Abstract. Introduction: Cutibacterium acnes is gaining recognition as a leading pathogen after orthopaedic shoulder procedures. Photodynamic therapy, a combination of light and a photosensitizer, has demonstrated antimicrobial activity against C. acnes in the treatment of acne vulgaris. We sought to evaluate the effect of photodynamic therapy using blue light and photosensitizers on C. acnes isolates from shoulder prosthetic joint infections.Methods: C. acnes strains isolated from 19 patients with shoulder PJI were exposed to blue light alone (415 nm) or in combination with photosensitizers (fluorescein, riboflavin and demeclocycline). C. acnes strains were divided into 4 categories: Highly Sensitive (HS), Sensitive (S), Weakly Sensitive (WS), Resistant to blue light.Results: 13 of 19 C. acnes strains (68%) were S or HS to blue light alone. Of these 19 strains tested, 11 were tested with blue light and fluorescein or blue light plus riboflavin. Fluorescein (1 µg/mL) enhanced the effect of blue light in 6 of 11 strains (55%). Blue light plus riboflavin (10 µg/mL) resulted enhanced killing in 3 of 11 strains (27%), but produced a paradoxical photoprotective effect in 4 of 11 strains (36%), resulting in a net decrease compared to blue light alone. Demeclocycline, however, enhanced the effect of blue light in 16 of 17 strains (94 %).Conclusions: Blue light with the addition of photosensitizers killed C. acnes from periprosthetic shoulder infections in vitro, with demeclocycline having the most pronounced effect.

Background Cutibacterium acnes (C. acnes) is a common shoulder periprosthetic joint infection (PJI). Blue light (BL) is effectively used in the dermatologic clinical setting against acne vulgaris caused by C. acnes. Photodynamic therapy (PDT) is the use of light source and photosensitizer (PS) to enhance antimicrobial activity. We studied the effect of PDT using BL and PS in vitro on shoulder PJI isolates of C. acnes.Methods19 strains were grown in thioglycollate medium and diluted in sterile normal saline (NS) to a turbidity of 0.5 McFarland standard; OD600 of 0.1 to 0.15. 250 µL with PS added were placed in 96-well plates at 37ºC, exposed to BL (415 nm) placed 1 cm above for 0 to 60 minutes at 15-minute intervals. Susceptibility to BL alone, and BL with PSs such as riboflavin (R, Vit B2), fluorescein (F) or demeclocycline (tetracycline antibiotic, “D”) were studied. After serial 10-fold dilution with NS, 3 µL of each well were spotted onto Brucella Blood Agar plates and incubated anaerobically for 48 hours. Eradication was defined as below the limit of detection. Definitions include Sensitive (S) if 3-log decrease in bacterial density or eradication at any time point, Weakly Sensitive (WS) with 1- to 3-log decrease and Resistant (R) with no decrease.Results Based on BL alone, (n = 19). 68% strains were S, 32% were resistant. BL+ R (10 μg/mL) effect in 25% (n = 3) and exerted a protective effect against 33% (n = 4). BL+ F (1 μg/mL) potentiated in 67%. BL+ D (0.1–1.5 μg/mL) in 83% of strains tested. The most resistant strain was eradicated using BL + D at an increased concentration of demeclocycline (2.5 μg/mL).ConclusionF and D enhanced the potential for eradication compared with BL exposure alone. R was a photo-protectant to BL for select strains. Prior studies have hypothesized endogenous intracellular porphyrins excited by BL causing energy transfer and production of highly cytotoxic reactive oxygen species causing bacterial death. Future clinical research evaluating the use of preoperative PS and surgical site exposure to BL as a preventative PJI strategy are needed. Our research shows that BL with the addition of PS significantly reduces the bacterial burden of clinically relevant PJI shoulder isolates of C. acnes in an in vitro model.Disclosures All authors: No reported disclosures.