Many yoghurts are loaded with live bacteria, and labelled with claims that consuming these microorganisms can be good for your health. But a study published today shows that such yoghurts have only subtle effects on the bacteria already in the gut and do not replace them. Nathan McNulty, a microbiologist at Washington University in St Louis, Missouri, recruited seven pairs of identical twins, and asked one in each pair to eat twice-daily servings of a popular yoghurt brand containing five strains of bacteria. By sequencing bacterial DNA in the twins' stool samples, the team showed that the yoghurt microbes neither took up residence in the volunteers' guts, nor affected the make-up of the local bacterial communities. Jeffrey Gordon, the microbiologist at Washington University who led the study, was not surprised. “We were only giving several billion bacterial cells in total to the twins, who harbour tens of trillions of gut microbes in their intestines,” he says.