Phototoxic effect of hypericin alone and in combination with acetylcysteine on Staphylococcus aureus biofilms

• We evaluated the in vitro bactericidal effect of HYP-PDI on clinical Staphylococcus aureus biofilms.
• In vitro bactericidal effect of HYP-PDI on S. aureus biofilms treated with acetylcysteine was also studied.
• HYP-PDI did not result in a reduction in viable count for each of the strains when grown in biofilms.
• HYP-PDI applied on biofilms treated with acetylcysteine was able to disrupt pre-formed biofilms.
• Acetylcysteine may enhance the efficacy of PDI by disrupting the biofilm structure.

SummaryBackgroundResistance of bacteria against antibiotics and antimicrobials is arising worldwide and there is an urgent need for strategies that are capable of inactivating biofilm-state pathogens with less potential of developing resistance in pathogens. A promising approach could be photodynamic inactivation (PDI) which uses light in combination with a photosensitizer to induce a phototoxic reaction. In this study, we evaluated the in vitro phototoxic effect of hypericin (HYP) alone and in combination with acetylcysteine (AC) on Staphylococcus aureus biofilms. AC, a mucolytic agent, reduces the production of extracellular polysaccharide matrix while promoting the disruption of mature biofilm.MethodsIn vitro phototoxic effect of HYP alone (0.5 μg/ml, light dose: 16 J/cm2), and in combination with AC (10 mg/ml) on ten clinical S. aureus isolates and S. aureus (ATCC 25923) biofilms was studied. Effect of HYP concentration (0.5 μg/ml) and light dose (8 J/cm2) on PDI of all eleven S. aureus strains in planktonic forms was also investigated.ResultsHYP-PDI did not result in a reduction in viable count for each of the strains when grown in biofilms. However, HYP-PDI applied on biofilms treated with AC was able to disrupt pre-formed biofilms (viable count reduction ranging from 5.2 to 6.3 log10-unit in comparison to controls in all tested strains). A 6.5 log killing was obtained for S. aureus (ATCC 25923) planktonic cells treated with 0.5 μg/ml at 8 J/cm2. For this set of PDI parameters, ten clinical S. aureus isolates showed 5.5–6.7 log killing.ConclusionHYP-PDI in combination with AC had significant ability to eradicate the pre-formed mature biofilms of S. aureus strains.