Conjugative R plasmids in Streptococcus faecium (group D).

Ten isolates of Streptococcus faecium were found to be resistant to penicillin, tetracycline, macrolides and related drugs, streptomycin, and kanamycin, and four strains were resistant to chloramphenicol. Six of these 10 strains transferred all their resistance markers (except penicillin) by conjugation at a low frequency (10(-7) to 10(-9)). Several plasmids of different molecular weights were found in each of the wild-type strains. In 5 of 11 transconjugant strains, R plasmids were detected which had molecular weights identical to those of the plasmids found in the corresponding donor strain. Each of the six other transconjugants harbored one plasmid with a size different from those found in the corresponding donor strain, suggesting the occurrence of molecular events during or after conjugative transfer. None of the five tetracycline-resistant transconjugants contained detectable satellite DNA, HindIII restriction enzyme fingerprints of S. faecium resistance plasmids were different from the HindIII patterns of macrolide, aminoglycoside, and tetracycline resistance plasmids from other strains of streptococci.