148 



BIOLOGY AND TECHNIQUE 



sufficient speed, the gelatin will harden in a thin layer on the inner 

 surface of the tube. The colonies will develop in this layer and may be 

 " fished " by means of a platinum wire with bent point introduced into the 

 tube. This method is useful for certain purposes, but is too inconvenient 

 for routine work. It is now rarely used. 



Separation of Bacteria by Surface Streaking. — When it is necessary to 

 isolate bacteria like the gonococcus. Bacillus influ- 

 enzae, the pneumococcus, and others, which, because 

 of great sensitiveness to environment and possibly a 

 preference • for free oxygen, are not readily grown 

 in pour plates, it is often advantageous first to 

 pour plates of suitable media, allow them to 

 harden, and then gently smear over their surfaces 

 dilutions of the infectious material, usually in three 

 or four parallel streaks. (See Fig. 31.) 



Upon such plates, if dilutions have been prop- 

 erly made, and this is only a c[uestion of judgment 

 based upon an estimation of the numbers of bac- 

 teria in the original material, discrete colonies of the 

 microorganisms sought for may develop, and can be 

 "fished" in the usual manner. 



The media most favorable for the cultivation of 

 various microorganisms will be discussed in the 

 sections dealing with the individual species. 



ANAEROBIC METHODS 



We have seen in a preceding chapter (p. 26) 



Pjg 32 Deep ^^^^ many bacteria, the so-called anaerobes, will 



Stae Cultivation, develop only in an environment, from which free 

 OF Anaerobic oxygen' has been excluded. 



Bacteria. "pj^g exclusion of oxygen for piirposes of anaero- 



bic cultivation may be accomplished by a variety of 

 methods, depending upon a few simple principles which have been 

 applied, either separately or in combination, by many workers. 



The earliest methods depended upon the simple exclusion of air 'by 

 mechanical devices. In other methods, the oxygen of the air is displaced 

 by inert gases (hydrogen), and others again depend upon the oxygen- 

 absorbing qualities of alkaline solutions of pyrogallol. 



Cultivation by the Mechanical Exclusion of Air. — Koch succeeded in 



