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

Background: Resistance 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.

Methods: In vitro phototoxic effect of HYP alone (0.5 μg/ml, light dose: 16 J/cm(2)), 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/cm(2)) on PDI of all eleven S. aureus strains in planktonic forms was also investigated.

Results: HYP-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/cm(2). For this set of PDI parameters, ten clinical S. aureus isolates showed 5.5-6.7 log killing.

Conclusion: HYP-PDI in combination with AC had significant ability to eradicate the pre-formed mature biofilms of S. aureus strains.