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Abstract Studies of microbial interactions often emphasize interactions with large, easily measurable growth differences and short-term ecological outcomes spanning just a few generations. However, more subtle interactions, such as those without obvious phenotypes, may play a significant role in shaping both the short-term ecological dynamics and the long-term evolutionary trajectories of microbial species. We used the cheese rind model microbiome to examine how two fungal species, Penicillium camemberti and Geotrichum candidum, impact global gene expression and genome evolution of the bacterium Pseudomonas carnis LP. Even though fungi had limited impacts on the growth of P. carnis LP, approximately 4–40% of its genome was differentially expressed, depending on the specific fungal partner. When we evolved this Pseudomonas strain alone or in co-culture with each of the fungi, we observed frequent mutations in global regulators of nitrogen regulation, secondary metabolite production, and motility, depending on the fungus. Strikingly, many strains with mutations in the nitrogen regulatory gene ntrB emerged when evolved alone or with G. candidum, but not with P. camemberti. Metabolomic and fitness experiments demonstrate that release of free amino acids by P. camemberti removes the fitness advantages conferred by ntrB mutations. Collectively, these results demonstrate that even in the absence of major short-term growth effects, fungi can have substantial impacts on the transcriptome and genomic evolution of bacterial species. 

Fermented foods and beverages have been produced around the world for millennia, providing humans with a range of gastronomic, cultural, health, and scientific benefits. Building on these traditional forms, a conver- gence of factors, including culinary innovation, globalization, shifts in consumer preferences, and advances in microbiome sciences, has led to the emergence of so-called ‘novel fermentations’. In this review, we define novel fermentation as the confluence of traditional food practices and rational microbiome design. Using principles of microbial ecology and evolution, we develop a microbiological framework that outlines several strategies for producing and characterizing novel fermentations, including switching substrates, engrafting target species, assembling whole-community chimeras, and generating novel phenotypes. A subsequent analysis of existing traditional ferments points to gaps in ‘fermentation space’ where novel ferments could potentially be produced using new combinations of microbes and food substrates. We highlight some impor- tant safety and sociocultural issues presented by the repurposing and modification of microbes from tradi- tional ferments that fermented-food producers and microbiologists need to address.