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Characterization of an erythromycin resistance (erm) plasmid in Streptococcus pyogenes

Three erythromyxin‐resistant Swedish isolates of Streptococcus pyogenes, representing different T‐types, were studied. Two of the strains showed constitutive high‐level (MIC>200 μg/ml) and one showed moderate (MIC 6.4 μg/ml) resistance; the latter strain was sensitive to lincosamide and clindamycin, and resistance was not induced by erythromycin. In each of the strains, a plasmid with an estimated Mw of 17.6±0.9×106 was isolated in addition to smaller cryptic plasmids. The three plasmids pSE701, pSE702 and pSE703 had very similar restriction enzyme cleavage patterns. Novobiocin curing of the high‐level resistance strain ER559 showed the resistance to be linked to its 17.6×106 plasmid, pSE703. Furthermore, by electroporation this rather large plasmid was reintroduced into an erythromycin‐sensitive cured derivative, acquiring resistance, and the plasmid was again recovered from the transconjugant. One of the plasmids, pSE702, was shown by filter mating to be conjugative within S. pyogenes. DNA‐DNA hybridization showed that the resistance determinant of the present three isolates was related to the erm gene on plasmid pAMpl of Enterococcus faecalis but not to that of plasmid pE194 of Staphylococcus aureus. The copy numbers of pSE702 and pSE703, derived from the two high‐level resistant strains, were 11±3 and 17±5 compared to 2±1 for pSE701, derived from the moderately resistant strain, possibly accounting for the phenotypic variation observed. The plasmids pSE702 and pAMpM showed about 80% homology in DNA‐DNA hybridization tests and high similarity in their restriction maps.