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Modification of penicillin-binding protein 5 associated with high-level ampicillin resistance in Enterococcus faecium
M Ligozzi1, F Pittaluga, R Fontana
1Istituto di Microbiologia, Università degli Studi di Verona, Italy.
Abstract:
High-level ampicillin resistance in Enterococcus faecium has been shown to be associated with the synthesis of a modified penicillin-binding protein 5 (PBP 5) which had apparently lost its penicillin-binding capability (R. Fontana, M. Aldegheri, M. Ligozzi, H. Lopez, A. Sucari, and G. Satta. Antimicrob. Agents Chemother. 38:1980-1983, 1994). The pbp5 gene of the highly resistant strain E. faecium 9439 was cloned and sequenced. The deduced amino acid sequence showed 77 and 54% homologies with the PBPs 5 of Enterococcus hirae and Enterococcus faecalis, respectively. A gene fragment coding for the C-terminal part of PBP 5 containing the penicillin-binding domain was also cloned from several E. faecium strains with different levels of ampicillin resistance. Sequence comparison revealed a few point mutations, some of which resulted in amino acid substitutions between SDN and KTG motifs in PBPs 5 of highly resistant strains. One of these converted a polar residue (the T residue at position 562 or 574) of PBP 5 produced by susceptible and moderately resistant strains into a nonpolar one (A or I). This alteration could be responsible for the altered phenotype of PBP 5 in highly resistant strains.
Insights
High-level ampicillin resistance in Enterococcus faecium is linked to changes in penicillin-binding protein 5 (PBP 5). Specific mutations in the PBP 5 gene alter its structure, potentially causing resistance.
Area of Science:
- Microbiology
- Molecular Biology
- Drug Resistance
Background:
- High-level ampicillin resistance in Enterococcus faecium is associated with modified penicillin-binding protein 5 (PBP 5).
- This modified PBP 5 appears to have lost its penicillin-binding capability.
Purpose of the Study:
- To investigate the genetic basis of ampicillin resistance in Enterococcus faecium.
- To clone and sequence the pbp5 gene from a highly resistant strain.
- To analyze mutations in the PBP 5 gene associated with varying levels of ampicillin resistance.
Main Methods:
- Cloning and sequencing of the pbp5 gene from Enterococcus faecium.
- Deduction of amino acid sequences and comparison with related species.
- Cloning and sequencing of a gene fragment encoding the C-terminal penicillin-binding domain of PBP 5.
- Sequence comparison to identify point mutations and amino acid substitutions.
Main Results:
- The deduced amino acid sequence of PBP 5 showed 77% homology with Enterococcus hirae and 54% with Enterococcus faecalis.
- Several point mutations were identified in the PBP 5 gene of resistant strains.
- Mutations resulted in amino acid substitutions, including the conversion of a polar threonine residue to a nonpolar alanine or isoleucine in highly resistant strains.
Conclusions:
- Specific point mutations in the pbp5 gene are associated with high-level ampicillin resistance in Enterococcus faecium.
- Alterations in the penicillin-binding domain of PBP 5, particularly the conversion of polar to nonpolar residues, likely contribute to the resistant phenotype.