FIG. 1.

Specific levels of L1 and L2 β-lactamase induction at difference inducer concentrations. Cells having an initial optical density at 600 nm of 0.4 were grown for 2 h in the presence of the inducer at the concentration stated prior to preparation of cell extracts and determination of specific activities of L1 (open triangles) and L2 (closed circles) in cell extracts as described previously (7). Briefly, total levels of nitrocefin-hydrolyzing activity (L1 and L2 activity combined) in cell extracts were determined, and the level of L2 activity alone was determined following addition of EDTA to reach a concentration of 100 μM (which specifically inhibits L1) to the cell extracts and reassaying. L1 activity was calculated by subtracting L2 activity from total activity. These specific activity values were divided by the value for the specific activity found in extracts of a culture treated identically but without addition of an inducer to give the severalfold induction values presented in the figure. Normally, we test β-lactamase inducibility in S. maltophilia by use of 100 mg/liter cefoxitin or 10 mg/liter imipenem, under which conditions each enzyme is maximally induced (as seen in Table 1). These data represent averages of the results of three experiments. There was less than 10% variation in the results obtained in these three repetitions in terms of severalfold induction levels determined for each concentration of inducer.