Reciprocal sharing of two classes of public goods facilitates Bacillus subtilis biofilm formation

Public goods support the development of microbial communities but are prone to exploitation. The extracellular matrix molecules of Bacillus subtilis are examples of public goods that are produced through the division of labour and support biofilm formation. Another class of public good that supports B. subtilis growth when nutrients are in a polymeric form are extracellular proteases. In this study, we investigated the role of extracellular proteases during B. subtilis biofilm formation. We established that the extracellular proteases support biofilm growth even when the nitrogen source is available as glutamic acid. Reviewing the contribution played by each protease identified that selected individual extracellular proteases have distinct functions, with AprE found to be crucial for development and WprA sufficient for biofilm formation. We found that changing the nutrient source affected biofilm structure, hydrophobicity, and architecture, likely due to the increased cost of protease dependency for growth. Co-culture of biofilm matrix and extracellular protease mutants rescued biofilm structure through public goods sharing, but the dependence on extracellular protease for growth influenced community coexistence. The intricate sharing among these two classes of public goods sheds light on microbial social dynamics during biofilm formation in different ecological contexts. ### Competing Interest Statement The authors have declared no competing interest.