ANTIGENIC STRUCTURE 687 



is slow and is not usually apparent for 2 to 10 days. Lactose-fermenting papillae 

 often develop on the original lactose-negative colonies obtained by primary plating. 

 Sucrose is also fermented late, though, according to Fors'yth (1933), Sh. dispar 

 acidifies sucrose before lactose. Sucrose is attacked by a small minority of Flexner 

 strains after subculture in the laboratory for a variable length of time, but never on 

 first isolation. Dulcitol is fermented rapidly by Sh. alkalescens, and slowly by 

 the Newcastle bacillus and the dispar-like organism known as Bad. ceylonense B, 

 The Newcastle bacillus is distinguished from other members by its ability to form 

 gas in glucose and dulcitol. The quantity of gas is very small, often amounting 

 to not more than a bubble in a Durham fermentation tube, and may be evident 

 only on first isolation. The closely allied Manchester bacillus forms a trace of 

 gas in mannitol as well as in glucose and dulcitol ; but the antigenically similar 

 organism met with in India and in the United States, known as Type 88, forms 

 no gas at all. Sh. alkalescens is distinguished from Sh. flexneri by the fermentation 

 of dulcitol and xylose ; but according to de Assis (1939a), the substance of greatest 

 differential value is 5 per cent, glycerol, which is always attacked by the alkalescens 

 and not by the flexneri type. Sh. alkalescens is said by Wood and Keeping (1944) 

 to be unique among the dysentery bacilli in its ability to form trimethylamine 

 from choline. Sh. dispar ferments xylose and sorbitol, thus differing from Sh. 

 sonnei ; it must be pointed out, however, that xylose-fermenting strains of 

 Sh. sonnei do occur (Bojlen 1934, Cruickshank and Swyer 1940), though the great 

 majority of strains isolated in Great Britain and the United States lack the power 

 to ferment this sugar. In litmus milk there is generally a slight acidity for a 

 few days. The reaction may remain permanently acid and go on to clotting, 

 as with Sh. sonnei and Sh. dispar, or it may revert to neutral, as with Sh. shigce, 

 Sh. schmitzi, and Sh. flexneri. Many strains oi flexneri, after a preliminary acidity, 

 turn milk alkaline. Sh. alkalescens produces an initial and lasting alkalinity. 



Indole formation is of some differential importance, serving to distinguish 

 Schmitz's from Shiga's bacillus, and Sh. dispar from Sh. sonnei. The Newcastle 

 bacillus does not form indole, but Sh. alkalescens does. As regards the Flexner 

 group, Gettings (1919) found that of 285 strains tested, 158 produced indole, and 

 127 did not. The methyl-red test is of limited value ; if the cultures are incu- 

 bated at 37° C, it may help to distinguish the positive Sh. dispar from the negative 

 Sh. sonnei ; but, according to Bamforth (1934), Sh. sonnei may give a positive 

 result if it is incubated at 30° C. 



All strains reduce nitrates to nitrites ; none forms HgS ; none grows in Koser's 

 citrate ; and none gives a positive Voges-Proskauer reaction. A list of biochemical 

 reactions is given in Table 45. (For reference to the more recent studies on these 

 reactions, see Lester 1926, Smith and Fraser 1928, Kerrin 1928, Nelson 1930, 

 Bojlen 1930, 1934, Johnston and Brown 1930, Buchanan and Roux 1930, Koser et al. 

 1930, Cann and de Navasquez 1931, Welch and Mickle 1932, 1934, Downie et al. 1933, 

 Forsyth 1933, Bamforth 1934, Whitehead and Scott 1934, Large and Sankaran 

 1934, Mandry 1935, McGinnes et al. 1936, Boyd 1931, 1932, 1936, 1938, Hazen 

 1938, Ali 1938, Hardy et al. 1940, Sachs 1943). 



Antigenic Structure. — The serological behaviour of the dysentery bacilli is com- 

 plicated. Of the non-mannitol fermenters the Shiga group is homogeneous ; all 

 strains of Sh. shigce are agglutinated by a specific serum prepared against any one 

 strain. An anti-shigse serum has some agglutinating action on- Schmitz's bacillus 

 and on some strains of the Flexner group, A serum prepared against Schmitz's 



