Following the study of Sakazaki, Tamura and Saito (1967), search was made for previously unrecognized enteropathogenic OK groups of Escherichia coli among 756 strains isolated from children and adults with diarrhoea. Eleven OK groups of E. coli were recognized as possible enteropathogens associated with diarrhoea in children and adults, based on their frequency of isolation, ligated gut-loop reactions in rabbits and keratoconjunctival tests in guinea pigs, together with published reports by other workers. These were: O6: K.; O6: K62; O18: K76; O18: K77; O20: K84; O25 : K1; O78: K80; O114: K90; O142: K86; O148: K.; and O152: K. Of the 11 OK groups, O25: K1 and O152: K., possessed Shigella-like enteropathogenicity, and the remaining 9, most of which had already been described as causative agents of infantile diarrhoea, were included in the “Salmonella-like group” of enteropathogenic E. coli by Sakazaki et al. (1967) .
The criteria for determination of the possible enteropathogenicity of a given E. coli OK group are discussed.

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The mechanism of enteropathogenicity of human enteropathogenic Escherichia coli (EEC) was studied by ligated gut loop tests in rabbits, keratoconjunctival tests in guinea pig eyes and feeding tests on human volunteers, and by examining for their ability to penetrate into HeLa cells.
Strains of invasive EEC (Shigella-like) serotypes produced inflammatory changes and fluid accumulation in the ligated rabbit gut loop, keratoconjunctivitis in guinea pig eyes, and penetration into HeLa cells. With few exceptions, no enterotoxic activity was demonstrated with cell-free filtrates of broth cultures. It was considered that EEC strains of this group may probably cause disease in man by a mechanism similar to that of Shigella organisms, and that enterotoxin production may not be concerned with the pathogenicity of these organisms.
When strains of noninvasive EEC including 27 OK group serotypes were studied, 46 of 67 cultures employed produced fluid accumulation and histopathological changes in ligated gut loop tests in rabbits, whereas none of them induced keratoconjunctivitis in guinea pig eyes nor penetration into HeLa cells. In addition, cell-free filtrates from broth cultures of 12 out of 20 strains produced alterations similar to those caused by living culture in the gut loop tests. When R-state cultures derived from three strains of noninvasive EEC serotypes were tested for their activity in rabbit gut loops, it was demonstrated that the mutants no longer had the ability to produce positive gut loop reactions, although filtrates of the R cultures were still able to cause pathological alterations in the loops. Cell cultures killed by acetone, chloroform, or heat failed to give positive gut loop reactions.
Positive gut loop reactions were also induced by some strains not belonging to the recognized EEC serotypes originating from sources other than gastroenteritis patients.
Nine strains consisting of four of noninvasive EEC and five enterotoxigenic strains not belonging to the recognized EEC serotypes were tested with human volunteers. All the former strains caused acute gastroenteritis in the volunteers, while the latter elicited on response.
These results suggest that enterotoxin production is not concerned in the mechanism of enteropathogenicity of noninvasive EEC. On the other hand, it seemed that the ability to multiply in the intestine is essential in the enteropathogenicity of noninvasive EEC strains, although an enterotoxic substance demonstrated in cell-free filtrates may play a role in the development of gastroenteritis.

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Enteropathogenic activity of Vibrio parahaemolyticus was studied with the ligated gut loop model in rabbits. Positive reaction of the gut loop was mostly produced with Kanagawa-positive organisms. Reaction was also induced by the cell-free filtrate from brain heart infusion cultures of the vibrio, thus suggesting the presence of an enterotoxic substance. With the filtrate from broth cultures, however, dilatation of the gut loop and inflammatory changes in the intestinal mucosa occurred regardless of the Kanagawa reaction of the original culture. The substance responsible for these changes was thermolabile, being different from the Kanagawa-hemolysin which failed to produce pathological changes in the gut loop. On the other hand, massive inocula acetone-killed, chloroform-killed, or heated cultures did not produce dilatation or pathological changes in the gut loops.
When the pathological changes in the intestinal mucosa of loops inoculated with cell-free filtrate from broth cultures were compared with those inoculated with live cultures, the only difference observed was less marked infiltration of granulocytes in the mucosa in the former.
These findings suggest that the ability of V. parahaemolyticus to multiply in the intestine which is generally associated with the Kanagawa reaction of the organism may possibly be essential to its enteropathogenicity, although an enterotoxin-like substance produced by the vibrio regardless of the Kanagawa reaction may play a role in inducing gastroenteritis and diarrhea.

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