Investigating Inhibitory Synergy between Blue Light Irradiation and Antibiotic Treatment of Staphylococcus aureus

Abstract

Staphylococcus aureus is a Gram-positive pathogen responsible for significant infections in both the hospital and the community. Rising antibiotic resistance has led to increased infection rates, while the ability of S. aureus to form protective biofilms on medical implants continues to contribute to life-threatening instances of recurrent bacteremia. In an effort to better combat these enhanced defenses, recent research has focused particularly on biofilm disruptors and other alternative treatment methods such as antimicrobial photodynamic therapy (aPDT), all of which have demonstrated synergistic inhibitory effects when combined with antibiotic therapy. This experiment was conducted to search for similar synergistic effects when irradiation with visible blue light, another emerging technique, is coupled with standard antibiotic treatments of oxacillin, neomycin, and ciprofloxacin. During testing, bacterial samples in 24-well plates were exposed to uniform 470nm blue light via an adjustable lightbox. The effects of combined therapy were ascertained through a modified broth microdilution assay with CFUs/mL measured periodically. Data from these tests reveal that the inhibitory effect of blue light alone is relatively dose-independent in the short term, with dosages as low as 5 minutes showing significant inhibition four hours after the beginning of treatment. Combined therapy is even more promising, evincing synergy between blue light and all tested antibiotics in the short-term and between blue light and all but neomycin in the long-term.