Checkerboard assays show synergy between BPEI and β-lactams on MRSA USA300 and MRSA MW2. BPEI had synergy with (A and B) oxacillin, (C and D) ceftizoxime, and (E and F) imipenem against (A, C, and E) MRSA USA300 and (B, D, and F) MRSA MW2. Each assay was performed as three separate trials, and the presented data are shown as the average change in OD600.