The dramatic rise in prevalence of multi-resistant enterococci in United States hospitals over the past decade has limited therapeutic options, affected morbidity and mortality and increased the cost of caring for seriously ill hospitalized patients. The expression of resistance to vancomycin has received the most attention during this time. However, it is equally problematic that virtually all vancomycin-resistant enterococci (VRE) are Enterococcus faecium that express resistance to high levels of ampicillin. While it is clear that ampicillin resistance in E. faecium requires expression of low affinity penicillin-binding protein 5 (PBP5), the correlation between the amounts of detectable PBP5 and the level of ampicillin resistance is not exact. Several point mutations in pbp5 have been identified in strains expressing high-level ampicillin resistance, but the specific contributions of these mutations to the levels of resistance have never been assessed. We have identified the first transferable ampicillin resistance described from E. faecium in a VRE strain from Northeast Ohio. The pbp5 gene conferring resistance in this isolate possesses several mutations that have been associated with high-level ampicillin resistance in other E. faecium isolates. Curiously, levels of ampicillin resistance expressed by transconjugant E. faecium strains are not equivalent to those expressed by the donor, despite documentation that equivalent amounts of PBP5 are produced. In the past two years, we have acquired evidence that levels of ampicillin resistance expressed correlate with transcription (but not necessarily translation) of an upstream open reading frame designated ftsWEf.
The specific aims of this proposal are to: 1) perform site directed mutagenesis of E. faecium pbp5 to determine the functional (MIC, affinity) and structural importance of specific mutations. With collaborations in France and Switzerland, we now possess the molecular expertise to create the mutants and analyze their functional impact and determine the crystal structure; 2) to investigate the role of the putative upstream repressor psr in regulating expression of ampicillin resistance in E. faecium; 3) to investigate the mechanisms by which transcription of ftsWEf impacts the levels of ampicillin resistance expressed by E. faecium; 4) to assess whether upstream open reading frames designated nanE-Ef and ywrF-Ef affect levels of ampicillin resistance expressed and 5) to determine whether the peptidoglycan precursors differ in sensitive and resistant strains. These investigations will yield new insights into what is arguably the most resistant nosocomial pathogen of our time by providing important structure-function correlations for PBP5, correlations which may be important for the development of newer and better inhibitory compounds. They will also yield important new information on mechanisms of cell wall synthesis in E. faecium and other Gram-positive bacteria as well as on the mechanisms by which ampicillin resistance in E. faecium is regulated.
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