CHAPTEK XIII. 



IDENTIFICATION OF BACTERIA CULTURAL CHARACTERISTICS- 

 ANIMAL EXPERIMENTS. 



IN practical bacteriological work it is not merely sufficient to 

 obtain a bacterium in pure culture from a pathologic product or 

 other source, but the organism must also be fully identified. This 

 may be a very easy matter, requiring perhaps only a simple animal 

 experiment or a simple serum test. On the other hand it may require 

 the study of the pure culture on a variety of media, and under varying 

 conditions, with careful observation of the naked-eye appearances 

 of the growth, tests for metabolic products and microscopic exam- 

 ination in the hanging drop and the examination of specimens stained 

 by different methods. Certain cultural peculiarities, like the lique- 

 factioh of gelatin and the production of acids and gases, have 

 already been considered. It is also necessary to note the effect of 

 certain growing bacteria on the fluid culture media. For example, 

 the media may appear clear, with the formation of a sediment or 

 cloudy, either slight or heavy. It should also be observed, when a 

 sediment is present, whether it is granular or not, whether a pellicle 

 is formed, and whether, in the latter case, streaks from it reach to 

 the bottom of the tube or flask; also, whether and when the pellicle 

 sinks to the bottom. The precipitation of the casein in milk and the 

 tendency of certain growths on a solid medium to adhere to the 

 platinum loop when introduced must also be noticed. Any and all of 

 these features may be quite characteristic of certain bacteria, and may 

 give considerable aid in identification. The colonies on plates and 

 slants present certain characteristics which must be observed in 

 reflected and transmitted light. Their size, color, dryness or moisture 

 pigments and early and late tendency to confluence are often important 

 characteristics. A number of descriptive terms applied to colonies 

 and bacterial growth as a whole in or on various culture media must 

 now be explained. 



CULTURAL CHARACTERISTICS. 



Stab Cultures. Stab cultures in gelatin which do not liquefy the 

 medium, and which show a uniform growth along the stab, without 

 any special characters, are known as filiform growths. The growth 

 is called nodose when it is composed of closely aggregated colonies, 



