Results of cytotoxicity neutralization assay. (A) Control: nucleus was normal. (B) Clostridium difficile toxin A (TcdA) treatment: condensation, fragmentation and dissolution of nuclei were found. (C) Anti-TcdA serum pre-treatment: cells became slightly round but still spindly. (D) Anti-TcdA serum treatment: cells became round and narrow; nuclei appeared condensed, but some cells were still spindly (×200 magnification).