Fig. (1) Schematic diagram of coumarin cephalosporin fluorescein (CCF) cleaved by β-lactamase. After the substrate CCF-acetoxymethylester (AM) enters the cell, it is converted into CCF by endogenous esterases, thereby trapping it inside the cell.Excitation of CCF at 409 nmleads to FRET (A) from the coumarin moiety to the fluorescein derivative that produces green fluorescencedetectable at 520 nm (green cells,B). In the presence of β-lactamase, the CCF substrate is cleaved to form two fluorophores that disrupts FRET. Excitation at 409 nm now produces a fluorescence detectable at 447 nm (blue cells, B) [8,9]