R factor tetracycline and chloramphenicol resistance in Escherichia coli K12 cmlB mutants

The isolation of Escherichia coli chromosomal mutants that increased the level of resistance of a partially tetracycline-sensitive mutant of RI00-I is described. Plasmid-less derivatives of these moderately resistant mutants were phenotypically similar to the cmlB mutants described by Reeve (1966, 1968), and also mapped in the same region. The level of intrinsic resistance to both chloramphenicol and tetracycline was increased about twofold. Also, the levels of R factor-determined resistance to these drugs were increased by this host mutation and tetracycline resistance was expressed constitutively. A cmlB accumulated tetracycline at a threefold lower rate than the wild-type strain, and it is proposed that the mutants have an altered permeability to the drugs and that this acts synergistically with the products of the R factor chloramphenicol and tetracycline resistance genes.