Relative substrate affinity index values: a method for identification of beta-lactamase enzymes and prediction of successful beta-lactam therapy.

Using a nitrocefin competition assay, I determined the relative substrate affinity index (RSAI) values of nine clinically significant beta-lactamase enzymes against a range of beta-lactams. Using selected beta-lactam substrates, I observed large differences in the RSAI values of the nine enzymes that were sufficient in many cases to positively identify specific enzymes. I made use of the unique RSAI values of SHV-1, TEM-1, and TEM-2 beta lactamases with cefoxitin to screen for the presence of these enzymes in Klebsiella aerogenes clinical isolates. The RSAI values also allow for the prediction of the outcome of beta-lactam therapy against specific beta-lactamase-producing isolates.