Summary: The conjugative plasmid R1 was introduced into ten strains of isolated from natural populations. Spontaneous nalidixic-acid-resistant mutants of the ten strains served as recipients. The ten donor and recipient strains were mated in all combinations and the rate at which R1 transferred between the strains was determined. The rate of transfer ranged from 5·2 × 10-1·1 × 10 ml per cell h, and averaged 1·3 × 10 ml per cell h. The results of these experiments suggest that the rates of conjugative transfer are far too low for plasmids to be maintained as parasites in their host populations. Infectious transfer is insufficient; plasmids must confer a selective advantage to their host to be maintained.


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