796 METHODS OF INVESTIGATING BACTERIA. 



individual species of bacteria from a mixture of organisms. 

 Solid nutrient media had been formerly frequently em- 

 ployed, but Koch was the first to use them with the de- 

 finite view of obtaining thereby pure cultivations. While 

 in the case of fluids the organisms which are introduced, 

 and any which may have accidentally entered, become 

 mixed with each other, so that those that are few in 

 number are scarcely recognisable among the larger 

 number of rapidly growing organisms, in the case of 

 solid substrata the individual species are much more 

 easily isolated. If we inoculate a species of bacterium at 

 various parts of the solid nutrient substratum, small 

 colonies, soon distinctly visible to the naked eye, develop 

 at each of the points of inoculation ; if extraneous or- 

 ganisms accidentally fall on the same nutrient soil, they 

 on their part form isolated colonies which do not usually 

 mix with those inoculated, and which are readily dis- 

 tinguishable from them by their colour, form, and con- 

 sistence ; should, however, the extraneous germ fall on 

 one of the colonies inoculated, and multiply at the same 

 part, they usually cause an alteration in the external ap- 

 pearance of this colony ; and we can ascertain by simple 

 microscopical observation whether at any part which we 

 wish to choose for the inoculation of fresh soil impuri- 

 ties are present. For the second inoculation we only 

 employ those parts which are found to be quite pure, 

 and it is in the certainty with which we can find the 

 material for each further inoculation that we have one 

 of the greatest advantages of solid as compared with fluid 

 nutrient substrata. If in the latter extraneous organisms 

 once enter they spread through the whole medium, and 

 it is pure chance if we succeed in avoiding one of these 

 extraneous organisms when we inoculate fresh soil ; the 

 previous examination of a drop under the microscope is 

 not sufficient ; for if we find on examination that other 

 organisms are present it is very difficult to obtain a pure 

 cultivation. In the case of the solid nutrient substrata, 

 on the other hand, it is not necessary to avoid altogether 

 the entrance of other organisms, for in this case, as we can 

 constantly control the selection of the part for re-inocu- 



