Extracellular and intracellular bactericidal activities of XF-70 against small-colony variant hemB mutants of meticillin-susceptible and meticillin-resistant Staphylococcus aureus

Small-colony variants (SCVs) of Staphylococcus aureus are phenotypic variants characterised by their small colony size and improved intracellular survival and are associated with persistent and relapsing infections. XF drugs are membrane-active, porphyrin-based antibacterial agents for topical administration, exerting rapid bactericidal activity against actively growing or resting, antibiotic-susceptible and multidrug-resistant strains of S. aureus. In this study, minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of XF-70 against isogenic, electron-transport deficient, SCV hemB mutants of one meticillin-susceptible S. aureus (MSSA) strain and one meticillin-resistant S. aureus (MRSA) strain were evaluated. Macrodilution MICs of XF-70 for MSSA strain 8325-4 and its hemB+-complemented derivative (0.5–1 mg/L) were reproducible and were slightly higher than that for the SCV hemB mutant (0.25–0.5 mg/L) and were not influenced by increasing inoculum size from 106 to 108 colony-forming units (CFU)/mL. MICs for MRSA strain COL, its SCV hemB mutant and hemB+-complemented derivative were equivalent (0.25–1 mg/L). MBCs of XF-70 were ≤2-fold higher than MICs for all isolates. Extensive killing (≥4 log reduction in CFU/mL) was produced by 2 mg/L XF-70 within 30 min against SCV hemB mutants both of 8325-4 and COL as well as their respective parent or hemB+-complemented derivatives. Pre-incubation of 107 CFU/mL of 8325-4 and its SCV hemB mutant with 5 × 106 polymorphonuclear neutrophils for 30 min markedly protected phagocytised organisms from rapid extensive killing by bactericidal levels (2 mg/L) of subsequently added XF-70. The rapid bactericidal activity of XF-70 at low concentrations both against SCV and normally growing S. aureus is remarkable and represents an attractive potential for the treatment of persistent localised infections.