Each of three isolates of Streptococcus faecalis subsp. zymogenes harbored three R plasmids and a hemolysin-bacteriocin plasmid. The plasmids carried by one of these strains were physically characterized after their conjugative transfer. In each strain one of the plasmids carried genetic markers for resistance to gentamicin, kanamycin, sisomicin, netilmicin, and tobramycin.

Two plasmids determining resistance to tetracycline .(RIP500) and to chloramphenicol, erythromycin, lincomycin, and pristinamycin I (RIP501) were isolated from a strain of Streptococcus agalactiae. The frequency-of-resistance loss is very low for RIP500 (<3 x 104) but higher for RIP501 (the efficiency was dependent upon the curing agents and incubation temperature and varied between 0.5 and 96%). Derivatives susceptible to all drugs were also obtained. RIP500 and RIP501 have similar molecular weights (17.9 x 106 and 20 x 106, respectively) and represent different percentages of total deoxyribonucleic acid (0.4 and 4%, respectively). The number of copies of RIP500 and RIP501 per cell is different, and these plasmids are likely replicated under different kinds of control (stringent and/or relaxed). No plasmid deoxyribonucleic acid was found in a derivative of strain B96 susceptible to all drugs.

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Wild-type strain A454 (Streptococcus pyogenes) transferred en bloc its erythromycin (Em) and tetracycline (Tc) resistance markers into several plasmid-free streptococcal recipients. No plasmid DNA was detected in either the wild-type or the transconjugant strains. Crosses were performed between A454 and S. faecalis Rec+ or Rec- recipients carrying hemolysin-bacteriocin plasmids, pIP964 or pAD1 . The Em Tc-resistant transconjugants obtained harbored either the parental plasmid or an Em Tc resistance plasmid derived from pIP964 or pAD1 . The restriction endonuclease analysis of 12 derivative plasmids showed insertions of various sizes into different fragments of pIP964 or pAD1 . A454 and the Em Tc-resistant plasmid-free transconjugants were found to contain two EcoRI DNA fragments, that shared homology with 32P-labeled pIP1077 , one of the Em Tc resistance derivative plasmids, but not with 32P-labeled pIP964 . No homology was detected between pIP1077 and the cellular DNA of the antibiotic-susceptible recipients.

Of 20 clinical isolates of group A, B, G, D (Streptococcus bovis), and viridans streptococci, 5 transferred their antibiotic resistance markers into streptococcal recipients at a low frequency (10(-4) to 10(-8)) in the apparent absence of extrachromosomal elements. All strains carried genetic markers for high-level resistance to streptomycin, kanamycin, neomycin, lividomycin A, and ribostamycin, as well as resistance to macrolides and related drugs, tetracycline, and chloramphenicol.