Carbon source diversity shapes bacterial interspecies interactions

Bacterial communities exhibit various classes of interspecies interactions, ranging from cooperative to competitive. As these interaction classes play a crucial role in determining characteristics of bacterial communities, including species composition and community stability, understanding of the mechanisms that shape them is warranted. While several studies have suggested that cooperative interactions are rare, a study focused on single-carbon-source environments reported them to be relatively common. This discrepancy highlights the potential role of carbon source diversity in shaping interaction classes, although the quantitative relationship remains unclear. To elucidate this relationship, we examined 896 interspecies interactions among 28 synthetic bacterial pairs, isolated from various environments, under 32 conditions with varying levels of carbon source diversity. As a result, we frequently observed cooperative interactions in single-carbon-source environments, with the interactions shifting to competitive as the carbon source diversity increased. Further analyses suggested that this shift was driven by intensified resource competition in more complex media. Our findings provide new insights into how environmental factors, particularly carbon source diversity, shape bacterial interactions in synthetic communities, offering potential strategies for manipulating bacterial communities in ecological and biotechnological contexts.