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Killing of Organisms Responsible for Wound Infections Using a Light-Activated Antimicrobial Agent

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Killing of Organisms Responsible for Wound Infections Using a Light-Activated Antimicrobial Agent Killing of organisms responsible for wound infections using a light-activated antimicrobial agent Submitted by Ghada Said Mohammed Omar BSc, MSc For the degree of Doctor of Philosophy in the Faculty of Medicine University College London 2010 Division of Microbial Diseases UCL Eastman Dental Institute University College London 256 Gray’s Inn Road London WC1X 8LD United Kingdom Declaration I, Ghada Omar certify that the work presented in this thesis is the result of my own investigations. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. Ghada Omar ii Acknowledgements First of all I would like to thank Allah for giving me the strength to complete this work. I would like to thank the Egyptian government for funding my studentship leading to this research work. I would like to express my deep-felt thanks and gratitude to my supervisors Professor Mike Wilson and Dr. Sean Nair for their continued support and guidance throughout my PhD. Mike for his technical advice, editorial assistance and endless enthusiasm and Sean for his practical guidance, encouragement and support which were an inspiration during even the most hectic times. I will always be grateful to both of them because they taught me with immense patience and understanding. I am also indebted to Mr Will Koning for his constructive technical assistance during biofilms’ experiments, to Dr. Derren Ready for his advice related to microbiology and to Miss Aviva Petrie for her help and guidance with statistical analyses. I would like to express my heartfelt thanks to all of my colleagues in the Division of Microbial Diseases who supported me throughout the ups and downs of my PhD and whose fostered friendship and music remedy were essential to overcome all the difficulties. In particular, I would like to acknowledge Ms Emily Earsman for her encouragement in the early days of my research and Dr. Farah Dalwai and Dr. Lena Ciric for their invaluable help and support during the writing-up stage. I would like to express my deepest gratitude to my family, without their love and support I could not accomplish this work. However my mother is far away her love and prayers always surrounded me, the echo of her sound telling me “you can do it Ghada” helped me to keep going even in the worst days. The believe of my parents in me makes me who I am today. Finally I would like to thank my best friend, Dr. Azam Arzoo, who provided a shoulder throughout the stress of writing-up. Without his forbearance, support and continuous encouragement I could not have accomplished this thesis. iii Abstract Infected wounds are a major cause of hospital-acquired infections and these are difficult to treat due to the emergence of antibiotic-resistant bacteria. This project is concerned with evaluating a novel antimicrobial approach involving the photosensitizer indocyanine green (ICG) which generates reactive oxygen species when irradiated with near-infrared (NIR) light which enables good tissue penetration. The photo-susceptibility of common wound-infecting organisms to ICG coupled with NIR-light was investigated. All species were susceptible to killing. ICG at a concentration of 25 µg/mL enabled the killing of the Gram-positive species (Staphylococcus aureus and Streptococcus pyogenes), higher concentrations (100-200µg/mL) were necessary to achieve substantial kills of the Gram-negative species (Pseudomonas aeruginosa and Escherichia coli). Both high and low fluences were able to kill 99.999% of the Gram-positive bacteria. High fluence irradiation was necessary to kill 99.99% of the Gram-negative bacteria. The pulsed-mode of irradiation was as effective as the continuous-mode for killing the Gram-positive species. Yet only the continuous-mode of irradiation was able to kill P. aeruginosa. Biofilms of Staph. aureus and P. aeruginosa were susceptible to disruption and killing by ICG-photosensitization. A significant enhancement of lethal photosensitization of Staph. aureus was achievable using gold-nanoparticles and antioxidants. Significant kills (>99%) were achieved in the presence of serum and 100 µg/mL ICG. A low oxygen concentration reduced the kills to 96.77% and 71.62% for Staph. aureus and Strep. pyogenes respectively. Mechanistic studies revealed that killing was mediated mainly by reactive-oxygen species. In vivo studies in mice showed that ICG and continuous-NIR light could achieve kills of 96%, 93% and 78-91% for P. aeruginosa, Strep. pyogenes and Staph. aureus respectively. The results of these in vitro and in vivo studies imply that ICG-PDT could be an effective means of decreasing the microbial burden in wounds. iv Table of Contents CHAPTER 1 INTRODUCTION .................................................................................... 1 1.1 WOUNDS ........................................................................................................... 2 1.1.1 Classification and types of wounds ........................................................... 2 1.1.2 Wound healing and factors adversely affecting it ...................................... 3 1.2 WOUND INFECTIONS .......................................................................................... 4 1.2.1 Historical background of wound infections ................................................ 4 1.2.2 Contamination, colonization and infection ................................................. 6 1.2.2.1 Definition of wound infection ............................................................................... 8 1.2.3 The impact of wound infections on both healthcare system and patients .. 9 1.2.4 Wound-associated organisms................................................................. 11 1.2.4.1 Organisms associated with non-infected wounds .............................................. 11 1.2.4.2 Wound-infecting organisms ................................................................................ 14 1.2.4.2.1 Surgical wound infection ............................................................. 16 1.2.4.2.2 Burn wound infection .................................................................. 17 1.2.4.2.3 Traumatic wound infection .......................................................... 17 1.2.4.2.4 Chronic wound infection .............................................................. 20 1.2.5 Management of wound infection ............................................................. 23 1.2.5.1 Debridement ....................................................................................................... 25 1.2.5.2 The role of antimicrobial agents in controlling wound infection ....................... 26 1.2.5.3 Antiseptics, disinfectants and topical antibiotics ............................................... 26 1.2.5.4 Systemic antibiotics ............................................................................................ 28 1.2.5.5 Disadvantage of topical antimicrobial Agents .................................................... 31 1.2.5.6 Emergence of antibiotic resistance in wounds ................................................... 31 1.2.5.7 Antimicrobial agents to control wound infection - an area of controversy ....... 32 1.2.6 Biofilm resistance to antimicrobial agents ............................................... 35 1.2.6.1 Biofilms in wounds .............................................................................................. 37 1.3 PHOTODYNAMIC THERAPY ................................................................................ 38 1.3.1 The components of photodynamic therapy ............................................. 40 1.3.1.1 Light .................................................................................................................... 40 1.3.1.1.1 Coherent Light (Laser) ................................................................ 41 1.3.1.1.2 Incoherent light ........................................................................... 44 1.3.1.1.3 Ultraviolet .................................................................................... 45 1.3.1.2 Photosensitizers .................................................................................................. 46 1.3.1.2.1 Phenothiaziniums ........................................................................ 47 1.3.1.2.2 Acridines ..................................................................................... 48 1.3.1.2.3 Cyanines ..................................................................................... 49 1.3.1.2.3.1 Indocyanine green ................................................................. 50 1.3.1.2.4 Macrocyclic photosensitizers....................................................... 51 1.3.1.2.4.1 Porphyrins ............................................................................. 51 1.3.1.2.4.2 Chlorins ................................................................................. 52 1.3.1.2.4.3 Phthalocyanines .................................................................... 53 1.3.1.2.5 Fullerenes ................................................................................... 54 1.3.1.2.6 Naturally occurring photosensitizers ............................................ 54 1.3.1.2.6.1 Psoralens .............................................................................. 54 1.3.1.3 The mode of action of photosensitizers ............................................................. 55 1.3.2 Mechanism of action: photophysical and photochemical process ........... 55 1.3.2.1 Light interaction
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