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F Factor-Mediated Immunity to Lethal Zygosis in Escherichia coli K-12

¹ Department of Microbiology, University of Adelaide, Adelaide, South Australia 5001

ABSTRACT

Recipient (F^–) cells of Escherichia coli are sensitive to an excess of Hfr donor cells. This phenomenon of lethal zygosis is associated with conjugation and is observed as a continuous fall in F^– viable cells during liquid mating, or as inhibition of F^– growth on solid media. One class of survivors, which arose in the zones of inhibition on solid media, was no longer sensitive to lethal zygosis and exhibited the following properties: sensitivity to male-specific phage, donor ability, and surface exclusion. Since these characteristics were sensitive to acridine orange treatment, the strains carry an F factor extrachromosomally. They are, however, defective in some way since they retain sensitivity to female-specific phage. Temporary sensitivity to lethal zygosis in these and in standard F⁺ strains can be induced by the formation of F^– phenocopies. We have suggested that there is an immunity to lethal zygosis (Ilz⁺) associated with the F factor and discuss the results in terms of this hypothesis.