title: Wavelength and bacterial density influence the bactericidal effect of blue light on methicillin-resistant Staphylococcus aureus (MRSA).
abstract: OBJECTIVE
  The purpose of this study was to investigate the effect of wavelength and methicillin-resistant Staphylococcus aureus (MRSA) density on the bactericidal effect of 405 and 470 nm light.
  
  
  BACKGROUND
  It is recognized that 405 and 470 nm light-emitting diode (LED) light kill MRSA in standard 5 × 10(6) colony-forming units (CFU)/mL cultures; however, the effect of bacterial density on the bactericidal effect of each wavelength is not known.
  
  
  METHODS
  In three experiments, we cultured and plated US300 MRSA at four densities. Then, we irradiated each plate once with either wavelength at 0, 1, 3, 45, 50, 55, 60, and 220 J/cm(2).
  
  
  RESULTS
  Irradiation with either wavelength reduced bacterial colonies at each density (p<0.05). More bacteria were cleared as density increased; however, the proportion of colonies cleared, inversely decreased as density increased--the maximum being 100%, 96%, and 78% for 3 × 10(6), 5 × 10(6), and 7 × 10(6) CFU/mL cultures, respectively. Both wavelengths had similar effects on the sparser 3 × 10(6) and 5 × 10(6) CFU/mL cultures, but in the denser 7 × 10(6) CFU/mL culture, 405 nm light cleared more bacteria at each fluence (p<0.001). To determine the effect of beam penetration, denser 8 × 10(6) and 12 × 10(6) CFU/mL culture plates were irradiated either from the top, the bottom, or both directions. More colonies were eradicated from plates irradiated from top and bottom, than from plates irradiated from top or bottom at the same sum total fluences (p<0.001).
  
  
  CONCLUSIONS
  The bactericidal effect of LED blue light is limited more by light penetration of bacterial layers than by bacterial density per se.
authors.affiliation: 1 College of Health Sciences, University of Wisconsin-Milwaukee , Milwaukee, Wisconsin.
authors.forename: Violet V
authors.initials: VV
authors.lastname: Bumah
authors.forename: Daniela S
authors.initials: DS
authors.lastname: Masson-Meyers
authors.forename: Susan E
authors.initials: SE
authors.lastname: Cashin
authors.forename: Chukuka S
authors.initials: CS
authors.lastname: Enwemeka
mesh_terms: D002478:Cells, Cultured; D015169:Colony Count, Microbial; D004307:Dose-Response Relationship, Radiation; D055624:Methicillin-Resistant Staphylococcus aureus / Q000528:radiation effects*; D050296:Microbial Viability / Q000528:radiation effects; D010789:Phototherapy / Q000379:methods*