Distribution in nature of R factors that increase susceptibility to rifampin of rif-r mutants in Escherichia coli.

Among 117 gram-negative bacteria isolated from pathological materials, 46 were found to carry antibiotic resistances transferable to Escherichia coli K-12; we therefore concluded that they carry infectious R factors. When transferred to a type of rifampin-resistant mutant of E. coli, all these R factors decreased the resistance to rifampin, but only 10% of them lowered the resistance to one-tenth or less that of the isogenic R(-) strain. The relevance of these facts for the epidemiology of R factors in gram-negative bacteria is discussed.