S. aureus SrtA (A) and B. anthracis SrtA (B), SrtB (C), or SrtC (D) were incubated with increasing concentrations of AAEK1 (●) or AAEK2 (○) followed by the addition of substrate (a-LPETG-d for SrtA, a-KTDNPKTGDEA-d for SrtB, or a-GEKLPNTASNN-d for SrtC) and fluorescence measurements. IC50 is the concentration of compound that inhibits 50% of sortase activity and is displayed in micromolar in the upper right corner. Data were fit using an nonlinear least squares method in GraphPad Prism®, and represent the mean of three independent determinations.