110 TEXT-BOOK OF BACTERIOLOGY. 



is, therefore, peculiarly suited for needle-stroke cultures on an 

 oblique surface; in fact, its chief use is for pure cultures of this de- 

 scription. The point of the infected platinum needle is drawn 

 slowly across the agar without penetrating- it, and then, within 

 twenty-four hours, we can have perfectly-developed cultures of the 

 parasitical micro-organisms, whose development, as already men- 

 tioned, may be accelerated by the aid of increased heat. 



The pure culture in the test-tube does riot, of course, last for an 

 unlimited time. The nutritive substances are consumed, and the 

 bacteria themselves excrete substances which act as a check to 

 their indefinite multiplication. 



This takes place more quickly with some, more slowly with 

 others. As a general rule, we may say that the cultures retain 

 vitality for three or four months, yet there are some which perish 

 much sooner, and it is always well to renew them about once in six 

 weeks i.e., to transplant them to a fresh food medium. 



VIII. CULTURE OF ANAEROBIC BACTERIA, INCUBATORS, 

 THERMO-REGULATORS, AND SAFETY BURNERS. 



It is necessary to know the ways and means by which we are 

 enabled to separate a mixture of bacteria into its component spe- 

 cies, and to breed them in pure cultures by the aid of transparent 

 solid food media. In this manner success will be attained with a 

 great number of micro-organisms, and there are only a few that 

 form exceptions, inasmuch as they require special appliances to in- 

 sure the success of their artificial cultivation. We refer to those 

 which only thrive in the complete absence of oxygen the anaerobia, 

 for whose culture, therefore, separate methods have had to be in- 

 vented. 



It may be readily imagined that the problem is a difficult one. 

 It is not an easy task to shut off a substance which is present in all 

 that surrounds us; we cannot succeed without special manipula- 

 tions, and often we shall require special instruments. We may, 

 therefore, at once say farewell to that laudable simplicity which is, 

 perhaps, the greatest merit of the usual process of culture. 



It is true that the difficulties are not great as long as we have 

 to deal with liquid media, but they increase from the moment we 

 attempt to employ the solid media. In particular, the separation 

 of mixtures of bacteria occasions much difficulty, and the great 

 number of means and ways that have been and are still continu- 

 ally suggested is in itself proof that as yet nothing has been 

 found to answer all the requirements of the case. Any one method 



