Synergistic bacterial compositions and methods of production and use thereof

A method of treating or preventing an occurrence or a recurrence of a Clostridium difficile infection, comprising administering to a human subject in need thereof an effective amount of a therapeutic composition comprising a first purified bacterial population consisting of bacteria comprising 16S rDNA sequence at least about 97% identical to a 16S rDNA sequence present in a reference Collinsella aerofaciens OTU, and a second purified bacterial population consisting of bacteria comprising 16S rDNA sequence at least about 97% identical to a 16S rDNA sequence present in a reference bacterium of the family Clostridiaceae listed in Table 1, wherein a synergistic combination of the first purified bacterial population and the second purified bacterial population is cytotoxic or cytostatic to a pathogenic bacterium, wherein the therapeutic composition is administered under conditions such that the first purified bacterial population and the second purified bacterial population exert an inhibitory effect on a pathogenic bacterium present in or entering into the gastrointestinal tract of the human subject, such that the number of Clostridium difficile bacteria present in the gastrointestinal tract is not detectably increased or is detectably decreased over a period of time.