Repeated Exposure of Aspergillus niger Spores to the Antifungal Bacterium Collimonas fungivorans Ter331 Selects for Delayed Spore Germination.

The bacterial strain Collimonas fungivorans Ter331 (CTer331) inhibits mycelial growth and spore germination in Aspergillus niger N402 (N402). The mechanisms underlying this antagonistic bacterial-fungal interaction have been extensively studied, but knowledge on the long-term outcome of this interaction is currently lacking. Here, we used experimental evolution to explore the dynamics of fungal adaptation to recurrent exposure to Ter331. Specifically, five single-spore isolates (SSIs) of N402 were evolved under three selection scenarios in liquid culture, i.e., (i) in the presence of Ter331 for 80 growth cycles, (ii) in the absence of the bacterium for 80 cycles, and (iii) in the presence of Ter331 for 40 cycles and then in its absence for 40 cycles. The evolved SSI lineages were then evaluated for phenotypic changes from the founder fungal strain, such as germinability with or without Ter331. The analysis showed that recurrent exposure to Ter331 selected for fungal lineages with reduced germinability and slower germination, even in the absence of Ter331. In contrast, when N402 evolved in the absence of the bacteria, lineages with increased germinability and faster germination were favored. SSIs that were first evolved in the presence of Ter331 and then in its absence showed intermediate phenotypes but overall were more similar to SSIs that evolved in the absence of Ter331 for 80 cycles. This suggests that traits acquired from exposure to Ter331 were reversible upon removal of the selection pressure. Overall, our study provides insights into the effects on fungi from the long-term coculture with bacteria. The use of antagonistic bacteria for managing fungal diseases is becoming increasingly popular, and thus there is a need to understand the implications of their long-term use against fungi. Most efforts have so far focused on characterizing the antifungal properties and mode of action of the bacterial antagonists, but the possible outcomes of the persisting interaction between antagonistic bacteria and fungi are not well understood. In this study, we used experimental evolution in order to explore the evolutionary aspects of an antagonistic bacterial-fungal interaction, using the antifungal bacterium Collimonas fungivorans and the fungus Aspergillus niger as a model system. We show that evolution in the presence or absence of the bacteria selects for fungal lineages with opposing and conditionally beneficial traits, such as slow and fast spore germination, respectively. Overall, our studies reveal that fungal responses to biotic factors related to antagonism could be to some extent predictable and reversible.