Characterization of a Streptococcus suis tet(O/W/32/O)-Carrying Element Transferable to Major Streptococcal Pathogens

Mosaic tetracycline resistance determinants are a recently discovered class of hybrids of ribosomal protectiontet genes. They may show different patterns of mosaicism, but their final size has remained unaltered. Initially thought to be confined to a small group of anaerobic bacteria, mosaictet genes were then found to be widespread. In the genusStreptococcus , a mosaictet gene [tet (O/W/32/O)] was first discovered inStreptococcus suis , an emerging drug-resistant pig and human pathogen. In this study, we report the molecular characterization of atet (O/W/32/O) gene-carrying mobile element from anS. suis isolate.tet (O/W/32/O) was detected, in tandem withtet (40), in a circular 14,741-bp genetic element (39.1% G+C; 17 open reading frames [ORFs] identified). The novel element, which we designated 15K, also carried the macrolide resistance determinanterm (B) and an aminoglycoside resistance four-gene cluster includingaadE (streptomycin) andaphA (kanamycin). 15K appeared to be an unstable genetic element that, in the absence of recombinases, is capable of undergoing spontaneous excision under standard growth conditions. In the integrated form, 15K was found inside a 54,879-bp integrative and conjugative element (ICE) (50.5% G+C; 55 ORFs), which we designated ICESsu32457 . An ∼1.3-kb segment that apparently served as theatt site for excision of the unstable 15K element was identified. The novel ICE was transferable at high frequency to recipients from pathogenicStreptococcus species (S. suis ,Streptococcus pyogenes ,Streptococcus pneumoniae , andStreptococcus agalactiae ), suggesting that the multiresistance 15K element can successfully spread within streptococcal populations.