PATHOGENICITY 695 



by the living bacilli but appears free in the culture medium only after autolysis of the 

 dead cells has begun. Again the imphed distinction between enterotoxin and neurotoxin 

 is misleading, since it is apparent that the so-called exotoxin, besides causing paralysis 

 of the limbs, leads also to serious changes in the intestine, perhaps by causing vasoconstric- 

 tion of the intestinal blood vessels (Penner and Bernheim 1942). For the present it is 

 better to refer to the so-called exotoxin by Steabben's term, neuro-enterotoxin. 



Summarizing our present knowledge, we may say that Sh. shigce gives rise to 

 a specific soluble neuro-enterotoxin, which is present in broth cultures about a 

 week old, in filtered autolysates of 24:-hour agar cultures, and in the dried bacterial 

 bodies. It is destroyed by a temperature of 75°-80° C. maintained for an hour. 

 If prepared by a method favouring bacillary autolysis (Hansen 1936, Takita 1939), 

 it proves fatal to mice inoculated intravenously in a dose of about 0-001 to 0-01 ml. 

 It is fatal to the rabbit, causing paralysis of the limbs, diarrhoea and collapse ; 

 but has less action on the guinea-pig, in which diarrhoea and collapse alone are 

 produced. It is a protein, and may be precipitated by trichloracetic acid (Boivin 

 and Mesrobeanu 1937a), and to a less extent by the addition of 40 per cent, solid 

 ammonium sulphate (Blake and Okell 1929). It gives rise to, and is neutralized 

 by, a specific antitoxin, which combines with it in constant proportions. In addition, 

 smooth strains of Shiga's bacillus contain a toxic somatic antigen, which is a lipo- 

 polysaccharide conjugated with a protein, resembling similar smooth antigens in 

 the Salmonella and in other members of the Shigella group. 



With the partial exception of Schmitz's bacillus, the high toxicity of Sh. shigce 

 is not rivalled by other dysentery bacilli. The ground-up bacterial bodies of 

 Sh. flexneri, Sh. sonnei, and Sh. dispar prove fatal on intravenous inoculation of 

 rabbits, but only in a dose that is about 20 times greater than the corresponding 

 fatal dose of dried Shiga cells. Schmitz's bacillus is variable in its toxicity. 

 According to Buchwald (1939), it may give rise to a thermolabile neurotoxin, 

 precipitable by trichloracetic acid from old broth cultures, and causing paralysis 

 of the extremities when inoculated into mice and rabbits ; but it appears to be 

 rather less potent than that formed by Shiga's bacillus. Sh. alkalescens is said 

 to be non-toxic. 



Pathogenicity. — Sk. shigae, Sh. schmitzi, Sh. flexneri, the Newcastle bacillus, 

 Sh. so)inei, and a number of types described by Boyd (1938, 1940), undoubtedly 

 give rise to dysentery in man. The role of Sh. alkalescens is still doubtful, though 

 it may be responsible for infections of the urinary tract. Sh. dispar appears to 

 be non-pathogenic. The evidence in favour of the pathogenicity of some members 

 of the para-Shiga group is strong, but further observations are desirable. (For 

 fuller information see Chapter 70.) 



With the exception of captive monkeys, which may carry Sh. flexneri in the 

 gut (see Lovell 1929), and which may sometimes, as on a vitamin-deficient diet 

 (Verder and Petran 1937, Janota and Dack 1939), develop dysenteric symptoms 

 (Preston and Clark 1938, David and Schirl 1939), and with the possible exception 

 of dogs which may be infected with either Shiga's or Flexner's bacillus (Dold 

 1916), animals do not appear to suffer from dysentery. It is not possible to repro- 

 duce the typical disease, as it occurs in man, by experimental inoculation or feed- 

 ing of the ordinary laboratory animals. Nevertheless many dysentery organisms, 

 especially Sh. shigce, are toxic to rabbits, horses, and mice, and to a less extent to 

 guinea-pigs. After subcutaneous inoculation into rabbits, dogs, and young pigs 



