Microtiter Plate-Based Assay for Inhibitors of Penicillin-Binding Protein 2a from Methicillin-Resistant Staphylococcus aureus

Penicillin-binding protein 2a (PBP2a), the molecular determinant for high-level beta-lactam resistance in methicillin-resistant Staphylococcus aureus (MRSA), is intrinsically resistant to most beta-lactam antibiotics. The development and characterization of new inhibitors targeting PBP2a would benefit from an effective and convenient assay for inhibitor binding. This study was directed toward the development of a fluorescently detected beta-lactam binding assay for PBP2a from MRSA. Biotinylated ampicillin and biotinylated cephalexin were tested as tagging reagents for fluorescence detection by using a streptavidin-horseradish peroxidase conjugate. Both bound surprisingly well to PBP2a, with binding constants of 1.6 +/- 0.4 mu M and 13.6 +/- 0.8 mu M, respectively. Two forms of the assay were developed, a one-step direct competition form of the assay and a two-step indirect competition form of the assay, and both forms of the assay gave comparable results. This assay was then used to characterize PBP2a binding to ceftobiprole, which gave results consistent with previous studies of ceftobiprole-PBP2a binding. This assay was also demonstrated for screening for PBP2a inhibitors by screening a set of 13 randomly selected beta-lactams for PBP2a inhibition at 750 mu M. Meropenem was observed to give substantial inhibition in this screen, and a follow-up titration experiment determined its apparent K(i) to be 480 +/- 70 mu M. The availability of convenient and sensitive microtiter-plate based assays for the screening and characterization of PBP2a inhibitors is expected to facilitate the discovery and development of new PBP2a inhibitors for use in combating the serious public health problem posed by MRSA.