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BACTERIOLOGICAL LABORATORY TECHNIC 



rotating the tube in a groove melted in a block of ice. The centrifu- 

 gal 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. Bacteria differ in very many ways in cultures. Some 

 grow rapidly and luxuriantly; 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 



Fig. 34. Esmarchs' method of making roll cultures on ice. (Williams.) 



by some; gas and acids produced by others. These various char- 

 acteristics 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 



