NATURE | Seres Therapeutics’ success with an industrially made bacterial mix in phase 3 trials against Clostridium difficile infection promises an alternative to fecal microbial transplantation in clinical practice. Seres’s SER-109 is a mix of bacterial spores of Firmicutes species extracted from the stool of healthy human donors, fractionated and packaged into a capsule. Positive data from the phase 3 trial has renewed hopes that purified, defined mixtures of bacteria can treat such infections as effectively as — and presumably more safely than — fecal microbial transplantation (FMT). Several other companies have now also shown favorable late-stage trial data in ‘C. diff.’ using customized stool-derived cocktails.