erm (B)-Carrying Elements in Tetracycline-Resistant Pneumococci and Correspondence between Tn 1545 and Tn 6003

Abstract

This study investigated the genetic organization of erm (B)-carrying transposons of Streptococcus pneumoniae and their distribution in tetracycline-resistant clinical isolates. By comparatively analyzing reference pneumococci carrying erm (B)/ tet (M) transposon Tn 1545 , Tn 6003 , Tn 6002 , or Tn 3872 , we demonstrated a substantial correspondence between Tn 1545 and Tn 6003 , which have the same resistance gene combination [ tet (M) (tetracycline), erm (B) (erythromycin), and aphA - 3 (kanamycin)]; share the macrolide-aminoglycoside-streptothricin element, containing a second erm (B); and only differ by a ca. 1.2-kb insertion (containing a putative IS 1239 insertion sequence) detected in Tn 1545 from S. pneumoniae reference strain BM4200. These results enabled elucidation of the structure of Tn 1545 , the first erm (B)-carrying transposon described in S. pneumoniae . A collection of 83 erythromycin- and tetracycline-resistant clinical pneumococci, representative of recent Italian isolates carrying erm (B) as the sole erythromycin resistance gene, was used to investigate the distribution of the different transposons. All 83 organisms were positive for tet (M) and bore an erm (B)/ tet (M) transposon that could be characterized by using a specific set of primer pairs; Tn 3872 was detected in 18 isolates, Tn 6002 in 59 isolates, and Tn 6003 in 6 (the sole kanamycin-resistant) isolates. The genetic organization of transposon Tn 1545 , with its specific insertion, was not detected in any of the isolates tested. The erm (B)-carrying elements of tetracycline-resistant pneumococci substantially corresponded to those [bearing a silent tet (M) gene] recently detected in tetracycline-susceptible pneumococci. Overall, in erm (B)-positive pneumococci, Tn 6003 was the least common erm (B)-carrying Tn 916 -related element and Tn 6002 the most common.