RELATIONS OF BACTERIA AMONG THEMSELVES 45 



injurious to each other, feed on the same substance, for each takes food 

 away from the other. 



By means of these changes all organic matter is made ready for 

 reabsorption into the bodies of plants, and later, as animals feed on 

 plants, into the bodies of animals. In this way putrefactive bacteria 

 act as true philanthropists, because, by their activity, the accumulation 

 of dead organic matter is prevented. Also, if the organic matter were 

 not changed, there would result a dearth of food for the plants, and 

 ultimately the extinction of all plants and animals would follow. The 

 exact method by which aid is given to plants cannot be explained until 

 we have considered the sulphixr-, the nitrite-, and nitrate-bacteria. 



§4. METHODS OF PURE CULTURE. 



A pure culture means a growth in which only one species is present. 

 Before 1878 pure cultures were unknown, so that descriptions of species 

 before this date are unreliable, as errors were made because cultures 

 apparently consisting of one species were really mixtures of several 

 kinds. The result was that the characteristics of several were described 

 as those of one species. In 1878, however. Lister devised a method 

 for isolating the various kinds of bacteria from a mixture containing 

 several species. This is called the Dilution method, and it consists in so 

 diluting the liquid containing the bacteria that a small portion extracted 

 from it by means of the platinum loop or wire will contain only one 

 kind. When the portion thus extracted is now transferred to a sterile 

 nutrient tube the resulting growth will consist of the derivatives of 

 one species only. Lister was thus the first to obtain a pure culture. 

 Pure cultures of yeast were first made by E. Chr. Hansen by this 

 method, a fact for which the brewers of the present day have great 

 reason to be thankful. 



This method is very seldom employed nowadays, as it is somewhat 

 cumbersome in comparison with the Plate method, and will separate out 

 only one form, whereas the plate method will separate several in one 

 experiment. 



A second method is that called the Fractional method of culture. In 

 a culture containing a mixture of bacteria one form will predominate, 

 and consequently be more numerous. When this is at the zenith of 

 its activity an inoculation is made from this culture to a second. In 

 this way the predominant bacteria start with an advantage over the 

 other competitors. As it is better adapted for this culture medium — 



