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Transfer of penicillin resistance from Streptococcus oralis to Streptococcus pneumoniae identifies murE as resistance

Katya Todorova1, Patrick Maurer1, Martin Rieger1

  • 1Department of Microbiology, University of Kaiserslautern, Kaiserslautern, Germany.

Molecular Microbiology
|May 27, 2015
PubMed

Insights

Beta-lactam resistance in Streptococcus pneumoniae can arise from altered penicillin-binding protein (PBP) genes or changes in peptidoglycan biosynthesis enzymes like MurM. This study reveals a novel mechanism involving the murEU gene transfer from Streptococcus oralis, conferring resistance.

Area of Science:

  • Microbiology
  • Molecular Biology
  • Antibiotic Resistance

Background:

  • Beta-lactam resistance in *Streptococcus pneumoniae* is often linked to alterations in penicillin-binding protein (PBP) genes and sometimes the murM gene, suggesting interspecies gene transfer.
  • MurM and MurN enzymes are crucial for synthesizing branched lipid II, a key substrate in the PBP-catalyzed transpeptidation reaction essential for bacterial cell wall synthesis.

Purpose of the Study:

  • To investigate the genetic basis of high-level beta-lactam resistance in *Streptococcus oralis* Uo5 by using it as a donor in transformation experiments with *Streptococcus pneumoniae* R6.
  • To identify specific genes and genetic elements responsible for conferring piperacillin resistance in *Streptococcus pneumoniae* following transformation with *Streptococcus oralis* DNA.

Main Methods:

  • Transformation experiments were conducted using a high-level beta-lactam resistant *Streptococcus oralis* Uo5 strain as the donor and a sensitive *Streptococcus pneumoniae* R6 strain as the recipient.
  • Analysis of transformants included sequencing of penicillin-binding protein (PBP) genes and other relevant genes, such as murEU, to identify genetic alterations responsible for the observed resistance phenotype.

Main Results:

  • Piperacillin-resistant *Streptococcus pneumoniae* transformants acquired the *murEUo5* gene from *S. oralis*, encoding UDP-N-acetylmuramyl tripeptide synthetase, rather than alterations in PBP genes.
  • Specific codons (83-183) of *murEUo5* were found to be sufficient for conferring the resistance phenotype, and its promoter also contributed to increased resistance.
  • While transformants contained *murEUo5* and some PBP genes (*pbp2xUo5*, *pbp1aUo5*, *pbp2bUo5*), the full resistance level of the donor strain was not achieved, indicating a complex resistance mechanism.

Conclusions:

  • The study demonstrates that *murEUo5* gene transfer from *S. oralis* is a significant mechanism contributing to beta-lactam resistance in *Streptococcus pneumoniae*, independent of PBP gene alterations.
  • The findings highlight the role of the UDP-N-acetylmuramyl tripeptide synthetase and its promoter in modulating beta-lactam resistance, suggesting a complex interplay of enzymes in peptidoglycan biosynthesis.
  • *Streptococcus oralis* Uo5 possesses an unusual *murM* and lacks *murN*, with peptidoglycan containing single L-Ala interpeptide bridges, further supporting a unique resistance mechanism.

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