The architecture of a mixed fungal–bacterial biofilm is modulated by quorum-sensing signals

Interkingdom communication is of particular relevance in polymicrobial biofilms. In this work, the ability of the fungus Ophiostoma piceae to form biofilms individually and in consortium with the bacterium Pseudomonas putida, as well as the effect of fungal and bacterial signal molecules on the architecture of the biofilms was evaluated. Pseudomonas putida KT2440 is able to form biofilms through the secretion of exopolysaccharides and two large extracellular adhesion proteins, LapA and LapF. It has two intercellular signalling systems, one mediated by dodecanoic acid and an orphan LuxR receptor that could participate in the response to AHL-type quorum sensing molecules (QSMs). Furthermore, the dimorphic fungus O. piceae uses farnesol as QSM to control its yeast to hyphae morphological transition. Results show for the first time the ability of this fungus to form biofilms alone and in mixed cultures with the bacterium. Biofilms were induced by bacterial and fungal QSMs. The essential role of LapA-LapF proteins in the architecture of biofilms was corroborated, LapA was induced by farnesol and dodecanol, while LapF by 3-oxo-C6-HSL and 3-oxo-C12-HSL. Our results indicate that fungal signals can induce a transient rise in the levels of the secondary messenger c-di-GMP, which control biofilm formation and architecture.