Total toxin amounts were quantified using 2 assays; a toxin end point titer assay and a commercial available ELISA (Ridascreen, Biopharma). The supernatants of culture samples (1 mL) were collected after centrifugation (30 min, 30006g, 4uC), filter sterilized (0.45 mM cellulose acetate membrane) and stored at 4uC.