The beneficial bacteria in our gut seem like a cooperative bunch: We give them a warm and nutrient-rich home, and they work together to keep us healthy. But with hundreds of microbial species crammed together in tight quarters, new research suggests that our intestinal innards might resemble a battlefield more than a conference room. Researchers have found that gut bacteria continually wage war on their neighbors, perhaps as a way to stake out space. The team injected different strains of Bacteroides fragilis, the species of gut-dwelling bacteria (pictured here), into mice that lacked their own microbes. When they analyzed the rodents’ stools over time, they found evidence that the strains were attacking each other. Different strains of B. fragilis inject different combinations of toxins into neighboring bacteria. Bacteria within the same strain fight back: They’re immune to the toxins secreted by their strain-mates. But different species or strains of bacteria can succumb to the assault, giving the attacking strain more room to spread out. Andrew Goodman of Yale University and colleagues estimate that a billion of these toxin transfers happen per minute per gram of stool inside any given gut, they report online today in the Proceedings of the National Academy of Sciences. The weaponry only works at close range, however: The warring bacteria have to actually make contact with each other. The contact-based approach may be a way for microbes to stake out a space in their immediate vicinity without affecting faraway bacteria that could be helpful.