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.