The aim of this study was to determine the photodynamic antimicrobial effect of hypericin on clinically isolated Staphylococcus aureus and Escherichia coli cells. Bacterial cells (10(8) cells per mL) were incubated with hypericin (0-40 μM) for 30 min and followed by light irradiation of 600-800 nm at 5-30 J cm(-2). Cell survival was determined by colony counting, cellular hypericin uptake examined by flow cytometer, and cell membrane damage examined by scanning electron microscopy and leakage assay. The effectiveness of hypericin-mediated photodynamic killing was strongly affected by cellular structure and photosensitizer uptake. The combination of hypericin and light irradiation could induce significant killing of Gram positive methicillin-sensitive and -resistant S. aureus cells (>6 log reduction), but was not effective on Gram negative E. coli cells (<0.2 log reduction). The difference was caused by different cell wall/membrane structures that directly affected cellular uptake of hypericin.

Photodynamic inactivation (PDI) has been investigated to cope with the increasing incidence of multidrug-resistant (MDR) pathogens. In Hong Kong, methicillin-resistant Staphylococcus aureus (MRSA) and extended-spectrum β-lactamase (ESBL)-producing Escherichia coli are the two commonest MDR pathogens. Here, we studied the photodynamic inactivation (PDI) mediated by poly-L-lysine chlorin(e6) conjugate (pL-ce6) and toluidine blue O (TBO) in clinical MRSA and ESBL producing E. coli, together with their corresponding American Type Culture Collection (ATCC) strains. Both pL-ce6 and TBO mediated a light-and drug dose-dependent efficacy for the four pathogens. pL-ce6 was more effective. pL-ce6 at 8 µM, 30 Jcm −2 , attained 5 log killing for ESBL-producing E. coli and E. coli (ATCC 25922); 4 log killing for MRSA, and 3 log killing for S. aureus (ATCC 25923). TBO at 80 µM, 30 Jcm −2 , only exhibited 3 log killing in MRSA and 2 log killing in S. aureus (ATCC 25923). TBO (400 µM, 30 Jcm −2 ) induced equal killing for ESBLproducing E. coli and E. coli (ATCC 25922). Our studied MRSA isolate responded better than S. aureus (ATCC 25923). Thus, pL-ce6-mediated PDI in other MRSA isolates deserves further investigation.