The relationship between two apparently different enterotoxins produced by enteropathogenic strains of Escherichia coli of porcine origin.

This enterotoxin retained most of its activity after 30 min. at 1OO"C, but was inactivated after 30 min. at 121°C. It will therefore be referred to as the heat-stable enterotoxin or ST. Although ST could occasionally be detected intracellularly in bacteria grown on soft agar cultures, it was mainly extracellular. In some strains of E. coIi its production was controlled by a transmissible plasmid designated Ent (Smith and Halls, 1968). Later work (Smith, unpublished) has revealed that the ability of ST to dilate ligated segments of pig intestine is not neutralised by antiserum. Kohler (1968) demonstrated an enterotoxin probably identical with ST that produced diarrhoea in young piglets after intragastric administration. Gyles and Barnum (1969) used the pig ligated intestine test to demonstrate what appeared to be an additional enterotoxin in cultures of some pig-enteropathogenic strains of E. coli. The additional enterotoxin differed sharply from ST in that it could be inactivated by antiserum and by heating to 60°C for 30 min; accordingly, it is here referred to as the heatlabile enterotoxin or LT. Although LT could sometimes be detected occurring extracellularly in cultures in liquid media, it was most consistently demonstrated in lysates of bacterial cells.