Covert <i>fi</i> ⁻ R Factors in <i>fi</i> ⁻ R ⁺ Strains of Bacteria

Abstract

The presence of an fi(-) sex factor can be detected by propagation of the I-specific phage If1. By use of this method of detection, a high proportion of strains with fi(+) R factors were shown also to carry an fi(-) factor which was frequently a second R factor. In some doubly R(+) strains, the fi(+) and the fi(-) factor were observed to be transferred independently at conjugation.