Distribution in Nature of R factors that Increase Susceptibility to Rifampin of rif-r Mutants in Escherichia coli

Abstract

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.In a previous paper we reported the existence of a class of rifampin-resistant mutants of Escherichia coli K-12 that become more susceptible to rifampin when infected with certain R factors, those of either the F-like or the I-like type (4). We reported also that such rif-r mutants, when R+, are more susceptible to erythromycin, to actinomycin D, and to some rifamycin derivatives which, because they possess a free carboxyl group on the side chain of the rifamycin molecule, do not penetrate efficiently into gram-negative bacteria (3). Since the molecular targets of erythromycin, actinomycin D, and rifamycins are different, and since the ribonucleic acid polymerases extracted from isogenic R-and R+ strains are equally susceptible to rifampin, we concluded that R factors increase the permeability of this class of rif-r mutants to these three antibiotics (3,4). This observation (that some R factors among our collection of strains increase the sensitivity of these rif-r mutants to some antibiotics) could become of practical interest if the phenomenon also applies to R factors in natural bacterial populations. We decided, therefore, to study the frequency distribution, in natural populations, of R factors capable of increasing the susceptibility to rifampin of this class of rif-r mutants. We isolated 117 gram-negative strains from pathological materials and tested them for presence of R factors transferable to K-12. The 46 strains found to carry R factors were conjugated to strain AJ4, a rif-r mutant belonging to the class of mutants whose susceptibility to rifampin is affected by R factors (3, 4), and the susceptibility of the R+ exconjugants was compared with that of the R-AJ4 strain. Since we wanted to work with the same rif-r mutant already used in previous work (3, 4), we were obliged to transfer the R factors in two steps: (i) to a chromosomally resistant auxotroph; and (ii) from the auxotroph to AJ4. MATERIALS AND METHODSStrains used. L286 is an F-nal-r derivative of J53 whose characters are metF-proA-. L286(R163 drd) R163 has the characters col I km fi-. L362 is an Fstr-r derivative of J62 whose characters are proA -histrp lac-. AJ4 is a merodiploid strain with a nonsense mutation in the rif chromosomal gene and with an F' named KLF10. The chromosomal characters of AJ4 are argG-metB-lacZ-,4 recA-, rif-0; the characters of KLF10 are metB+ rif-r.The origin of these strains has been reported previously (4). Strains were cloned, and a clone of each was lyophilized after verification ...