Horizontal gene transfer and host specificity of beta-haemolytic streptococci: the role of a putative composite transposon containing scpB and lmb

Beta-haemolytic streptococci are important human and animal pathogens: thei r genetic traits that are associated with the ability to infect human hosts remain, however, unclear. The surface protein, Lmb, mediates the adherence of Streptococcus agalactiae to human laminin. For further analysis of the corresponding gene, the adjacent genomic regions were sequenced. Lmb is loc alized on a putative composite transposon of 16 kb and is flanked by two co pies of a novel insertion sequence element (ISSag2). It harbours the genes scpB and Imb, which are 98% identical with the respective genes of Streptoc occus pyogenes. Analysis of the distribution of these genes and ISSag2 amon g 131 streptococcal strains revealed that all of the human isolates, but on ly 20% (12 of 61) of the animal isolates, contained scpB and Imb or their h omologues. To investigate if the putative transposon can be mobilized, an e rythromycin resistance marker was incorporated into the Imb gene of S. agal actiae. Screening for mutant strains with a regained susceptibility for ery thromycin identified strains with a deletion of scpB, Imb, and one copy of ISSag2. We hypothesize that a horizontal gene transfer caused the exchange of scpB and Imb and that the ability of S. pyogenes, S. agalactiae and grou p C and G streptococcal strains to colonize or infect human hosts is depend ent on their presence.