Fighting Aspergillus infection using biocontrol bacteria: A proof-of-concept of environmental interference in a translational setting

Aspergillus fungi are opportunistic pathogens that affect millions of people worldwide. Aspergilli produce organic acids to optimize the environmental pH and match the needs of their enzymatic machinery. In this study, we tested the hypothesis that this also occurs during infection. By producing oxalic acid (OA), Aspergillus would manipulate pH during lung infection and thus, interfering with this process could control the pathogen. To test this hypothesis, we assessed in silico the potential for OA production in a wide range of Aspergilli. A genetic marker for AO production was detected in most of the species including prevalent human pathogens. We tested OA production in vitro in four strains of A. niger and A. fumigatus , but only one of the A. niger strains produced OA consistently. For this fungal strain, oxalotrophic bacteria were able to control fungal growth via OA consumption. To translate this observation into a pre-clinical system, increasingly complex experiments were performed. In 3D-cell cultures, A. niger also secreted OA and modified pH and free Ca ²⁺ . Co-inoculation of the oxalotrophic bacterium inhibited the development of the fungus. However, biocontrol could not be replicated in Galleria mellonella , which is often used as an infection model. In contrast, the bacterium improved disease score and the absence of oxalate crystals in the lungs in the mouse model. This biocontrol interaction between oxalotrophic bacteria and A. niger represents a paradigm shift in the fight against opportunistic fungal pathogens, where the goal is to render the host environment less permissive to pathogen development