Numerous methods, not one of which can be regard- 

 ed as absolutely reliable, have been proposed 

 for distinguishing IB. typhosus from the closely 

 allied B. coli. Amongst them the use of coloured me- 

 dia holds a prominent position. 



The idea of making use of colour change in micro- 

 biology originated from Helmholtz 1 ) who in 1843 em- 

 ployed a litmus gluten solution to detect putrefaction. 

 He found that before this could be perceived by smell, 

 decoloration of the solution took place. 



Buchner 2 ) in 1885 introduced coloured media into the 

 bacteriological technique. Since then, their utility has be- 

 come more and more recognised, not only for purposes 

 of diagnosis, but also for demonstrating certain charac- 

 teristics of a given micro-organism. Buchner himself 

 used a litmus medium to distinguish Vb. cholerae from 

 similar forms. 



Cahen 3 ) maintained, that together with the change 

 of colour produced in the medium by the altered reac- 

 tion, a reduction of the dye itself took place. 



Spina 4 ) regarded this reduction as a deoxidation be- 

 cause the colour returned if the medium was shaken 

 with atmospheric air. 



Ra/csahegyi 5 ) made a large number of experiments 

 with gelatine stained with methylene-blue or methyl- 

 violet for the purpose of differentiating various bacilli. 



