Clostridium difficile toxin A was purified to homogeneity and was used to raise monospecific antiserum in rabbits. A gene bank of C. difficile DNA in Escherichia coli was constructed by cloning Sau3A-cleaved clostridial DNA fragments into the bacteriophage vector λEMBL3. Out of 4500 plaques screened with antitoxin A, 9 clones were positively identified. One of these clones λtA5 expressed a 235 kDa protein which exhibited a cytotonic effect on Chinese hamster ovary cells, and had the ability to haemagglutinate rabbit erythrocytes, both properties characteristic of toxin A. The size of the λtA5 insert DNA was 14.3 kb.
