Non-iron Hemin Analogs in Antimicrobial Photodynamic therapy, Stress tolerance, and Heme Uptake Mechanisms

Bacteria often develop tolerance upon prolonged exposure to antibiotics. Tolerance development is associated with the failure of antibiotic regimens and cause chronic infections. Tolerance is often overshadowed by persister cells and resistant bacteria. However, there is an increasing understanding that tolerances serve as a percussor for resistance development. This work focuses on non-iron hemin analogs (Al-, Ga-, and In-PpIX) as photosensitizer against S. aureus, methicillin resistant S. aureus (MRSA), and tolerant small colony variants (SCVs). Al-PpIX and Ga-PpIX are potent photodynamic agents against S. aureus and MRSA with antimicrobial activity at nanomolar range. The potency of Al-PpIX and Ga-PpIX drops against H₂O₂ tolerant bacteria, but the potency is enhanced against H₂O₂ sensitive bacteria. Ga-PpIX is effective as an antimicrobial agent against H₂O₂ and antibiotic tolerant SCVs with 3-log reduction in sub-micromolar range. In addition, the fluorescent properties of Al- and Ga-PpIX are utilized to study the heme uptake systems in Gram-positive and Gram-negative bacteria by time-dependent flow cytometry. Flow cytometry analysis allowed to differentiate between active cell surface dependent hemin acquisition (Type 1) and heme harvesting (Type 2) bacteria.