Chromosomal gentamicin resistance transposon Tn3706 in Streptococcus agalactiae B128

Streptococcus agalactiae B128 is the only highly gentamicin-resistant group B streptococcal (GBS) strain described so far. This strain carries a chromosomal gentamicin resistance transposon, designated Tn3706, which is similar, if not identical, to the Tn4001 and Tn5281 transpons detected in Staphylococcus aureus and Enterococcus faecalis, respectively. Transposition of Tn3706 occurred onto the GBS plasmid pIP501 in two different loci of its 7.5-kb AvaII fragment carrying the genes for chloramphenicol and erythromycin resistance. Molecular analysis of pIP501 derivatives showed that Tn3706 is composed of a central fragment containing the aac6'-aph2" gene; this fragment is flanked by two tandemly repeated copies of IS256 at the 5' extremity of the resistance gene and a single inverted copy of IS256 at its 3' extremity. The two tandemly repeated copies of IS256 were separated by a 6-bp segment identical to that found, in the same orientation, in the IS256-aac6'-aph2" junction. The hybrid replicons pIP501::Tn3706 were found to be structurally unstable following conjugative transfer between GBS strains. Numerous individual copies of IS256 were detected in B128, but this insertion sequence was not found in the 11 wild-type, gentamicin-susceptible GBS strains studied.