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Genetic and molecular characterisation of resistance determinants in methicillin-resistant Staphylococcus-aureus
Abstract:
A genetic analysis of resistance to antibiotics in methicillin-resistant Staphylococcus aureus was performed. Demonstration of plasmid-specific DNA either in transductants that had received antibiotic-resistance markers from multiply-resistant strains, or in segregants of methicillin-resistant strains that had lost unstable determinants except the one under study, indicated that markers of resistance to penicillin, chloramphenicol and neomycin are present on separate, mutually compatible plasmids. Absence of covalently closed circular DNA was demonstrated in transductants that were resistant to methicillin, tetracycline, erythromycin and streptomycin, as well as in segregants that had lost the penicillinase, chloramphenicol and neomycin plasmid, but were still resistant to methicillin, tetracycline, erythromycin, streptomycin and the sulphonamides. Analysis of plasmid DNA either in a 5-20% neutral sucrose gradient or by electron microscopy revealed the presence of three readily distinguishable plasmids. The molecular weights of these plasmids were estimated by comparing the sedimentation rate constants with those of known reference plasmids and by contour-length measurements. The molecular weight of the penicillinase plasmid was estimated to be 20 X 10(6) daltons, that of the chloramphenicol plasmid 3 X 10(6) daltons and that of the plasmid carrying the neomycin resistance marker 37 X 10(6) daltons.
Insights
Antibiotic resistance in methicillin-resistant Staphylococcus aureus is linked to specific plasmids. Penicillin, chloramphenicol, and neomycin resistance markers reside on separate, compatible plasmids, while other resistances are not plasmid-associated.
Area of Science:
- Microbiology
- Genetics
- Molecular Biology
Background:
- Methicillin-resistant Staphylococcus aureus (MRSA) poses a significant public health threat due to its resistance to multiple antibiotics.
- Understanding the genetic basis of antibiotic resistance in MRSA is crucial for developing effective treatment strategies.
Purpose of the Study:
- To genetically analyze the antibiotic resistance determinants in methicillin-resistant Staphylococcus aureus.
- To identify and characterize the plasmids carrying specific antibiotic resistance markers.
Main Methods:
- Genetic analysis of transductants and segregants of MRSA strains.
- Plasmid DNA analysis using neutral sucrose gradient centrifugation and electron microscopy.
- Estimation of plasmid molecular weights via sedimentation rate constants and contour-length measurements.
Main Results:
- Resistance markers for penicillin, chloramphenicol, and neomycin were found on separate, compatible plasmids.
- Absence of covalently closed circular DNA was observed in strains resistant to methicillin, tetracycline, erythromycin, and streptomycin, suggesting non-plasmid-mediated resistance for these agents.
- Three distinct plasmids were identified with estimated molecular weights: penicillinase plasmid (20 x 10^6 daltons), chloramphenicol plasmid (3 x 10^6 daltons), and neomycin resistance plasmid (37 x 10^6 daltons).
Conclusions:
- The study elucidates the plasmid-based genetic architecture of resistance to penicillin, chloramphenicol, and neomycin in MRSA.
- Different resistance mechanisms, some plasmid-dependent and others not, contribute to the multi-drug resistance profile of MRSA.
- Characterization of these resistance plasmids provides valuable insights into MRSA evolution and epidemiology.