Lacey and Chopra investigated genetic mechanisms of multi-antibiotic resistance in Staphylococcus aureus by constructing a strain (649MR) carrying eight plasmids conferring resistance to streptomycin, tetracycline, neomycin, chloramphenicol, methicillin, heavy metal ions, and penicillin/erythromycin. Using transduction with bacteriophage 88, they determined that the strain maintained multiple plasmids simultaneously, though with limitations. Notably, chloramphenicol resistance and pigment production were less stable in the multi-resistant strain than in single-plasmid derivatives. The researchers found that individual plasmid copies could confer full phenotypic resistance despite being present in multiple copies per cell. Recombination between dissimilar plasmids was rare, and they identified incompatibility interactions between certain plasmid pairs, particularly the penicillinase and heavy-metal-resistance plasmids. The study revealed that total extrachromosomal DNA content is restricted in staphylococcal cells, suggesting natural limits to accumulation of resistance plasmids.

Key findings

• Multiple plasmids can be maintained simultaneously in S. aureus, but total extrachromosomal DNA content is limited, preventing indefinite plasmid accumulation
• Chloramphenicol resistance and pigment production are less stable and less efficiently transduced from multi-resistant cells than from single-plasmid strains, indicating compatibility problems with other plasmids
• Recombination between dissimilar plasmids is rare; simultaneous transduction of separate plasmids is also uncommon
• One or very few plasmid copies are sufficient for full phenotypic expression of antibiotic resistance, despite plasmids being present in multiple copies per cell
• Specific plasmid incompatibility interactions occur, particularly between penicillinase/erythromycin and heavy-metal-resistance plasmids, resulting in reduced plasmid DNA content