title: Effect of sub-lethal photodynamic inactivation on the antibiotic susceptibility and biofilm formation of clinical Staphylococcus aureus isolates.
abstract: BACKGROUND
  A promising approach to kill antibiotic-resistant bacteria uses light in combination with a photosensitizer to induce a phototoxic reaction. A major concern with the use of any non-antibiotic antimicrobial treatment is that exposure of bacteria to sub-lethal concentrations will lead to the development of resistance to antibiotics. This study aimed to determine the effect of sub-lethal photodynamic inactivation (PDI) on the antibiotic susceptibility and biofilm formation of clinical Staphylococcus aureus isolates.
  
  
  METHODS
  Forty clinical S. aureus isolates were exposed to PDI with toluidine blue O (TBO) and methylene blue (MB). After exposure, susceptibility of surviving organisms to a range of antibiotics was determined and compared with the susceptibility of an untreated control. PDI experiments were done during three generations for assessment of biofilm formation, to determine if biofilm formation was affected by exposure to PDI.
  
  
  RESULTS
  It was observed that the effect of sub-lethal PDI on the antibiotic sensitivity was strain-dependent. In general, exposure to sub-lethal MB/TBO-PDI increased resistance to erythromycin, amoxicillin-clavulanate and amikacin. Biofilm formation ability of studied clinical isolates increased after second sub-lethal PDI regimen compared to that before PDI.
  
  
  CONCLUSIONS
  S. aureus cells may develop resistance by growing in the presence of sub-lethal MB/TBO-PDI.
authors.affiliation: Department of Microbiology, School of Biology, College of Science, University of Tehran, Tehran, Iran. Electronic address: kashefn@khayam.ut.ac.ir.
authors.forename: Nasim
authors.initials: N
authors.lastname: Kashef
authors.forename: Mahboobeh
authors.initials: M
authors.lastname: Akbarizare
authors.forename: Seyed Kamran
authors.initials: SK
authors.lastname: Kamrava
mesh_terms: D000900:Anti-Bacterial Agents / Q000008:administration & dosage; D018441:Biofilms / Q000187:drug effects* / Q000254:growth & development / Q000528:radiation effects*; D004305:Dose-Response Relationship, Drug; D004307:Dose-Response Relationship, Radiation; D010778:Photochemotherapy / Q000379:methods*; D017319:Photosensitizing Agents / Q000008:administration & dosage*; D011836:Radiation Tolerance / Q000187:drug effects* / Q000502:physiology; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000502:physiology* / Q000528:radiation effects; D013242:Sterilization / Q000379:methods*
chemical_list: D000900:Anti-Bacterial Agents; D017319:Photosensitizing Agents
keywords: Phenothiazinium dyes; Staphylococcus aureus; Sub-lethal photodynamic inactivation