Abstract
Strains of <i>E. coli </i>involved in pig enterotoxemia were demonstrated to undergo a particular type of dissociation. Dissociated colonies were distinguished from the original colony forms by an illumination procedure as described by Henry. Dissociated cultures possessed an antigen, which, when injected into rabbits, provoked high serum agglutination titers. The heat stability of antigen X was similar to the K antigen. The presence of X antibodies in diagnostic ‘OK’ sera and of antigen X in strains of several serotypes interfered with the identification of the K antigen in these cultures. Dissociation did not affect the O, B and L antigens. Whereas cultures from freshly isolated strains of <i>E. coli </i>did not contain X<sup>+</sup> colonies, a high percentage of strains having the K antigen 88 (L) were found to be dissociated following <i>in vitro </i>maintenance. It is suggested that colony morphology could be used to recognize cultures with X antigen and so help to eliminate difficulties in serological typing.
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