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SUMMARY Two enterotoxins formed by enteropathogenic strains of Escherichia coli of pig origin were studied. One of them (LT) was heat-labile and antigenic and the other (ST) was heat-stable and apparently non-antigenic. They were identified by their ability to dilate ligated segments of pig intestine. The ST component was produced by all of 40 enteropathogenic pig strains examined and the LT component was produced only by those strains that naturally possessed, or had possessed, K88 antigen. Both enterotoxins were found to be controlled by transmissible plasmids (Ent). In transmission experiments with donor strains that produced both LT and ST, it was not possible to isolate recipient strains that produced only one or other of these two enterotoxins; they all produced both. As a group, strains that had received Ent (Ent+) from donor strains that produced only ST dilated ligated segments to the same extent as did similar strains that had received Ent from donor strains that produced both ST and LT. The dilating effect of LT was neutralised by antisera prepared against live organisms of strains that produced LT and ST and, when larger doses were used, by antisera prepared against strains that had been shown to produce ST only. None of the antisera examined neutralised ST. Antisera prepared against living organisms neutralised the dilating effect of these organisms. This neutralising effect was bactericidal, not antitoxic, in character and it appeared to be largely strain-specific. ST and LT preparations of Ent+ strains produced a similar type of severe diarrhoea when given by mouth to piglets. Similar proparations of the corresponding Ent− strains did not. It is concluded that LT and ST are probably two forms of essentially the same enterotoxin.
Smith et al. (Sat,) studied this question.