Infections as ecosystems: community metabolic interactions in microbial pathogenesis

Microbes rarely exist alone; instead, they live in dynamic multi-species communities with a range of metabolic capacities. To establish within a polymicrobial community, an organism must compete with the other members of the community for space and nutrients. In addition, microbes form complex metabolic interdependencies in polymicrobial environments, and these nutrient exchanges are central to overall community function. Interactions between microbial community members dictate key processes, including nutrient cycling, tolerance to disturbances, and disease progression, and these interactions are known to depend on the environment in which they are measured. Therefore, understanding these ecological interactions is fundamental to our understanding of community composition, function, and impacts on disease. In this mini-review, we will describe the mechanisms microbes use to exchange nutrients in host-associated environments, with a focus on the oral and respiratory tracts. We will particularly emphasize the environmental factors that influence community composition and how interactions between organisms, ranging from cooperation to competition, impact nutrient bioavailability and overall community function during infection.