FIG 3.

AmpC is the β-lactamase that confers ΔampD cells elevated ampicillin resistance. (A) An ampicillin susceptibility assay was performed by spotting dilutions. Briefly, exponentially grown cultures were serially diluted, spotted on solid medium containing no ampicillin (No AMP) or ampicillin at 25 μg/mL (AMP 25), and incubated at 28°C for ~40 h before imaging. (B) Quantitative analysis of phase-contrast microscopy of exponentially growing strains untreated (No AMP) or treated with ampicillin at 25 μg/mL (AMP 25). The percent phenotype was calculated by counting the number of cells displaying one of the indicated phenotypes (1 cell = 1 phenotype) and dividing it by the total number of cells for each strain. (C) Determination of β-lactamase production was performed by a nitrocefin assay using cell lysates. No AMP or AMP 25 indicates cells untreated or treated with ampicillin at 25 μg/mL, respectively, for 2 h before the generation of cell lysates. Data represent one of two biological replicates.