ROLL CULTURE 127 



the tubes by quickly rotating the tube in a groove melted in a 

 block of ice. The centrifugal force distributes the gelatine over 

 the glass, and the ice hardens it rapidly while in contact with the 

 glass. Such tubes are veritable plates, and in them colonies of 

 bacteria often grow as well as on the plates and may be fished 

 out. 



The various characteristics of bacterial growth may be studied 

 in cultures. Some organismal cultures grow rapidly and luxuri- 

 antly; some discretely and slowly; colors and odors are produced 

 by some; gelatine is liquefied by many, while others do not liquefy 

 gelatine. Milk is curdled and digested by some; gas and acids 

 produced by others. These various characteristics enable us to 

 identify and differentiate bacteria. 



The cultivation of bacteria in the laboratory has for its purpose 

 a demonstration of their vital activities. This may indicate only 

 their botanical character or it may show their relation to disease. 

 In order that we may classify germs systematically certain criteria 

 have been established which when added together permit us to 

 identify and name the organisms. This is called determinative 

 bacteriology. The principal characters to be noted are com- 

 plete morphology, staining characters, particularly with Gram's 

 method, colonial growth on agar and gelatine, potato, blood 

 serum, milk, sometimes inorganic salt solutions, the enzymic 

 products as indicated by fermentation of carbohydrates and 

 solution of proteins like milk curd and gelatine. With this last 

 comes ammonia and nitrite productions. The optimum tempera- 

 ture and media, and resistance to physical and chemical agencies 

 must be taken into consideration. For pathogenic bacteria we 

 establish as far as possible the relations with lower animals. 

 This includes, of course, the production of soluble toxins and 

 endotoxins. 



The chemical activities of many bacteria are well displayed in 

 litmus milk culture in which decolorization, acid or alkali forma- 

 tion, coagulation and clot digestion are the important ones. 



