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

<i>Pseudomonas aeruginosa</i> and <i>Scedosporium/Lomentospora</i> often coexist in the lungs of cystic fibrosis patients, where their interaction can affect disease outcomes. Our group has recently demonstrated that <i>P. aeruginosa</i> suppresses the growth of <i>Scedosporium/Lomentospora</i> species partly through mechanisms involving iron sequestration. In this study, we have investigated how molecules secreted by <i>P. aeruginosa</i> under high (36 µM) and low (3.6 µM) iron conditions affect the planktonic growth and biofilm formation by <i>S. apiospermum</i>, <i>S. minutisporum</i>, <i>S. aurantiacum</i> and <i>L. prolificans</i>. Although <i>P. aeruginosa</i> 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 <i>P. aeruginosa</i> 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 <i>Tenebrio molitor</i> infection model, resulting in 100% larval mortality within 7 days. Collectively, these results indicate that the antifungal activity of <i>P. aeruginosa</i> 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 <i>P. aeruginosa</i> interactions with <i>Scedosporium/Lomentospora</i> species. Such iron-dependent dynamics may influence the progression and severity of respiratory co-infections, with important implications for patient management and therapeutic interventions.