6+ PRACTICAL BACTERIOLOGY. 



If a puncture cultivation is to be made, a test-tube, 

 in which the nutrient medium is horizontal, is taken, 

 and is inoculated in precisely the same manner as the 

 stroke cultivation, only that the platinum wire is 

 pushed right down to the bottom of the tube. It is 

 very importaat to keep the wire carefully in the 

 middle of the medium, for if the puncture is wavy and 

 excentric, not only does the culture have an ugly 

 careless appearance, but very often its specific charac- 

 teristics are incorrectly or incompletely shown. 



In puncture cultivations the different species show 

 greater and more striking peculiarities than in stroke 

 cultures. To start with, some species flourish equally 

 well upon or below the surface ; that is to say, they 

 can live with or without oxygen. Then there are a 

 very large number of species which can only develop 

 on the sui'face, or for a very few millimetres along 

 the track of the needle; i.e., they can only live where 

 oxygen is to be obtained. The first are called facul- 

 tative anaerobes, the second, aerobes ; a third class, 

 the obligatory or strongly anaerobic bacteria, do not 

 occur on cultivation plates, and other methods, which 

 will be described in the next chapter, must be em- 

 ployed for their cultivation. Moreover, bacteria wTiich 

 grow only on the surface also show differences in their 

 manner of growth; some keep close to the point 

 where the wire entered the medium, others form a 

 small flat droplet there, and yet others form a more 



