BIOCHEMICAL CHARACTERS 



517 



by adding pure sulphuric acid to a 24-liour peptone water culture of the 

 organism ; if positive, a red coloration appears almost immediately. It depends 

 on the ability of the organism to produce indole and reduce nitrates, so 

 that on the addition of sulphuric acid the nitroso-indole reaction occurs. Many 

 organisms not belonging to the Vibrio group are able to form indole and reduce 

 nitrates, but most of them break down the nitrite to NH3 (Maassen 1902). The 

 reaction is given not only by V. cliolerce, but by a number of other vibrios, so that it 

 no longer has the diagnostic significance with which it was at first accredited. For 

 its production it is essential to use a brand of peptone that contains a small 

 amount of nitrate ; not all peptones are suitable. Paladino-Blandini (1906) 

 states that the indole and the nitrite must be formed in certain proportions ; if 

 either is in excess, the reaction fails : but if the culture is heated to 60-70° C. 

 for 15 minutes, the colour appears. All the members appear to form catalase 

 and ammonia. Some reduce methylene blue, and some form HjS — though not 

 for several days. 



Nobechi (1925), who studied a large number of cholera and cholera-like vibrios, 

 concluded that it was impossible to distinguish them by their biochemical reactions. 

 Heiberg (1935, 1936a), however, who studied 384 strains of vibrios, was able to 

 classify them into six groups by the use of mannose, sucrose, and arabinose. He 

 found that all true cholera vibrios fell into one group, which he calls Type I, but 

 which, owing to the almost exclusive restriction of the word " type " to antigenic 

 variants, we propose to refer to as Group I. His scheme of classification is as 

 follows : 



Unfortunately Heiberg's series of strains was insufiiciently representative. Num- 

 erous workers, such as Pollitzer (1935-36), Taylor, Read and Pandit (1936), 

 Combiesco-Popesco and Cocioba (1936), Taylor and Ahuja (1938-39), and Pasricha, 

 Chatter] ee and Das (1938-39), who have repeated Heiberg's work, have found 

 that, though all true agglutinable cholera strains fall into Group I, a considerable 

 proportion of other vibrio strains from human sources and from water having 

 no connection with cholera fall likewise into this group. It may be said, therefore, 

 that a strain falling into Groups II to VI may be excluded from the cholera species, 

 but that a strain falling into Group I may or may not be a true cholera vibrio ; 

 serological examination alone can determine its identity. It may be mentioned, 

 however, that Taylor, Pandit and Read (1937) find that, whereas true cholera 

 vibrios give a positive cholera red and a negative Voges-Proskauer reaction, the 

 great majority of inagglutinable vibrios falling into Heiberg's Group I react 

 positively to both tests — provided Barritt's (1936) modification is used for the 



