Stable coexistence of Rfi− factors in Escherichia coli

Abstract

Strains of Escherichia coli K12 constructed to bear combinations of pairs of Rfi− factors originating from other strains, have been found to maintain them stably in antibiotic-free media. Compatibility between these factors of the same fi type, which were all able to propagate a phage specific for one of them, was inferred from the absence of entry exclusion for the donor factor and their ability to coexist in the same cell without any selective pressure. The findings reported here indicate heterogeneity among this group of Rfi− factors. This conclusion is further supported by observations that some pairs of these Rfi− factors show no evidence of recombination.