22 TEXT-BOOK OF BACTERIOLOGY. 



teria in the economy of nature, in consequence of their toeing the 

 only agents for causing the putrefaction of organic substances. It 

 is understood that by putrefaction is meant the stinking decompo- 

 sition of matter containing albumin. Pasteur referred this proc- 

 ess mainly to the activity of anaerobic bacteria. He thought that 

 the exclusion of oxygen or its removal by aerobic micro-organ- 

 isms simultaneously present was an essential condition of putre- 

 faction. Yet this view has not been fully confirmed. We know 

 aerobic bacteria also which can cause real putrefaction, and it is 

 probable that very many different bacteria are able to do the 

 same. 



Putrefaction is not a specific process that might be caused by 

 the action of one particular species of bacteria, but the general 

 name for a number of separate phenomena which produce the same 

 results, being all reduction processes, and all resulting in the for- 

 mation of ammonia, sulphuretted hydrogen, etc. 



On this accounb we may class putrefaction among those changes 

 that we observe in the soil and call nitrifaction and nitration. 

 These also depend on the action of bacteria, and show a decompo- 

 sition of organic matter into its simplest component parts, and 

 these in turn causing the production of higher combinations from 

 their union. 



In this manner alone is the soil enabled to serve.as nourishment 

 to ^the higher orders of plants, and the importance of the micro- 

 organisms for the development of vegetation is therefore ex- 

 tremely great. 



Nearly related to putrefaction is a process which becomes fa- 

 miliar in the course of breeding experiments. 



As is already known, the artificial food-solution which we usu- 

 ally employ is changed by the addition of gelatin (an extract of 

 calf's bone and other substances, rich in chondrin and gluten) 

 into a mass capable of solidification. Numbers of bacteria, motile 

 and non-motile bacilli' and micrococci, possess the capacity of de- 

 composing the gelatin; to digest it, or, as we may say, to peptonize 

 it. It thereby loses its solid consistence and becomes fluid. 



This is so remarkable a fact that it has even been employed to 

 make a division, for practical use, of the bacteria into two classes: 

 the " liquefying " and the " non-liquefying." 



Generally, however, it is not the bacteria themselves which 

 directly cause the liquefaction of the food-medium, but, in the 

 great majority of cases, the products of their metabolism. The 

 liquefying species produce a sort of ferment which we can sepa- 

 rate from the micro-organisms either by killing them or by means 



