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Bacteria commonly live in diverse communities where each species can affect the growth and survival of others (, ). These species interactions are central to bacterial ecology and have diverse implications for health, agriculture, and industry. To understand and manipulate bacterial communities, it is therefore critical to know how species interact. There is an emerging controversy about the importance of positive interactions, such as cooperation (or mutualism), between bacterial species (–). However, once the standard ecological measures are applied, a clear picture emerges. Negative interactions prevail, and cooperation, where two species both benefit, is typically rare. The prevalence of competition gives hope for bacterial community engineering strategies that seek to eliminate pathogens without the need for antibiotics.