Phototoxicity of phenothiazinium dyes against methicillin-resistant Staphylococcus aureus and multi-drug resistant Escherichia coli

Summary Background Photodynamic inactivation (PDI) has been investigated to cope with the increasing incidence of multidrug-resistant (MDR) pathogens. Here we studied the PDI mediated by methylene blue (MB) and toluidine blue O (TBO) in clinical methicillin-resistant Staphylococcus aureus and MDR Escherichia coli , together with their corresponding American Type Culture Collection (ATCC) strains. Methods Effect of photosensitizer concentration (12.5,25,50 μg/ml) and laser irradiation time (10, 20 and 30 min) on lethal photosensitization was investigated. Results TBO was more effective. TBO at 50 μg/ml, 46.8 J cm−2 , exhibited 0.7 log killing for MDR E. coli and 1.7 log killing for E. coli (ATCC 25922 ); 3.1 log killing for MRSA, and 4.2 log killing for S. aureus (ATCC 25923 ). MB at 50 μg/ml, 163.8 J cm−2 , only exhibited 2.2 log killing in MRSA and 3.1 log killing in S. aureus (ATCC 25923 ). MB (50 μg/ml, 163.8 J cm−2 ) induced 0.2 log killing for MDR E. coli and 0.3 log killing for E. coli (ATCC 25922 ). After TBO-PDI, MDR isolates were more susceptible to some antibiotics than control groups. Conclusion Our studied clinical isolates were more resistant to PDI-mediated killing than their ATCC reference strains. Thus, TBO/MB-mediated PDI in other MDR isolates deserves further investigation.