Isolation of Salmonella typhimurium Mutants with Increased Recipient Ability by the Use of R Factor

Abstract

SEXUAL recombination occurs only at very low frequencies between Escherichia coli Hfr and Salmonella typhimurium recipients^1–5. Frequencies of transfer of R factors⁶ from E. coli R⁺ to S. typhimurium R⁻ are also very low (10⁻⁷/ donor)⁷. Thus a characteristic feature of the mating of E. coli with S. typhimurium is the low recipient ability of Salmonella strains. The low recipient ability of Salmonella was overcome by isolating fertile mutants by an indirect selection method in S. typhimurium LT-7 mut⁵. We reported that these fertile mutants are restrictionless and modificationless for foreign deoxyribonucleic acids (DNA) such as phage P22 DNA, R factors, F factor and bacterial chromosomes⁸. Eisenstark⁹ found that the frequencies of hybridization between E. coli Hfr and S. typhimurium LT-2 are increased by treatment of LT-2 recipients with N-methyl-N′-nitro-N-mtrosoguanidine (NG). We have found that two types of mutants of LT-2 selected by the transfer of an R factor from E. coli K-12 behaved, after elimination of the R factor with acridine orange, as good recipients of E. coli chromosomes, in the same way as was shown for K-12 (P1) by means of F-prime factor transfer¹⁰.