HOW TO AVOID CONTAMINATIONS. 



103 



THE MAKING AND TRANSFERENCE OK PURE CULTURES. 



In addition to what has been said under Pathogenesis, the following suggestions 

 may be of service to the beginner. 



For the making of plate cultures and for the transfer of organisms from one 

 culture medium to another, select a still day and, if possible, a day when a gentle rain 

 or snow is falling. This offers ideal conditions, since the earth is wet, the outside 

 air has been washed free from dust, and there is no wind to stir up dust within the 

 laboratory. A strict adherence to this rule is sometimes very inconvenient and it is 





Fig. 88.* 



not meant to be iron-clad. It is, however, of immense service in keeping cultures 

 free from contaminations, and those who propose to disregard it should remember 

 that haste in the beginning of an experiment often leads to vexation and delay in 

 the end, especially when the success of the experiment depends absolutely upon 

 the purity of the culture. 



*Fic. 88. Soft rot of green cucumbers inoculated by needle-punctures from a pure culture of 

 Bacillus carotovorus. The only parts not softened are those through which the infected needle en- 

 tered, i. e., the parts rubbed with mercuric-chloride water. In each a little button of tissue under 

 the disinfected area did not decay. The sound fruit at the right was punctured at the same time, 

 but with a sterile needle. The cucumbers had been removed from the vine, but were not flabby. 

 They were exposed after inoculation to the ordinary air of the laboratory. The photograph was 

 made on the seventh day. About two-fifths natural size. 



