82 APPLIED BACTERIOLOGY 



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 Micko-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 

 method^ 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 

 to different dyes, this property serves to distinguish between 

 various kinds of organisms which are not otherwise to be 

 differentiated by simple microscopical examination. 



Weigert, in 1876, found that bacteria could be stained 

 with the basic aniline dyes, but not by the acid dyes or the 

 natural colouring matters. Koch and other workers at 

 once recognised the value of this discovery, and rapidly 

 investigated the matter, and devised many of the methods 

 now in use. 



All the basic aniline dyes, such as gentian violet, f uchsine, 

 methylene blue, methyl violet, Bismarck brown, etc., have 



