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A Microtiter-based Assay for the Determination of ID50s of b-lactamase Inhibitors Employing Reporter Substrates Detected at UV or Visible Wavelengths (MaxLine Application Note #20)


INTRODUCTION
β -lactamases are either plasmid or chromosomally encoded bacterial enzymes which hydrolyze β -lactam antibiotics. The majority of β -lactamases produced by clinical isolates are serine active site enzymes. One successful approach to combating the serine active site β -lactamases is the use of β -lactamase inhibitors, such as clavulanic acid, in combination with a β -lactam antibiotic4.

There is also a small group of β -lactamases which have a metal ion at their active site (metallo-β -lactamases). These enzymes are not sensitive to serine β lactamase inhibitors and have a much broader substrate profile than the serine enzymes. Metallo-β -lactamases (MBLs) are capable of hydrolyzing the majority of clinically important β -lactam antibiotics, including carbapenems.

To date, more than 190 different β -lactamases have been identified1 and we will continue to observe clinical isolates which produce new types of enzymes, such as MBLs. Therefore, to enable the clinical spectra of established β -lactamase inhibitor/β -lactam combinations to be defined, it is necessary to evaluate these inhibitors against all new β -lactamases. In addition, novel inhibitors with improved potency and spectra are required to combat the current plethora of enzymes (both metallo and serine active site β -lactamases) and such compounds require rapid and effective evaluation.

This application note describes a simple procedure to determine the inhibition (ID50 value) of β -lactamases by various agents using the chromogenic cephalosporin nitrocefin (λ max = 482 nm) and the carbapenem antibiotic imipenem (λ max = 299 nm) as reporter substrates. Imipenem was used for those carbapenemases which did not have significant hydrolytic activity against nitrocefin. The method described us
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