Studies of Pseudomonas aeruginosa Mutants Indicate Pyoverdine as the Central Factor in Inhibition of Aspergillus fumigatus Biofilm

Abstract

<i>Pseudomonas aeruginosa</i> and <i>Aspergillus fumigatus</i> are common opportunistic bacterial and fungal pathogens, respectively. They often coexist in airways of immunocompromised patients and individuals with cystic fibrosis, where they form biofilms and cause acute and chronic illnesses. Hence, the interactions between them have long been of interest and it is known that <i>P. aeruginosa</i> can inhibit <i>A. fumigatus</i><i>in vitro</i> We have approached the definition of the inhibitory <i>P. aeruginosa</i> molecules by studying 24 <i>P. aeruginosa</i> mutants with various virulence genes deleted for the ability to inhibit <i>A. fumigatus</i> biofilms. The ability of <i>P. aeruginosa</i> cells or their extracellular products produced during planktonic or biofilm growth to affect <i>A. fumigatus</i> biofilm metabolism or planktonic <i>A. fumigatus</i> growth was studied in agar and liquid assays using conidia or hyphae. Four mutants, the <i>pvdD pchE</i>, <i>pvdD</i>, <i>lasR rhlR</i>, and <i>lasR</i> mutants, were shown to be defective in various assays. This suggested the <i>P. aeruginosa</i> siderophore pyoverdine as the key inhibitory molecule, although additional quorum sensing-regulated factors likely contribute to the deficiency of the latter two mutants. Studies of pure pyoverdine substantiated these conclusions and included the restoration of inhibition by the pyoverdine deletion mutants. A correlation between the concentration of pyoverdine produced and antifungal activity was also observed in clinical <i>P. aeruginosa</i> isolates derived from lungs of cystic fibrosis patients. The key inhibitory mechanism of pyoverdine was chelation of iron and denial of iron to <i>A. fumigatus</i><b>IMPORTANCE</b> Interactions between human pathogens found in the same body locale are of vast interest. These interactions could result in exacerbation or amelioration of diseases. The bacterium <i>Pseudomonas aeruginosa</i> affects the growth of the fungus <i>Aspergillus fumigatus</i> Both pathogens form biofilms that are resistant to therapeutic drugs and host immunity. <i>P. aeruginosa</i> and <i>A. fumigatus</i> biofilms are found <i>in vivo</i>, e.g., in the lungs of cystic fibrosis patients. Studying 24 <i>P. aeruginosa</i> mutants, we identified pyoverdine as the major anti-<i>A. fumigatus</i> compound produced by <i>P. aeruginosa</i> Pyoverdine captures iron from the environment, thus depriving <i>A. fumigatus</i> of a nutrient essential for its growth and metabolism. We show how microbes of different kingdoms compete for essential resources. Iron deprivation could be a therapeutic approach to the control of pathogen growth.