USE OF EXTRACTS VERSUS WHOLE-CELL BACTERIAL SUSPENSIONS IN THE IDENTIFICATION OF STAPHYLOCOCCUS-AUREUS BETA-LACTAMASE VARIANTS

We previously have shown that extracts of S. aureus isolates which produce the recognized serotypes of staphylococcal β-lactamase (A, B, C, D) differ in the rates at which they hydrolyze selected cephalosporins, exhibiting substrate profiles which are distinctive for each serotype. In an effort to simplify the methods employed in identifying the different staphylococcal β-lactamases, we evaluated whether distinctive substrate profiles could be obtained by using whole-cell suspensions of 115 β-lactamase-producing isolates of S. aureus. Compared with extracts from the same strains, the whole-cell bacterial suspensions not only were simpler to prepare but enabled β-lactamase typing of a higher proportion of the evaluated strains (86 versus 97%, respectively). Furthermore, the use of whole-cell bacterial suspensions enabled the simultaneous quantitation of the β-lactamase activity exhibited by each strain. Additionally, by comparing the quantitative activity of β-lactamase-induced and -uninduced preparations of the same strain, induction ratios (i.e., induced/uninduced activity) could be derived, yielding information regarding the regulation of β-lactamase production by each strain. We believe that the utilization of whole-cell methods, such as those employed in this study, will facilitate the investigation of qualitative and quantitative differences in β-lactamase production among clinical and reference isolates of S. aureus.