الفهرس 
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Abstract
Antimicrobial resistance has emerged as one of the leading public health threats of the 21st century. Increasing global antibiotic use and access to nonprescription antibiotics may partially explain the rise in AMR. It poses a threat to the effectiveness and efficiency of healthcare systems with increasing morbidity, mortality, and healthcare costs. The emergence of MDR strains makes the treatment options challenging. Poor infection control measures as well as continued exposure of humans and animals to antibiotics have resulted in the dissemination of MDR organisms. This in turn limits the choice of treatment for MDR infections and leads to enormous increases in antibiotic use for therapeutic, prophylactic and empirical strategies with more resistance rates. Physicians are limited by the treatment options they can use in MDR infections. In addition to the increasing resistance to available treatments, concerns regarding the limited number of new treatments under development have also been raised. MDR demands immediate implementation of novel therapeutic strategies as PDT. Antimicrobial photodynamic therapy has the ability to kill various microorganisms when the appropriate PS and light are combined in the presence of oxygen, generating reactive oxygen species that induce destruction of the target cell. Photosensitizers in PDT are the vessels that allow for the transfer and translation of light energy into a chemical reaction. For PDT to be successful for antimicrobial purposes, it is essential to select an appropriate and effective non-toxic PS capable of high absorption in the light length used. Phenothiazinium is a subgroup of synthetic dyes with a maximum absorption wavelength in the red light spectrum ranging from 630-640 nm. The most commonly used phenothiazinium dyes are MB, NMBN and TBO. Methylene blue is a well-known blue dye which is capable of inactivating both Gram-positive and Gram-negative bacteria. The ease to obtain MB, the possibility of using non-laser polychromatic light sources and low cost makes MB a potential PDT sensitizer that could be used in a larger population for the treatment of a variety of diseases. New Methylene Blue N is a new generation of MB with more lipophilic nature. It has phototherapeutic potential due to the fact that its absorption wavelength is in the range of maximum penetration of light in the host tissue. Toluidine Blue O is a hydrophilic cationic phenothiazinium dye that has been widely studied for the inactivation of pathogenic microorganisms. It is a broad spectrum bactericidal for many microorganisms. Skin functions as a crucial barrier against the environment and microbial invasions. The disruption of skin properties could be due to surgical procedures, chemical ,physical and thermal actions. Skin wounds are at risk of colonization of pathogenic microorganisms involving gram-negative and gram-positive bacteria. The conventional strategies for combating microbial infections for skin infections treatment are primarily based on antibiotics as the most efficient method. The abuse or overuse of antibiotics in hospitals generates MDR bacteria which is hard to treat . The aim of our study was to isolate pathogenic micro-organisms from sites of superficial skin infection, evaluate antimicrobial effect of PDT on the MDR isolates with different PSs as MB, NMBN and TBO, with different exposure times and compare the effect of PDT to the effect of PSs alone on bacteria. This study was carried out in Medical Microbiology and Immunology department, Faculty of Medicine Tanta University on 50 samples collected from patients admitted to the inpatient departments of Tanta University Hospitals during the period of research. All patients were subjected to thorough history taking, clinical examination, samples were cultured on nutrient, MacConkey, blood, Sabouraud’s dextrose, mannitol salt and bile esculin agar. Gram stained smears were prepared and microscopically examined after culturing the plates to avoid the possibility of contaminating the samples, then all cultivated plates were incubated at 37°c for 24 hrs., after incubation, the plates were observed for growth and the isolated colonies were identified by morphological and biochemical characteristics. Antimicrobial susceptibility of all bacterial isolates was determined by using disc diffusion method on Muller Hinton agar plate, results were interpreted according to the CLSI 2021 guidelines. Only the MDR isolates were kept at -80 refrigerator. For determination of the antibacterial effect of PDT on MDR isolates, three experiments were done. The first one was using red LED alone on all the MDR isolates for 10,20,30 and 60 minutes. The second one was testing each of the three PSs in three concentrations 50μ, 100μ and 200μ on all the MDR isolates for 10,20,30 and 60 minutes. The third experiment was the PDT in which the MDR isolate were treated with both red LED and PSs with different concentrations for 10,20,30 and 60 minutes. After each experiment colony count was made to determine the antibacterial effect of each experiment. Among 50 specimens, the most frequently isolated organisms were Pseudomonas spp. and S. aureus ( 18%, each), followed by Klebsiella spp. (16%) then E.coli (12%) followed by CoNS (10%) then Candida spp. (6%) and finally Proteus spp.(4%). As regard the antibacterial effect of PDT using MB, NMBN and TBO, all showed reduction in colony count of all MDR isolated bacteria with variable degrees, the results were different with different concentrations of the PSs used and with different durations. PDT with TBO showed the best colony count reduction especially with S. aureus. Using the PSs alone also had an antibacterial effect, however, it was lower than the effect obtained from PDT. The MDR isolates treated with red LED only showed initial reduction of colony count followed by a slight increase with increasing the exposure duration. The results of the present study indicated that PDT comparatively more efficient in reduction of bacterial colony count in comparison with using the PSs alone. This could be explained by the fact that PSs produce more toxic metabolites when activated by the appropriate wavelength light more than dark toxicity. Increasing concentration and increasing exposure duration has increased the antibacterial effect of PSs against MDR isolates with variable degrees, with 200μ concentration and 60 minutes duration giving the best results regarding colony count reduction.