# Role of Iron Availability in Modulating Pseudomonas aeruginosa’s Antifungal Effects on Planktonic and Biofilm Growth of Scedosporium/Lomentospora Under Cystic Fibrosis-Mimicking Conditions

## TL;DR

This study shows how iron levels affect how Pseudomonas aeruginosa interacts with a fungus in cystic fibrosis patients, influencing both antifungal effects and toxicity.

## Contribution

The study reveals that iron availability modulates Pseudomonas aeruginosa's antifungal and toxic effects on fungal species in cystic fibrosis-like conditions.

## Key findings

- Low-iron conditions enhance P. aeruginosa's antifungal activity against Scedosporium/Lomentospora species.
- Low-iron supernatants show significant toxicity to mammalian cells and insect larvae.
- Iron availability shapes the interaction dynamics between P. aeruginosa and fungi in respiratory co-infections.

## Abstract

Pseudomonas aeruginosa and Scedosporium/Lomentospora often coexist in the lungs of cystic fibrosis patients, where their interaction can affect disease outcomes. Our group has recently demonstrated that P. aeruginosa suppresses the growth of Scedosporium/Lomentospora species partly through mechanisms involving iron sequestration. In this study, we have investigated how molecules secreted by P. aeruginosa under high (36 µM) and low (3.6 µM) iron conditions affect the planktonic growth and biofilm formation by S. apiospermum, S. minutisporum, S. aurantiacum and L. prolificans. Although P. aeruginosa exhibited enhanced proliferation under high-iron conditions, spectrophotometric analyses revealed a marked increase in phenazine and pyoverdine production under low-iron conditions, with siderophore activity confirmed by Chrome Azurol S assays. Supporting these findings, supernatants from P. aeruginosa cells grown under iron limitation markedly inhibited fungal growth (≈30%) and biofilm formation (≈70%), whereas those from high-iron cultures were less effective. Notably, low-iron bacterial-free supernatants exhibited pronounced cytotoxic effects on mammalian cells, reducing metabolic activity by an average of 20% in A549 lung epithelial cells and 40% in THP-1 macrophages, and significantly compromising survival in the Tenebrio molitor infection model, resulting in 100% larval mortality within 7 days. Collectively, these results indicate that the antifungal activity of P. aeruginosa is closely coupled with increased host toxicity. Moreover, the results demonstrate that environmental iron availability plays a critical role in modulating both antifungal activity and toxicity, thereby shaping P. aeruginosa interactions with Scedosporium/Lomentospora species. Such iron-dependent dynamics may influence the progression and severity of respiratory co-infections, with important implications for patient management and therapeutic interventions.