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Updated: Jul 17, 2026
Methodology for the Study of Horizontal Gene Transfer in Staphylococcus aureus
Published on: March 10, 2017
A conjugative plasmid encoding production of a diffusible pigment and resistance to aminoglycosides and macrolides in
Abstract:
A methicillin-resistant strain of Staphylococcus aureus, WG512, was isolated in 1968 and has been shown to harbour a plasmid, pWG14, which encodes the production of a diffusible pigment and resistance to kanamycin, neomycin, streptomycin, lincomycin, erythromycin and spectinomycin. Plasmid pWG14 has only been detected as a slowly migrating band of DNA during agarose-gel electrophoresis, and this was thought to be the open-circular form of the plasmid with an estimated molecular weight of 30-34 Mdal. The transfer properties of this plasmid were compared with a non-conjugative plasmid encoding tetracycline resistance and the class 2, conjugative plasmid described by Archer and Johnston (1983). Plasmid pWG14 shared many of the conjugative properties of the latter plasmid, including the ability to transfer at high frequency on the surface of a filter membrane and to transfer between non-lysogenic strains in the absence of Ca++ and in the presence of citrate of DNase I. However, unlike the class 2 plasmid, pWG14 was able to transfer in broth culture at low frequency and did not mobilise the non-conjugative plasmid. Furthermore, the class 2 plasmid and pWG14 were shown to belong to different incompatibility groups. A derivative of WG512, which had lost the ability to produce the diffusible pigment along with plasmid pWG14 but had retained the resistance determinants of plasmid pWG14 in the chromosome, was used to demonstrate that the conjugative mechanism of pWG14 was a property of the plasmid and not the resistance determinants.
Insights
Methicillin-resistant Staphylococcus aureus plasmid pWG14 mediates pigment production and antibiotic resistance. This conjugative plasmid transfers efficiently on filters but poorly in broth, differing from other plasmids.
Area of Science:
- Microbiology
- Molecular Biology
- Genetics
Background:
- Methicillin-resistant Staphylococcus aureus (MRSA) strain WG512, isolated in 1968, harbors plasmid pWG14.
- Plasmid pWG14 encodes pigment production and resistance to multiple antibiotics, including kanamycin and erythromycin.
- The plasmid was previously characterized as a slowly migrating DNA band, presumed to be its open-circular form.
Purpose of the Study:
- To compare the transfer properties of plasmid pWG14 with other known plasmids.
- To investigate the conjugative mechanism of pWG14.
- To determine if resistance determinants influence plasmid conjugation.
Main Methods:
- Agarose-gel electrophoresis to visualize plasmid DNA.
- Conjugation experiments comparing transfer frequencies on filter membranes and in broth.
- Incompatibility grouping to differentiate plasmids.
- Analysis of a derivative strain lacking the plasmid to assess the role of resistance determinants.
Main Results:
- Plasmid pWG14 exhibited high-frequency transfer on filter membranes and Ca++-independent, citrate/DNase I-resistant transfer between strains.
- Unlike a class 2 plasmid, pWG14 showed low-frequency transfer in broth and did not mobilize a non-conjugative plasmid.
- pWG14 and the class 2 plasmid belonged to different incompatibility groups.
- Conjugation was confirmed as a plasmid-encoded property, independent of resistance determinants.
Conclusions:
- Plasmid pWG14 possesses unique conjugative properties distinct from other characterized plasmids.
- The conjugative ability of pWG14 is intrinsic to the plasmid itself.
- Understanding plasmid transfer mechanisms is crucial for controlling antibiotic resistance spread in MRSA.
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