REPLACING missing gut bacteria in a mouse model of autism reverses adverse social behaviours and gut disorders associated with the condition. Last year, Sarkis Mazmanian and Paul Patterson at the California Institute of Technology in Pasadena found that infecting pregnant mice with molecules from a flu virus caused autism-like symptoms in their offspring. The pups were less social, squeaked less and displayed repetitive behaviours. They also had a “leaky” gastrointestinal tract that allowed bacteria to move in and out of the lining. In addition, the bacteria present in their gut were significantly different from that found in mice without autism-like behaviour.