BACTERIOLOGICAL TECHNIQUE 31 



This method of examination is particularly useful to determine 

 whether the bacteria are capable of movement or not, that is, 

 whether they belong to the class of motile bacteria. Bacteria 

 are more simply observed when they are dried and stained 

 with suitable dyes, which render them more clearly observable. 

 There are a number of methods in use for staining bacteria, 

 varying according to the medium in which they are observed, 

 especially, e.g., in tissues, and also for the purpose of bringing 

 out such features as the flagellae or thread-hke processes, 

 which are characteristic of certain organisms, e.g., the typhoid 

 bacillus. Special methods also are necessary for staining 

 spores. It will be sufficient here briefly to indicate a simple 

 method of staining a pure culture. A carefully cleaned cover 

 glass is taken, and held in a pair of specially constructed 

 forceps, a drop of clean water is placed on the slip and a small 

 portion of the culture mixed with the water and spread in a 

 thin film over the glass by means of a sterile platinum wire ; 

 the film is now carefully dried by passing the glass several 

 times through a Bunsen flame with the film uppermost 

 (Fig. 6 b). The cover glass should never be made hotter than 

 can be easily borne by the finger if the under side of the glass 

 is pressed down on it. When the film is dry a drop of stain is 

 placed on the slide, ordinary magenta (rose-aniline) or gentian 

 violet are commonly used. The stain is allowed to remain 

 for a minute or two in contact with the glass and then washed 

 off in a gentle stream of water or by immersion in a large 

 volume of clean water. The preparation is again carefully 

 dried, and a drop of Canada balsam placed on the film side of 

 the cover glass, which is then carefully placed in contact with 

 the ordinary mounting slide. With a little care only such a 

 quantity of Canada balsam is dropped on to the cover glass 

 as will just suffice to reach to its edge when it is pressed 

 down upon the mounting sHde. Care should be taken to 

 remove all air bubbles from between the cover glass and the 

 shde. 



