Bacterial strains and culture conditions

The bacterial strains, MRSA (ATCC 43300, ATCC BAA-42, ATCC BAA-43, ATCC BAA-44) and two antibiotic-resistant SA strains, APH2″-AAC6′ and RN4220/pUL5054, were included for testing. The APH2″-AAC6′ strain expresses the bi-functional enzyme AAC(6′)-APH(2″), which is an aminoglycoside-modifying enzyme conferring high-level gentamicin resistance (MIC: >128 μg/mL). The RN4220/pUL5054 strain over-expresses the msr(A) gene encoding for an ATP-binding cassette (ABC) transporter that induces resistance against erythromycin (MIC: 128 μg/mL) [40]. Ten non-duplicate clinical isolates, namely five hospital (HA)- and five community-associated (CA)-MRSA were included. They include important clonal types ST239, ST30, and ST59 previously documented to be prevalent in Hong Kong and in neighbouring countries [41–43]^. All MRSA strains were grown in Mueller Hinton Broth (MHB) for 18 h at 37°C. The overnight culture suspension was adjusted to McFarland 0.5 and suspended in MHB to make a final concentration of 1.0 × 10⁶ CFU/mL. Altogether, 16 MRSA strains (6 ATCC type strains and 10 clinical non-duplicate isolates) were included for in vitro aPDT studies.