ABA binding and neutralizing activities. Serially diluted ABA (ng ml⁻¹) was added to microplate wells coated with TcdA (a) or TcdB (b). The binding of ABA to the toxins was determined by an HRP-conjugated anti-E-tag antibody. (c) Serially diluted TcdA or TcdB mixed with or without 1 μg ml⁻¹ of ABA was added to mRG1 monolayers in a 96-well plate. The cell rounding was examined under a phase-contrast microscope after 24 h incubation.