Iron Depletion Enhances Production of Antimicrobials by Pseudomonas aeruginosa

Abstract

Cystic fibrosis (CF) is a heritable disease characterized by chronic, polymicrobial lung infections. While Staphylococcus aureus is the dominant lung pathogen in young CF patients, Pseudomonas aeruginosa becomes predominant by adulthood. P. aeruginosa produces a variety of antimicrobials that likely contribute to this shift in microbial populations. In particular, secretion of 2-alkyl-4(1H)-quinolones (AQs) contributes to lysis of S. aureus in coculture, providing an iron source to P. aeruginosa both in vitro and in vivo. We previously showed that production of one such AQ, the Pseudomonas quinolone signal (PQS), is enhanced by iron depletion and that this induction is dependent upon the iron-responsive PrrF small RNAs (sRNAs). Here, we demonstrate that antimicrobial activity against S. aureus during coculture is also enhanced by iron depletion, and we provide evidence that multiple AQs contribute to this activity. Strikingly, a P. aeruginosa ⌬prrF mutant, which produces very little PQS in monoculture, was capable of mediating iron-regulated growth suppression of S. aureus. We show that the presence of S. aureus suppresses the ⌬prrF1,2 mutant's defect in iron-regulated PQS production, indicating that a PrrF-independent iron regulatory pathway mediates AQ production in coculture. We further demonstrate that iron-regulated antimicrobial production is conserved in multiple P. aeruginosa strains, including clinical isolates from CF patients. These results demonstrate that iron plays a central role in modulating interactions of P. aeruginosa with S. aureus. Moreover, our studies suggest that established iron regulatory pathways of these pathogens are significantly altered during polymicrobial infections. IMPORTANCEChronic polymicrobial infections involving Pseudomonas aeruginosa and Staphylococcus aureus are a significant cause of morbidity and mortality, as the interplay between these two organisms exacerbates infection. This is in part due to enhanced production of antimicrobial metabolites by P. aeruginosa when these two species are cocultured. Using both established and newly developed coculture techniques, this report demonstrates that iron depletion increases P. aeruginosa's ability to suppress growth of S. aureus. These findings present a novel role for iron in modulating microbial interaction and provide the basis for understanding how essential nutrients drive polymicrobial infections.

The Pseudomonas aeruginosa PrrF Small RNAs Regulate Iron Homeostasis during Acute Murine Lung Infection

“…2B) and previous (43) in vitro expression studies, expression of the antR mRNA was not upregulated in the ΔprrF1-prrF2 mutant during infection. PQS and its biosynthetic precursor, HHQ, are quorum sensing molecules that activate the expression of several virulence traits (33, 34, 44), and a variety of AQ metabolites are thought to contribute to interactions of P. aeruginosa with other bacterial pathogens during chronic, polymicrobial infections (45)(46)(47). To the best of our knowledge, though, the role of these metabolites in acute mammalian infections has not been addressed.…”

Iron-Mediated Control of Pseudomonas aeruginosa-Staphylococcus aureus Interactions in the Cystic Fibrosis Lung

“…The shift proves detrimental to S. aureus growth rates and fitness, while these conditions are more than hospitable for P. aeruginosa, which preferentially feeds on the lactate that its victim produces (7). Relevant to this model, Nguyen et al also demonstrate that free iron levels regulate P. aeruginosa's inhibition of S. aureus in coculture (9). This group had previously shown that P. aeruginosa's production of specific 2-alkyl-4(1H)-quinolones, antimicrobial compounds capable of killing S. aureus, is increased in low-iron environments (5).…”

“…Moreover, when these two bacteria are grown together in coculture, iron's ability to suppress antimicrobial production is restored to CF isolates previously thought to be defective for this activity. These findings suggest that bacterial ironregulated pathways become altered in polymicrobial infections and may contribute significantly to pathogenesis (9).…”

Coculture of Staphylococcus aureus with Pseudomonas aeruginosa Drives S. aureus towards Fermentative Metabolism and Reduced Viability in a Cystic Fibrosis Model

“…As previously reported, P. aeruginosainduced S. aureus cell lysis causes the release of iron, which can be further utilized by P. aeruginosa (43) and thus provide an additional competitive advantage to P. aeruginosa over S. aureus. The accompanying paper by Nguyen et al (72) further explores the role of iron in the transition from S. aureus to P. aeruginosa predominance.…”