The group of dysentery organisms is a large and. varied one. It may be divided, as is well known, into two main sub-groups, the "Shiga-Kruse" and the "mannit-fermenters," which are of equal importance and entirely distinct. If we take decided differences in agglutination, as determined by absorptions, as a criterion of specificity, the latter group includes at least four distinct species with a number of sub-varieties and transition forms. Two of the above species are found among organisms which split either dextrose, mannit, and saccharose or in addition maltose. The other two species are represented by the bacillus "Y" of Hiss and Russell and the "Flexner-Manila" bacillus. If we adopt Ford's proposal, that a new species should be made for each culture showing a constant cultural or agglutinative difference, the number would increase considerably. At the best, however, classification of bacteria is at present very artificial. Although so many types appear among the " mannit-fermenters," there is no reason why they should not all be included in the dysentery group of bacilli, provided the reaction in litmus milk be typical. This cultural test, as Duval has emphasized, is the most constant one which we have at present, but it should extend over several weeks in order to exclude lactose fermenters.
In contradistinction to the heterogeneity of the "mannit-fermenters," the homogeneity of the "Shiga-Kruse" type stands in marked contrast. Every culture of the latter which. was tested (about twenty in all) reacted in all media and agglutinated with all the various sera exactly alike. One is tempted to explain this diversity of the "mannit-fermenters" by accepting the suggestion of Flexner, that they may be occasional, if not constant, inhabitants of the normal intestine. Certainly Duval has isolated the "mannit-fermenters" from the mildest cases of diarrhœa, to say nothing of the two apparently normal infants from which he also obtained them. Furthermore, the agglutinins for the mannit group in normal blood might be accounted for on this hypothesis, as well as the many differences in fermentation and agglutination. For, as has been well said by Smith and Reagh, one " should keep in mind the various adaptations in the intestine—it may be to the food remains in the large intestine, to the mucus on the surface of the epithelium, to the contents of the tubules and the larger flask-shaped glands, and further, with this progressive adaptation are associated modifications of biological characters which most likely involve agglutinative capacities as well." The fact that dysentery bacilli have not been isolated frequently from the normal intestine might be explained by their habitat being possibly within the mucus tubules and their number few under normal condition, for it is well known that they are most frequently found in association with mucus. When abnormal conditions, however, arise and the production of mucus increases, these organisms would possibly multiply in number pari passu and find themselves in a favorable position to attack any slight injury which might occur in the wall of the intestine.
The "Shiga-Kruse," or Group I, type, on the other hand, seems to present the characters of a true parasite. Only an insignificant amount of agglutinin is present for it in normal human blood and that of laboratory animals, according to most authorities; it agglutinates alike in various immune sera; it has a very marked toxicity for laboratory animals; it has never been isolated from the normal human intestine, although an organism in some respects like it has been described by Ford as being an inhabitant of the normal intestine; it produces no indol; it splits only the monosaccharids, through which it falls in line with the conclusion of Smith and Reagh, that the less a bacillus acts on sugars the more pronounced is its parasitic character. Whether or not it ever invades the blood in numbers is a question yet to be decided.