Drug resistance of staphylococci. VII. Genetic determinants responsible for the resistance to tetracycline, streptomycin, sulfanilamide, and penicillin

Association of a penicillin resistance gene with a tetracycline resistance plasmid (PTP-2) in Staphylococcus aureus

On transduction with a lysogenic strain of Straphylococcus aureus isolated from a clinical specimen and having tetracycline (TC)-penicillin (PC)-chloramphenicol (CP)-resistant plasmids, the three-drug-resistant strain was frequently obtained. By repeatedly transducing from this strain, a strain (TP-2) having stable resistance to TC and PC could be obtained. In transformation with the deoxyribonucleic acid (DNA) of TP-2 as donor, all of the transformants obtained by selecting with either TC or PC were both TC and PC resistant. According to electron microscopy study of the covalently closed circular DNA of TP-2, the plasmid DNA size was 1.37 +/- 0.03 mum (2.84 x 10(6) daltons). The plasmid (P(TP-2)) is presumed to be a new plasmid in which the PC resistance gene was integrated into the TC-resistant plasmid.

Plasmid interactions in Staphylococcus aureus: nonadditivity of compatible plasmid DNA pools

Six different Staphylococcus aureus plasmids have been examined for compatibility relationships and their intracellular DNA pools have been measured singly and in various combinations. All six were mutually compatible, but contrary to expectation, their intracellular DNA pools were not additive; instead, there appeared to be a maximum level of extrachromosomal DNA that could be supported by the cell, and the plasmids studied approached this level individually as well as in varous combinations. One exception was encountered: a plasmid encoding kanamycin/neomycin resistance was present in a small but constant number of copies regardless of the presence of other plasmids.

Co-transduction of plasmids mediating resistance to tetracycline and chloramphenicol in Staphylococcus aureus

Independent plasmids mediating resistance either to tetracycline (Tc) or chloramphenicol (Cm) were transduced successively into Staphylococcus aureus strain 8325. From this doubly resistant donor strain, Tc was co-transduced with a frequency of 40 to 50% when Cm was selected. Co-transduction of Cm was 5 to 10% with Tc selection. Plasmid elimination was infrequent and restricted to the Cm plasmid. A variant, doubly resistant strain gave 100% co-transduction of Tc and Cm and a high rate of joint elimination of both plasmid markers. Co-transduction of the plasmids from recombination-deficient donor strains was much reduced if the plasmids had been introduced separately into the donor strain, but occurred at the normal high rate if they had been introduced jointly. The plasmids were co-transformed at relatively low rates with closed circular deoxyribonucleic acid (DNA) from doubly resistant donors, but not with DNA from a mixed lysate of singly resistant strains. Our evidence favored a hypothesis of recombination-dependent, reversible linkage between the two plasmids as the basis for their co-transduction. Examination of plasmid DNA from the doubly resistant strains by ultracentrifugal and electron microscopic methods did not disclose any physical differences between singly and doubly resistant strains that might account for the observed co-transduction.

Isolation of the rec mutants in Staphylococcus aureus

A histidine auxotroph (his-) of Staphylococcus aureus MS3937 and mutants sensitive to ultraviolet (UV) irradiation were obtained. The UV-sensitive mutants were found also to be sensitive to N-methyl-N'-nitro-N-nitrosoguanidine and mitomycin C, and their sensitivity was accounted for by a defect in deoxyribonucleic acid repair. Transduction of either chromosomal or plasmid markers to UV-sensitive mutants showed that these staphylococcus mutants are of the recA (reckless) type mutants as reported in Escherichia coli and Salmonella typhimurium; therefore the UV-sensitive mutants (uvr-) were renamed recombination-deficient mutants (rec-). The biochemical and genetic properties of these mutants are described, and their usefulness for studies of staphylococcal plasmids is discussed.