FIG. 1.

(A) A typical PAE experiment for E. coli ATCC 25922 after exposure to ceftriaxone or ciprofloxacin (each at a concentration equivalent to twice the MIC), as determined by viability counting. As shown, no PAE was seen after ceftriaxone exposure, but ciprofloxacin induced a PAE of 1.9 h. The organisms were stained with propidium iodide and examined by fluorescence microscopy. (B through D) Photomicrographs of untreated control organisms (B), ceftriaxone-exposed organisms 35 min after drug removal (C), and ciprofloxacin-exposed organisms 270 min after drug removal (D). Both antibiotics induced filamentation, but this morphological form persisted past the classically defined PAE in organisms exposed to ciprofloxacin. Magnification, ×880.