Genetic Studies of A Multi-Resistant Strain of Staphylococcus Aureus

SUMMARY A multi-resistant strain of Staphylococcus aureus (no. 649MR) containing eight plasmids was constructed in vitro. Chloramphenicol resistance and pigment production were less stable in strain 649MR than when present singly in strain 649. Chloramphenicol resistance could not be transduced from strain 649MR to other strains. Recombination occurred between only two of the plasmids in strain 649MR; these determined (1) metal-ion resistance only (mir-r) and (2) resistance to erythromycin and production of penicillinase in addition to metal-ion resistance (pen +). The plasmid-DNA content of the multi-resistant strain was only 60% of that expected from analysis of the individual plasmids in the host strain, and this could be only partially explained by interaction between the mir-r and pen + plasmids. These findings suggest that under natural conditions the number of plasmids that a staphylococcal cell can maintain will be limited and that recombination will be confined to the “penicillinase plasmids”. The mechanism or mechanisms that limit the number of plasmids within the cell are not known, but factors additional to plasmid incompatibility must operate. The existence of plasmids as multiple copies cannot be accounted for entirely as a means for ensuring their distribution at cell division, nor as provision of maximum levels of antibiotic resistance, but may be related to transfer between cells by transduction.