Using photodynamic therapy to combat bacterial biofilms and reduce infection on orthopaedic implants

摘要

Introduction: Periprosthetic Joint Infection (PJI) is one of the greatest challenges to orthopaedics. The quoted rate for deep infection varies from 0.28% to 4% for a primary hip replacement, 0.39% to 3.9% for primary total knee replacement, and a higher incidence of infection in spinal implants . It is associated with high patient morbidity and has huge financial cost. Current treatments are long, invasive, and often ineffective. Increasing antibiotic resistance of microorganisms and the inefficacy of antibiotic treatment against biofilms on prosthetic implants means alternative treatments are needed. Photodynamic Therapy (PDT) is a known broad-spectrum antimicrobial treatment providing rapid results with few systemic side effects and no bacterial resistance. It uses a photosensitiser that targets and kills bacterial cells following activation by an appropriate light source . This study investigates the use of PDT as a means of eradicating strains of bacteria that commonly cause prosthetic joint infections (methicillin sensitive staphylococcus aureus (MSSA), methicillin resistant staphylococcus aureus (MRSA), staphylococcus epidermidis, and pseudomonas aeruginosa both as planktonic culture and in a biofllm on titanium alloy and hydroxyapatite (HA)-coated titanium. Hypothesis: Photodynamic therapy is an effective means of eradicating common strains of bacteria that cause biofilms on orthopaedic implants. Methods: The concentration of the photosensitizer Methylene Blue (MB), and the laser power that was the most effective was determined using lawns of all bacteria in plate culture. Planktonic cultures of 4 bacteria grown in well-plates with4 treatment regimes - PDT (MB+L+), photo sensitizer alone (MB+L-), laser alone (MB-L+), and control (MB-L) using 0.3mM MB and 35jcm-2 laser. Following treatment the wells were analysed for the number of bacteria present using a standard serial dilution. Biofilms were formed for each of the 4 bacteria by placing titanium discs in bacterial suspension, which were oscillated and incubated for 3 days. The attached biofilms were treated (as per planktonic culture). Following treatment the discs were washed and sonicated to displace biofilm bacteria and standard serial dilution was performed to quantify remaining bacteria. This experiment was then repeated for P. aeruginosa biofilms on HA-coated discs Results: Staphylococci colony forming units were eradicated at the lowest strength of MB. With P. aeruginosa and A. baumannii, increasing the MB concentration had an improved bactericidal effect (Figure 1). The higher the power of the laser used in PDT the more bacteria were eradicated. Laser power ≥35 jcm-2 eradicated all bacterial colonies (figure2) but below this level there were slight differences in the susceptibility of different species. PDT had a significant bactericidal effect against planktonic MRSA and S. epidermidis compared to MB alone, laser alone, or the control. PDT was shown to have significantly higher bactericidal effects than MB alone or laser alone for all four strains of bacterial biofilms. For P. aeruginosa on HA-coated titanium discs, PDT was shown to have significantly higher bactericidal effects than photosensitiser alone, laser alone and the control however the effects of PDT were not as large as those seen with a titanium alloy surface (Figure 3). Discussion: This study demonstrates that PDT is an effective treatment for killing common gram-positive and gram-negative bacteria that cause PJI. It is effective not just for planktonic bacteria but bacteria within a biofilm on a prosthetic surface. Its speed of action and lack of bacterial resistance makes it an attractive therapy. The future challenge will be to find the optimal way of delivering this treatment.