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Evidence for natural gene transfer from gram-positive cocci to Escherichia coli
A Brisson-Noël1, M Arthur, P Courvalin
1Unité des Agents Antibactériens, Institut Pasteur, Paris, France.
Abstract:
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.
Insights
High-level macrolide-lincosamide-streptogramin B (MLS) resistance in E. coli is linked to the ermBC gene on plasmid pIP1527. This gene, similar to those in Streptococcus and Enterococcus, shows constitutive expression in E. coli due to regulatory mutations, suggesting recent bacterial transfer.
Area of Science:
- Microbiology
- Molecular Biology
- Genetics
Background:
- Macrolide-lincosamide-streptogramin type B (MLS) resistance is a significant challenge in bacterial infections.
- The ermBC gene confers high-level MLS resistance and has been identified on conjugative plasmids in various bacterial species.
Purpose of the Study:
- To investigate the genetic basis of MLS resistance in Escherichia coli BM2570.
- To characterize the ermBC gene and its regulatory elements on plasmid pIP1527.
- To explore the evolutionary origins and transfer mechanisms of the ermBC determinant.
Main Methods:
- Plasmid DNA isolation and characterization.
- Gene sequencing and comparative analysis of ermBC with related genes.
- Reporter gene assays to study promoter activity and gene expression.
- Bioinformatic analysis to identify insertion sequences and regulatory regions.
Main Results:
- The MLS resistance in E. coli BM2570 is conferred by the ermBC gene located on plasmid pIP1527.
- The ermBC gene is highly similar to ermAM from Streptococcus sanguis and ermB from Enterococcus faecalis.
- Constitutive expression of ermBC in E. coli and Bacillus subtilis is attributed to mutations in its regulatory region, unlike the inducible expression in streptococci.
- Transcription initiates from three promoters (P1, P2, P3), with P1 being a hybrid promoter involving an insertion sequence (IS15).
- Evidence suggests recent horizontal gene transfer of ermBC from Streptococcus species to E. coli, potentially mediated by transposons.
Conclusions:
- The ermBC gene on plasmid pIP1527 represents a recent acquisition in E. coli, likely transferred from Gram-positive bacteria.
- Mutations in the regulatory region lead to constitutive expression, enhancing the dissemination of MLS resistance.
- Insertion sequences IS1 and IS15 may play a role in the expression and spread of ermBC within enterobacteria.