ESBL-producing isolates were incubated with cefotaxime (0, 0.01, 1, 10, and 100 μg/ml) for 12 hours at 30°C. At that point, the culture was centrifuged (13,000 rpm, 5 min) to separate the supernatant and whole cells. The supernatant was removed, and whole cells were resuspended in fresh M9. A portion of the whole cells was removed and sonicated (20 amp, duration of 1 min at 4°C) to release Bla from the periplasmic space. Bla activity in each component (supernatant, sonicated whole cells, and whole cells) was quantified by adding 1 μM fluorocillin and monitoring the change in green fluorescence with a plate reader. A 96-well plate was loaded with 200 μl of each mixture and topped with 50 μl of mineral oil. The plate was loaded into a Tecan Infinite M200 PRO microplate reader (chamber temperature maintained at 25°C), and OD600 and GFP readings were measured every 10 min for 1.5 hours. The GFP measurements of the samples were plotted over time, and the slope was calculated. The slope was normalized by the whole cell’s OD measurement.

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