Pathogenic behavior of Escherichia coli has been explored with regard to necrotizing power, hemolysis, toxicity for mice, and other characteristics. On the other hand, the presence of certain pathogenic Escherichia culture has been clarified by serological investigation. However, when a culture of Escherichia coli, the sero-type of which is different from that of any known pathogenic organism, is isolated from gastro-enteritis, it is very difficult to determine whether the culture is the causative agent of the disease or not.
Sjöstedt (1946) found that Escherichia cultures belonging to certain 0 groups (2, 4, and 6) were frequently hemolytic, confirming Ewertsen's finding (1946) that hemolysis and the capacity of producing necrosis were related to each other.
Recently, De and Chatterje (1953) performed rabbit loop experiments to study the action of Vibrio cholerae on the intestine. De, Bhattacharya and Sarker (1956) extended the experiments to determine the pathogenicity of Escherichia cultures isolated from acute and chronic enteritis. From the results of their experiments, they suggested that the serological properties of the cultures might not be correlated with their pathogenicity for the rabbit intestine.
The authors (Namioka, Urushido and Sakazaki, 1958) carried out an experiment similar to that of De et al. using about 40 cultures isolated from transmissible gastroenteritis in pigs caused by a viral agent, and concluded that, in this case, toxic Escherichia organisms, which might have normally been harbored in the large intestine, migrated into the small intestine where they aggravated enteritis in association with the causative agent. Moreover, so far as Escherichia organisms isolated from transmissible gastroenteritis are concerned, no remarkable relationships were observed among 0 inagglutinability, hemolytic and necrotizing powers, and pathogenicity for the rabbit intestine.
Rolle and his co-workers (Rolle and Kalich, 1955; Botze, 1956) described that various toxic strains of Escherichia possessed an ability of agglutinating the sperm of the dog, while atoxic (non-pathogenic) organisms were lacking in such an ability.
Many attempts have been made, however, to differentiate the culture of pathogenic Escherichia organisms from that of non-pathogenic.
It would be very difficult to distinguish one from the other with any known procedure, although differentiation was possible among various types of disease caused by these organisms in man and animals.
It is well known that by using some reagents the invasiveness of certain organisms is enhanced directly or indirectly and, consequently, the virulence of the organisms seems to be intensified. Nungester et al. (1936) reported that Salmonella typhosa and some other organisms, when treated with mucin, increased their virulence. Such enhancement of pathogenicity was also demonstrated with the spreading factors discovered by Duran-Reynals (1942) . Chain (1939, 1940) described that the diffusibility of the organisms into certain tissue was accelerated when the host was administered with hyaluronidase. This effect is presumed to be due to an increase in tissue permeability which results from the enzymatic hydrolysis of hyaluronic acid and with facilitates infiltration and spread of pathogenes.
Recently, an assumption has been made on the possible presence of a wetting agent, a kind of surface activate agent, by the function of which the surface tension of tissue cells is depressed and capillary permeability increased. Consequently, this agent might facilitate the access of organisms to the tissues of their hosts. Christovao (1957) descovered that the pathogenicity of Shigella dysenteriae and Sh. flexneri for mice was consistently increased as shown by their LD₅₀ 100 to 2, 000 times as small as those of the controls.