Evidence for Conjugal Transfer of a Streptococcus faecalis Transposon (Tn916) from a Chromosomal Site in the Absence of Plasmid DNA

Streptococcus faecalis strain DS16, a multiply drugresistant clinical isolate obtained from St. Joseph's Mercy Hospital (Ann Arbor), was previously shown to harbor two plasmids (Clewell et al. 1978; Tomich et al. 1979b). One of the plasmids, pAD1 (35 megadaltons MD), is conjugative and determines hemolysin and bacteriocin activities; the other plasmid, pAD2 (15 MD), is nonconjugative and determines resistance to streptomycin (Sm), kanamycin (Km), and erythromycin (Em) (inducible). The last resistance is located on a transposon (Tn917) that can be induced to transpose by exposing the cells to a low (0.5 µg/ml) concentration of erythromycin (Tomich et al. 1979a, 1980). A tetracycline-resistance (Tcr) determinant was presumed to be chromosome-borne, since derivatives cured of pAD1 and pAD2 and devoid of plasmid DNA (on the basis of physical analysis) maintained a resistance to Tc (minimum inhibitory concentration was 30 µg/ml). We report here that the Tc resistance is located on a transposon...