Accepting that many interactions with cell wall targets are possible with the ampicillin-sulbactam combination, the low IC(50)s of ampicillin and sulbactam for PBP3 may contribute to understanding why this combination is effective against A. A molecular model with ampicillin and sulbactam positioned in the active site of PBP3 reveals that both compounds interact similarly with residues Thr526, Thr528, and Ser390. In contrast, PBP3 demonstrated a narrower range of IC(50)s against β-lactamase inhibitors than PBP1a (ranges, 4 to 5 versus 8 to 144 μM, respectively). ranged from 1 to 5 μM for a series of β-lactams. baumannii and PBP3 from Acinetobacter sp. Using an in vitro assay with a reporter β-lactam, Bocillin, we determined that the 50% inhibitory concentrations (IC(50)s) for PBP1a from A. Decreased expression of PBPs as well as loss of binding of β-lactams to PBPs was previously shown to promote β-lactam resistance in A. Little is known about the contributions of penicillin binding proteins (PBPs), the target of β-lactam antibiotics, to β-lactam-sulbactam susceptibility and β-lactam resistance in A. Acinetobacter baumannii is an increasingly problematic pathogen in United States hospitals.
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