132 MANUAL OF BACTERIOLOGY. 



the breaking up of complex organic compounds by micro- 

 organisms with the formation of simpler compounds. Fer- 

 mentation refers especially to the formation of useful prod- 

 ucts like alcohol. The term putrefaction is employed chiefly 

 for the breaking up of nitrogenous compounds with the 

 development of foul-smelling gases. The term fermentation 

 is also applied to the decomposition of complex substances 

 through the influence of unorganized ferments or enzymes. 



The work of bacteria in fermentation and putrefaction is 

 indispensable to the existence of the organic world as we 

 find it. Green plants convert the stable compounds of nitro- 

 gen, the carbon dioxide of the atmosphere, and water into 

 the complex and unstable albumins and carbohydrates which 

 serve as food for animals. Animals, on the other hand, con- 

 vert these unstable and complex compounds back into simpler 

 forms. The work of changing them back into the simple 

 and stable condition, in which they serve as the food for 

 plants, is performed by animal life in part only, and its com- 

 pletion is left to the activities of bacteria. It is the work of 

 bacteria in this direction which we call fermentation and 

 putrefaction. Without that work, as we understand it, the 

 existence of life upon the earth would soon come to an end, 

 and the dead and undecomposed bodies of living things and 

 their products of all kinds would lie about unchanged, as 

 they had fallen. 



Bacterium tcnno is the name formerly given to a sup- 

 posed species of bacteria which was credited with being 

 the producer of putrefaction. The individuals were rep- 

 resented as being short rods, mostly going in pairs, and 

 actively motile. The term has been abandoned since it 

 appears to have included a number of different species. 



