multiple resistance, to streptomycin and those three other antibiotics, was coded on an episome. They gave the episome a name: resistance transfer factor. It became known as R factor, for short, in parallel to Esther Lederberg’s F factor. This R factor could be transferred by conjugation. It could be transferred (at least in lab experiments) by transduction. It explained how harmless bacteria such as ordinary Escherichia coli could convey genes for multiple antibiotic resistance, across species boundaries, into dangerous bacteria such as Shigella dysenteriae,
