282 



PRACTICAL DAIRY BACTERIOLOGY 



made in No. 12 will be a pure culture. By this term is meant 

 that it will contain only one kind of bacteria. If the colony on 

 the plate comes from a single bacterium, as has been assumed, 

 the colony of bacteria will contain a single kind of organism. 

 It frequently happens, however, that a single colony may come 

 from two bacteria, side by side, in which case it 

 may not be a pure culture. In order to be sure of 

 having a pure culture, it is always necessary to 

 purify it, as follows: 



a. Fill some test tubes with about 10 c.c. of 

 water, and sterilize one hour in an autoclav. These 

 are water blanks. 



b. After the cultures of No. 12 have grown from 

 one to two days, remove with a platinum needle a 

 small amount of the bacterial growth and transfer 

 it to a water blank. Shake thoroughly so as to 

 distribute the bacteria uniformly. Transfer with 

 a platinum loop, two loopfuls of this to a second 

 water blank, for the purpose of more highly diluting 

 the bacteria. Again after a thorough shaking, trans- 

 fer two loopfuls of this second tube to a tube of 

 melted gelatin. Shake gently, to distribute the drop 

 of water uniformly through the gelatin, and pour 

 out into a Petri dish. In about two days the plate 

 will be dotted with colonies. Pick out one with a 

 platinum needle, inoculate upon an agar slant, and 



almost invariably be pure. This purification 

 of cultures must be done in all subsequent work. 

 If it is not done, errors are sure to result from the mixture 

 of different kinds of bacteria together, and all the work will 

 be rendered worthless. 



*No. 14. Preservation of Cultures. After having once been 

 planted on the surface of agar slant, agar culture may be kept 

 alive, as long as the agar remains moist; and it may be studied 



FIG. 79 

 PLATINUM j t 



NEEDLES 



