Heterogeneous Response to 470 nm Blue Light Amongst 25 Staphylococcus aureus Isolates

Abstract

Staphylococcus aureus infections are an increasing concern for the United States as it is currently the most prevalent cause of hospital acquired infections. The growing antibiotic resistance amongst these bacteria has called for new treatments, including photodynamic therapy, which uses light to kill microorganisms. We tested the variance of the response to blue light of 25 different isolates of S. aureus. Using 470 nm blue LEDs with an approximate forward power of 80 mW and an average luminance of 4.22 klux, we exposed 1 ml S. aureus cultures for 30 minutes and measured the optical density at 600 nm 18 hours after treatment. We compared the blue light treatment to a control group and found that there was a wide degree of variance with inhibition values ranging from 97% to an increased growth rate of 8%. Using flow cytometry, we measured cell counts at 4 hours after exposure to blue light for 3 strains identified as highly sensitive and 3 strains identified as highly resistant. There was a range of 80-99.9% inhibition when examining live cell counts. However, the discrepancy between the more and less sensitive strains was still apparent when examining live to dead cell ratios. A variation in susceptibility to blue light indicates a genetic trait may be responsible for this phenotype. Future studies will explore the genetic differences in these strains.