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Alterations in penicillin-binding protein 2B from penicillin-resistant wild-type strains of Streptococcus pneumoniae
1Department of Medical Microbiology, School of Pathology, South African Institute for Medical Research, Johannesburg.
Abstract:
The 1.5-kb transpeptidase-encoding region (TER) of penicillin-binding protein (PBP) 2B was amplified and sequenced from 18 penicillin-resistant isolates of Streptococcus pneumoniae, with each isolate representing a different DNA fingerprint profile of the TER. PBP 2B TERs from penicillin-resistant isolates revealed extensive sequence divergence from the penicillin-susceptible R6 strain, differing by up to 170 nucleotide substitutions and resulting in up to 38 alterations in the amino acid sequence of the protein. All penicillin-resistant isolates showed sequence divergence within a +/- 300-bp area at the center of the PBP 2B TER. Although a number of amino acid substitutions were found within this central area of PBP 2B, only two substitutions were common to all resistant isolates, namely, Thr-252 replacement by Ala and Glu-282 replacement by Gly. These two substitutions appear to be essentially associated with a decreased affinity of PBP 2B for penicillin. A second block of divergent nucleotide sequence was prominent amongst isolates with high levels of resistance. This was a +/- 100-bp area of the TER around nucleotide 1300 and included the substitution of Gly for Asp-431, which was the only amino acid substitution within this area that was common to all isolates. These data may assist in the definition of the structural changes in the penicillin-binding site of PBP 2B associated with penicillin resistance in S. pneumoniae.
Insights
Penicillin resistance in Streptococcus pneumoniae is linked to specific changes in the penicillin-binding protein 2B (PBP 2B) transpeptidase-encoding region (TER). Key amino acid substitutions in PBP 2B reduce its affinity for penicillin, contributing to resistance.
Area of Science:
- Microbiology
- Molecular Biology
- Genetics
Background:
- Penicillin resistance in Streptococcus pneumoniae is a growing public health concern.
- Penicillin-binding proteins (PBPs) are the primary targets of beta-lactam antibiotics, including penicillin.
- Alterations in PBP 2B are implicated in reduced susceptibility to penicillin in S. pneumoniae.
Purpose of the Study:
- To investigate the sequence variations within the transpeptidase-encoding region (TER) of penicillin-binding protein (PBP) 2B in penicillin-resistant Streptococcus pneumoniae isolates.
- To identify specific amino acid substitutions in PBP 2B associated with penicillin resistance.
- To correlate structural changes in PBP 2B with decreased penicillin binding affinity.
Main Methods:
- Amplification and sequencing of the 1.5-kb TER of PBP 2B from 18 penicillin-resistant S. pneumoniae isolates.
- Comparison of TER sequences from resistant isolates with a penicillin-susceptible strain (R6).
- Analysis of nucleotide and amino acid sequence divergence to identify common substitutions.
Main Results:
- Extensive sequence divergence was observed in the PBP 2B TER of resistant isolates compared to the susceptible R6 strain, with up to 170 nucleotide and 38 amino acid changes.
- All resistant isolates showed divergence within a central +/- 300-bp region of the TER.
- Two key amino acid substitutions (Thr-252 to Ala and Glu-282 to Gly) were common to all resistant isolates and linked to reduced penicillin affinity.
- A second divergent region around nucleotide 1300, containing a Gly for Asp-431 substitution, was associated with high-level resistance.
Conclusions:
- Specific sequence variations in the PBP 2B TER are strongly associated with penicillin resistance in Streptococcus pneumoniae.
- The identified amino acid substitutions, particularly Thr-252-Ala and Glu-282-Gly, are crucial for decreased PBP 2B affinity for penicillin.
- These findings contribute to understanding the structural basis of penicillin resistance in S. pneumoniae and may aid in defining the penicillin-binding site alterations.