The Journal of Medical Microbiology, Vol 9, Issue 2 137-148, Copyright © 1976 by Society for General Microbiology
Genetic and molecular characterisation of resistance determinants in methicillin-resistant Staphylococcus-aureus
F. H. Kayser, J. Wust and P. Santanam
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.
