METHODS OF STORING CULTURES 75 



3. The colour of the growth, and whether the growth itself or the medium 

 around it is pigmented. 



4. The smell of the culture. 



SECTION V. THE METHODS OF STORING CULTURES. 



When growth has ceased, the organisms retain their vitality for a certain 

 length of time varying according to the species from a few days to several 

 months and even years, but ultimately they die and sub-cultures sown from 

 them remain sterile. 



The weakening and ultimate disappearance of vitality are in a large measure 

 the result of the prolonged action of the oxygen of the atmosphere on organisms 

 in an old culture medium, and which are not actively multiplying ; to keep 

 organisms alive therefore it is necessary to sow them from time to time on 

 a new medium. But the same result is obtained by removing the organism, 

 once growth has finished, from the action of the air ; this may be done as 

 follows : 



1. Sow a broth culture, incubate it at the optimum temperature of growth 

 until no further development of the organism takes place (the time required 

 will obviously vary with different organisms). 



2. Take a Pasteur pipette, and make a constriction just below the wool 

 plug by heating it in the flame and drawing it out a little (a, fig. 67). 



FIG. 67. Method of sealing up a culture in a pipette. 



3. When the pipette has cooled, dip the narrow end with the usual pre- 

 cautions into the culture and suck up the broth until it reaches the 

 constriction a. 



4. Seal the pipette both at a and at the other end as quickly as possible 

 in the blow-pipe. 



In this way a small tube is obtained, which is filled with the growth and 

 sealed at both ends. This should be put away in the dark. 



[In many cases when the tubes or flasks containing the culture are plugged 

 with wool, it will be quite sufficient to pour melted paraffin wax over the wool 

 and the lips of the opening in order to preserve the organisms alive for an 

 indefinite period.] 



