way to kill microorganisms in water, yet the majority of photosensitising metal complexes uses transition metal complexes which are compromised when immobilised to a surface, meaning photosensitiser purification remains problematic. These findings presented here for the first time show that a copper-based photosensitiser immobilised onto the surface of silica is effective in reducing both Gram-positive and Gram-negative bacteria in solution. This example of an immobilised copper photosensitiser used for light driven bacterial killing demonstrates the potential of transition metal complexes as low-cost efficient photo- antimicrobials which has the potential for use in water purification systems. The second manuscript investigates the susceptibility of the foodborne pathogen Campylobacter jejuni to killing by photodynamic inactivation. We have shown that the presence of oxygen sensitive enzymes coupled with an abundance of light absorbing photosensitiser makes C. jejuni much more susceptible to the effects of PDI than other Gram-negative pathogens. In the absence of endogenous photosensitisers, we have shown Copper-conjugate photosensitisers can also be applied to efficiently kill bacterial pathogens by visible-light excitation. Given we are approaching the end of the antibiotic era, the application and understanding of these alternative pathogen reduction treatments is becoming ever more important. The findings presented here demonstrate that photodynamic inactivation could be used to remove C. jejuni from the surface of chicken skin which in turn could reduce the prevalence of this widespread pathogen. In this study, we aimed to characterise the global bacterial response to photooxidative stress, uncovering the molecular targets which become inactivated during PDI, as well as the cellular responses expressed to protect these enzymes. Given the microaerophilic nature of C. jejuni, this pathogen has an innate sensitivity to stress, yet little is understood how this stress-sensitive pathogen can survive across the range of oxygen tensions required to become a successful gastrointestinal pathogen. In the third chapter of unpublished work, we aim to utilise new techniques to uncover novel regulatory proteins which control the bacteria’s ability to survive and thrive across a range of oxygen tensions as it passes between its hosts. The results described herein further the knowledge of this widespread foodborne pathogen and highlight the potential of PDI to kill pathogenic bacteria.