80 APPLIED BACTERIOLOGY 



concentrated sulphuric acid. If indol is present, a rose to a 

 deep-red coloration is produced. 



Stoddart {The Analyst, May, 1897) applies the test as 

 follows : A warm solution of nitrous acid prepared by 

 adding one part of pure sulphuric acid to ten parts of 0"02 

 per cent, solution of potassium nitrite, is poured over an 

 agar culture. If the organism gives the reaction, a pink 

 coloration appears in the superficial layers. 



In the case of cultures of the cholera spirillum, only the 

 addition of the sulphuric acid is required to bring about the 

 reaction, as the necessary nitrite is already present, having 

 been formed by the reduction of the nitrate which is in- 

 variably contained in the peptone used in the culture 

 medium. This test is sometimes called the cholera-red 

 reaction. 



The Staining and Mounting of Micbo-oeganisms. 



Owing to the very great difficulty of observing bacteria in 

 their natural condition, even with the best microscopes, it 

 becomes necessary to treat them in such a way as to make 

 them easier of observation. This is done by staining them 

 with various dyes. Staining constitutes an indispensable 

 aid to the study of the finer details of the various organisms, 

 and the great advance which has been made in our know- 

 ledge of the bacteria is largely due to the many ingenious 

 methods of differential staining which have been devised for 

 their identification. 



Owing to the avidity with which the bacteria take up 

 certain aniline dyes, it becomes possible to recognise them 

 amongst the tissues of the animal body and in other places 

 where otherwise they would escape notice. Not only by 

 the use of staining reagents is much of the internal 

 structure and other details, as spore formation, made out, 

 but as the behaviour of various organisms is not the same 



