Altered Pseudomonas Strategies to Inhibit Surface Aspergillus Colonies.

Pseudomonas aeruginosa and Aspergillus fumigatus infections frequently co-localize in lungs of immunocompromised patients, and individuals with cystic fibrosis (CF). Anti-fungal activity of P. aeruginosa has been described for its filtrates. Pyoverdine and pyocyanin are the principal anti-fungal P. aeruginosa molecules active against A. fumigatus biofilm metabolism present in iron-limited or iron-replete planktonic P. aeruginosa culture filtrates, respectively. Using various P. aeruginosa laboratory wildtype strains (PA14, PAO1, PAK) we found anti-fungal activity against Aspergillus colonies on agar. Comparing 36 PA14 and 7 PAO1 mutants we found that mutants lacking both major siderophores, pyoverdine and pyochelin, display higher anti-fungal activity on agar than their wildtypes, while quorum sensing mutants lost anti-fungal activity. Addition of ferric iron, but not calcium or magnesium, reduced anti-fungal effects of P. aeruginosa on agar, whereas iron-poor agar enhanced anti-fungal effects. Anti-fungal activity on agar was mediated by PQS and HHQ, via MvfR. Among the MvfR downstream factors, rhamnolipids and elastase were produced in larger quantities by pyoverdine-pyochelin double mutants, and showed anti-fungal activity on agar. In summary, anti-fungal factors produced by P. aeruginosa on agar differ from those produced by bacteria grown in liquid cultures, are dependent on quorum sensing, and are down-regulated by the availability of ferric iron. Rhamnolipids and elastase seem to be major mediators of Pseudomonas’ anti-fungal activity on a solid surface.