In your mouth there are billions of bacterial cells growing on your teeth and below your gumline. These microbes are not each alone in a little bubble, but instead interact with each other and with your human cells. Undoubtedly, these interactions are important; they determine the behavior of individual microbes, the composition of your oral microbiome, and ultimately, the development of cavities and gum disease. But conceptualizing these interactions and understanding their impact is a huge challenge.

As a microbial ecologist, there is no way I can study the millions of possible pairwise interactions among the hundreds of species in our oral microbiota, not to mention any higher-level interactions. Instead, I must prioritize my efforts. I assume that due to the spatial structure of the oral microbiota, not all of these possible interactions are actually occurring. Further, many of the interactions likely have only a minuscule impact on the overall behavior of the community, the evolution of the microbes involved, or the progression of diseases. Yet, this still leaves an unknown, and possibly vast number of important interactions to try to understand, leading me to ponder basic, unanswered questions. When and how do microbes within a community interact? And potentially even more importantly, when and how do those interactions matter?

These questions are not unique to the oral microbiota. A better understanding of microbial interactions is important across environments, from our microbiome, to soil and plant-associated communities, to the microbes driving carbon cycling in our oceans. There is a basic science value in understanding these important, complex communities. Additionally, scientists hypothesize that altering key interactions can allow us to shift our communities, for example from a diseased state to a healthy state. To determine whether this is actually feasible requires researchers to identify the key interactions and to understand the implications of microbial interactions on both ecological and evolutionary timescales. Thus, while I research the oral microbiota because of its importance to human health, ease of study, and interesting spatial and temporal community dynamics, I hope that my work has broad relevance across systems.

Recently, in collaboration with my co-authors, I studied the interactions formed by the oral pathogen Aggregatibacter actinomycetemcomitans (Aa) in pairwise coinfection with 25 different microbes in a mouse abscess infection model. Each of these 25 microbes altered the set of essential genes that Aa needed to survive in the abscess, implying that each microbe interacted with Aa in some manner. Further, we found that each interaction altered Aa’s essential genes in a unique way.

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