Evidence for natural gene transfer from gram-positive cocci to Escherichia coli

High-level resistance to macrolide-lincosamide-streptogramin type B (MLS) antibiotics in Escherichia coli BM2570 is due to the presence on the conjugative plasmid pIP1527 of the MLS resistance determinant ermBC, which is almost identical to the erm genes previously described in plasmid pAM77 from Streptococcus sanguis (ermAM) and in transposon Tn917 from Enterococcus faecalis (ermB). This gene and its regulatory region are located downstream from the insertion sequence IS1. The 23S rRNA methylase encoded by pIP1527 differs by three and six amino acids from those encoded by Tn917 and pAM77, respectively. Unlike the streptococcal elements which confer the inducible MLS phenotype, the ermBC gene is expressed constitutively in E. coli and Bacillus subtilis, due to several mutations in the regulatory region. Transcription of the ermBC gene starts from three different sites following three overlapping promoters which function in both E. coli and B. subtilis. Promoters P2 and P3 are located within the region homologous to pAM77 and Tn917, and P1 is a hybrid promoter constituted by -35 and -10 sequences located at the end of IS15 and in the streptococcal region, respectively. These results constitute evidence for the recent in vivo transfer from Streptococcus spp. to E. coli. This transfer could have been mediated by transposons such as Tn917 or Tn1545 from Streptococcus pneumoniae, which also bears an MLS determinant that is homologous to ermB. We speculate that the insertion sequences IS15 and IS1 could have played a role in the expression and dissemination of ermBC, which has been found in numerous strains of enterobacteria.