R plasmids in Streptococcus agalactiae (group B).

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 10(4)) 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 10(6) and 20 x 10(6), 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.