Characterization of two tetracycline resistance determinants in Streptococcus faecalis JH1.

Streptococcus faecalis strain JH1 harbors two conjugative plasmids: pJH1, an R plasmid mediating resistance to kanamycin, streptomycin, gentamicin, erythromycin, and tetracycline, and pJH2, a hemolysin-bacteriocin plasmid. Studies of plasmid-cured derivatives of strain JH1 and of transconjugates obtained after mixed incubation of JH1 with the plasmid-free S. faecalis strain JH2-2 revealed the presence of two tetracycline resistance determinants in strain JH1. One determinant mediated constitutive resistance to 40 micrograms of tetracycline per ml and was associated with plasmid pJH1. The second determinant, either on the chromosome of strain JH1 or on an undetectable plasmid, was inducible by tetracycline and enabled the host strain, in the absence of pJH1, to grow in the presence of 80 micrograms of tetracycline per ml. One transconjugant, strain DL172, was resistant to 80 micrograms of tetracycline per ml, sensitive to kanamycin, streptomycin, and erythromycin, and hemolytic in the presence, but not in the absence, of tetracycline. A single plasmid, pDL172, from this strain consisted of plasmid pJH2 and a 17.8-kilobase segment of DNA homologous to total cell DNA from strain JH1 but did not contain plasmid pJH1. Whether the addition of heterologous DNA to plasmid pJH2 occurred by translocation of a 17.8-kilobase tetracycline resistance transposon or by classical recombination with pJH2 has not been determined.