/Extracellular and intracellular concentrations of TcdA were measured in strains VPI10463, UK1, and 630. VPI10463 and UK1 showed 330- and 2,200-fold higher extracellular TcdA at 12h compared to 630. Intracellular TcdA in 630 became comparable to VPI10463 and UK1 by 24h, but extracellular TcdA remained significantly higher in the clinical strains (Fig. 1b).