New Tn916-related elements causing erm(B)-mediated erythromycin resistance in tetracycline-susceptible pneumococci.

OBJECTIVES

To analyse the as yet unexplored genetic elements encoding erm(B)-mediated erythromycin resistance in tetracycline-susceptible pneumococci.

METHODS

Sixteen Streptococcus pneumoniae clinical isolates sharing erm(B)-mediated erythromycin resistance and susceptibility to tetracycline were used. Gene detection was performed by PCR using both established and specially designed primers. S. pneumoniae R6, Streptococcus pyogenes 12RF and Enterococcus faecalis JH2-2 were used as recipients in mating experiments.

RESULTS

Of the 16 test strains, 14 bore an unexpressed tet(M) gene which in 13 strains had a genetic linkage with erm(B). Three isolates yielded a 3.2 kb and 10 an 11.9 kb erm(B)/tet(M) amplicon. The former three showed genetic organizations similar to that of the composite element Tn3872, where the erm(B)-carrying Tn917 transposon is inserted into a Tn916-like element. Of the latter 10 isolates, 9 showed genetic organizations substantially overlapping with that of Tn6002, a newly sequenced erm(B)-containing Tn916-related transposon. The tenth isolate carried a novel composite element (designated Tn6003) resulting from the insertion into a Tn6002-like transposon of a fragment [designated macrolide-aminoglycoside-streptothricin (MAS) element] containing a second erm(B) (lacking the stop codon) and a variant of the aadE-sat4-aphA-3 cluster. The two tet(M)-negative isolates had different Tn3872-related elements, one containing a complete and one a deleted MAS fragment. Conjugative transfer was obtained from donors carrying Tn6002-related elements, not from donors carrying Tn3872-related elements.

CONCLUSIONS

In tetracycline-susceptible pneumococci with erm(B)-mediated erythromycin resistance, the erm(B) gene is carried on a variety of Tn916-related genetic elements either lacking tet(M) or, more often, carrying an unexpressed tet(M) gene.