This disclosure relates to the discovery and isolation of the entire cluster of genes encoding R-type high molecular weight bacteriocins that specifically kill Clostridium difficile bacteria, dangerous human pathogens. Also disclosed are methods of producing the R-type bacteriocins in innocuous producer cells that, unlike C. difficile, do not die in the presence of oxygen. Disclosed also is the specific gene of the isolated gene cluster that determines the killing spectrum of the R-type bacteriocin and the demonstration that the killing spectra of diffocins can be altered by engineering orf1374 of the diffocin genetic locus. This invention offers a potent bactericidal agent and a means to make it in order to kill selectively C. difficile bacteria in the environment of the gastrointestinal tract where they can cause great harm and even death of the infected patient or farm animal.