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J Bacteriol. 1974 February; 117(2): 360-372
Copyright © 1974 American Society for Microbiology. All Rights Reserved.

Conjugal Transfer of Plasmid-Borne Multiple Antibiotic Resistance in Streptococcus faecalis var. zymogenes

Alan E. Jacob and Susan J. Hobbs

1 Department of Bacteriology, Royal Postgraduate Medical School, Ducane Road, London W12 OHS, England

ABSTRACT

A strain of Streptococcus faecalis var. zymogenes, designated JH1, had high-level resistance to the antibiotics streptomycin, kanamycin, neomycin, erythromycin, and tetracycline. These resistances were lost en bloc from approximately 0.1% of cells grown in nutrient broth at 45 C. The frequency of resistance loss was not increased by growth in the presence of the "curing" agents acriflavine or acridine orange, but after prolonged storage in nutrient agar 17% of cells became antibiotic sensitive. Covalently closed circular deoxyribonucleic acid (DNA) molecules were isolated from the parental strain and from antibiotic-sensitive segregants by using cesium chloride-ethidium bromide gradients. DNA molecular species were identified by using neutral sucrose gradients. Strain JH1 contained two covalently closed circular DNA species of molecular weights 50 x 106 and 38 x 106. An antibiotic-sensitive segregant, strain JH1-9, had lost the larger molecular species. A second sensitive segregant, strain JH1-5, had also lost the larger molecular species but a new molecular species of approximate molecular weight 6 x 106 was present. The antibiotic resistances that were curable from the parental strain were transferred to antibiotic-sensitive strains of S. faecalis and to strain JH1-9, during mixed incubation in nutrient broth at 37 C. Data to be described are interpreted to suggest that the transfer is by a conjugal mechanism. Analysis of the plasmid species in recipient clones showed that all had received the plasmid of molecular weight 50 x 106. Strain JH1-5 was not a good recipient. Analysis of one successful recipient clone of JH1-5 revealed that it had gained the 50 x 106 molecular weight plasmid but lost the 6 x 106 molecular weight species. These data are interpreted to mean that the multiple antibiotic resistance is borne by a transferable plasmid of 50 x 106 molecular weight, and that in clone JH1-5 this plasmid suffered a large deletion leaving only a 6 x 106 remnant which was incompatible with the complete replicon.

J Bacteriol. 1974 February; 117(2): 360-372
Copyright © 1974 American Society for Microbiology. All Rights Reserved.



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