Gallium-Protoporphyrin IX Inhibits Pseudomonas aeruginosa Growth by Targeting Cytochromes

Abstract

<i>Pseudomonas aeruginosa</i> is a challenging pathogen due to both innate and acquired resistance to antibiotics. It is capable of causing a variety of infections, including chronic lung infection in cystic fibrosis (CF) patients. Given the importance of iron in bacterial physiology and pathogenicity, iron-uptake and metabolism have become attractive targets for the development of new antibacterial compounds. <i>P. aeruginosa</i> can acquire iron from a variety of sources to fulfill its nutritional requirements both in the environment and in the infected host. The adaptation of <i>P. aeruginosa</i> to heme iron acquisition in the CF lung makes heme utilization pathways a promising target for the development of new anti-<i>Pseudomonas</i> drugs. Gallium [Ga(III)] is an iron mimetic metal which inhibits <i>P. aeruginosa</i> growth by interfering with iron-dependent metabolism. The Ga(III) complex of the heme precursor protoporphyrin IX (GaPPIX) showed enhanced antibacterial activity against several bacterial species, although no inhibitory effect has been reported on <i>P. aeruginosa</i>. Here, we demonstrate that GaPPIX is indeed capable of inhibiting the growth of clinical <i>P. aeruginosa</i> strains under iron-deplete conditions, as those encountered by bacteria during infection, and that GaPPIX inhibition is reversed by iron. Using <i>P. aeruginosa</i> PAO1 as model organism, we show that GaPPIX enters cells through both the heme-uptake systems <i>has</i> and <i>phu</i>, primarily <i>via</i> the PhuR receptor which plays a crucial role in <i>P. aeruginosa</i> adaptation to the CF lung. We also demonstrate that intracellular GaPPIX inhibits the aerobic growth of <i>P. aeruginosa</i> by targeting cytochromes, thus interfering with cellular respiration.