Summary: Non-transferring plasmids containing the genetic determinants for resistance to streptomycin (Sm), sulphonamides (Su) and ampicillin (Ap), colicine production (Col) and α-haemolysin production (Hly) were incorporated in the same strain of K12. A plasmid coding for tetracycline (Tc) resistance was then co-transferred to it by eleven transfer factors, including F and I.

Some of the transfer factors differed greatly in the rate at which they transferred the determinants from this TcColHlySmSuAp donor strain to another strain of K12 and some of the determinants differed greatly in the rate at which they were transferred. Only F and one other transfer factor were found to transmit Hly. Some transfer factors were not detected to transfer Col or Ap. Linkage was demonstrated between Tc and three transfer factors, including F, and between Sm and Su. The other plasmids and transfer factors appeared to exist as separate units. In mating studies between the donor strain possessing both F and I, and recipient strains possessing different combinations of these transfer factors, the determinants most greatly affected by the choice of recipient were the F-linked Tc and Hly.

When a multiple antibiotic resistance plasmid, instead of the Tc one, was co-transferred to the donor strain by I or F, then I, but not F, established a linkage with this plasmid. Although this linkage was probably responsible for the higher rate at which the plasmid was transferred from the I than from the F strain, its actual transfer rate was lower than that of the unlinked Col plasmid also possessed by the I strain.


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