METHODS OF EXAMINATION OF BACTERIA 49 



were at first grayish and finally lemon-yellow. All these cultures came 

 from the same source and yet we have four colours represented. In the 

 same way it has been observed by bacteriologists that colour-forming 

 bacteria often lose this power by being grown for several bacterial 

 generations in laboratory cultures. Still another example of their 

 plasticity is the change in size which the individuals undergo when 

 their environment changes. When circumstances are unfavourable, 

 they not infrequently diminish very much in size. This is usually the 

 precursor of death, but not always, as sometimes a protracted existence 

 can follow after a partial adaptation has taken place. Instances can be 

 multiplied, but these are sufficient to show that, in the study of any 

 species, reliance must be placed on the sum total of characteristics, 

 and a knowledge of the whole life-history. The following methods 

 of cultivation and observation are those most commonly used : 



1. Gelatine Plate-culture. The method of procedure is that 

 described under the section dealing with the methods of obtaining 

 pure cultures The colonies are examined and the following points 

 noted : 



(1) Colour, size, and shape of colonies. 



(2) Rate of growth. 



(3) Whether growth is on surface, under surface, or both. 



(4) Whether liquefaction takes place. 



(5) Microscopically the size, shape, and motility of the cells. 



(6) Microscopically the appearance of the edge of the colony and 



the various modifications of form assumed by it. 

 It will be found that No. 6 needs careful attention, as the appearance 

 of the different colonies of the same species, even in the same plate, 

 is often very various, sometimes leading to the impression that more 

 than one form is present. 



2. Agar Plate-culture. Prepared in the same way. It has the 

 advantage that plates can be placed in incubators at fairly high 

 temperature, and so more rapid growth is obtained. It has the dis- 

 advantage that growth over the surface takes place very rapidly, so 

 the plates are soon spoiled for observation purposes. The points to 

 be noted are those given above for gelatine plate-cultures. 



3. Gelatine Stab-cultures. A test-tube is filled with about 6 cms. of 

 nutrient-gelatine, which, when solid, is stabbed with a platinum needle 

 which has been dipped in a solution containing the species in question. 

 The following points are noted : 



(1) Whether growth on surface only, along the stab only, or on 



both. 



D 



