Top panel – Experimental checkerboard matrix matching the top 3 R. typhi whole cell aptamers from ELASA screening in Table 2. Sandwich assay capture and reporter aptamer pairs were assigned combination numbers 1-9 as shown. Bottom panel – results of a typical screening experiment in which the fluorescent enzyme-linked aptamer-magnetic bead sandwich assay was assessed for each combination shown in the above panel with zero (blank, white bars) and a 1:100 dilution of stock R. typhi cells (~ 1,000 cells per ml, black bars).