The characteristic of nonmotility is of greatest differentiating value and the reports of slight motility are probably from misinterpretation of molecular movement as motility. The dysentery bacilli do not form those thread or whip-like filaments so characteristic of typhoid cultures and are somewhat plumper. The dysentery bacillus is not found in the blood and hence is not eliminated in the urine, although recently there have been reported rare cases where dysentery bacilli were isolated from the blood. It is found in mesenteric glands. In dysentery patients agglutination phenomena do not show themselves until about the tenth day from the onset. Hence, this procedure is of no particular value in diagnosis. It is of value, however, to identify an organism isolated from the stools at the commencement of the attack, using serum from an immunized animal or a human convalescent for the agglutination test.
There seems to be only moderate agglutination power in the serum of convalescents from Shiga strains. Flexner strains give higher agglutinations, but early in convalescence the serum is not apt to have a titre of more than 1-150.
Dysentery bacilli produce a coagulation necrosis of the mucous membrane of the large intestine and occasionally of the lower part of the ileum. Polymorphonuclears are contained in the fibrin exudate.
It was formerly thought that these lesions were of local origin, but the present view is that toxins are produced which, being absorbed, are eliminated by the large intestine with resulting necrosis. Flexner, by injecting rabbits intravenously with a toxic autolysate, produced characteristic intestinal lesions. The toxin withstands a temperature of 70°C. without being destroyed. The toxin may cause joint trouble.
There are two main types of dysentery bacilli:
1. Those producing acid in mannite media—the acid strains (Flexner-Strong types).
2. Those not developing acid in mannite (Shiga-Kruse types). The Shiga type is very toxic in cultures, while the Flexner type seems to be less so. An organism resembling the Shiga bacillus but producing indol is the Schmitz bacillus. It does not appear to be pathogenic.
In immunizing horses for the production of antidysenteric serum it is customary to use both Flexner and Shiga strains, thus producing a polyvalent serum.
Lentz recognizes 4 types of dysentery bacilli for the differentiation of which he uses mannite, maltose and saccharose bouillon with litmus as an indicator.