Index >> Bacterial Structure >> Staphylococcal Plasmids

Plasmids are small pieces of DNA resident within a bacterial cell that are both separate from the chromosome, and capable of replicating under their own control.

In certain cases, plasmids are the reason why a bacterial infection may no longer respond to treatment with antibiotics: plasmids can carry genes that confer resistance on their bacterial host.

Plasmids may also have the ability to transfer from their current host to a plasmid-free recipient strain, thus spreading antibiotic resistance throughout a population.

Plasmid pSA5700 from Staphylococcus aureus coding for erythromycin (EmR) and chloramphenicol (CmR) resistance was transformed into Streptococcus pneumoniae.

High-copy-number and EmR constitutive mutants of this plasmid were isolated. Transformation frequencies in S. pneumoniae as high as 70% were obtained with a constructive plasmid as donor DNA, into a recipient cell containing a resident, inducible, high-copy-number plasmid.

With the aid of these high frequencies, the site of constitutive mutations could be mapped via a simple marker rescue technique that uses purified restriction endonuclease-generated fragments.

One of the EmR constitutive mutants, pFB9, a plasmid originating from a Gram-positive host, was shown to replicate and express EmR and CmR in a Gram-negative organism, Escherichia coli.