2C)6 PRACTICAL DAIRY BACTERIOLOGY 



The question of the purity of the commercial cultures is of 

 the greatest importance. Inasmuch as they consist of living 

 organisms it is absolutely necessary that the cultures should 

 contain the organisms desired and no others. It would be of 

 no great value to add to a lot of cream a mass of bacteria, un- 

 less it contained the desired organisms and no others. During 

 the first years that such cultures were put on the market they 

 were very apt to be impure, and even at the present time they 

 are not absolutely reliable. It is an extremely difficult matter 

 to produce bacteria in quantity without their becoming con- 

 taminated with undesirable species; and in the commercial 

 cultures it occasionally happens that unexpected and undesired 

 species of bacteria or molds find their way into the cultures, 

 and injure or ruin them. It also sometimes happens that the 

 culture, though pure, may lose its vigor, for bacteria retain 

 their greatest vigor only under the most favorable conditions. 

 As better methods of cultivating bacteria are learned, the diffi- 

 culties of impure cultures are disappearing, and the cultures are 

 becoming more and more reliable each year. At the present 

 time the commercial cultures can be fairly well depended upon. 

 But even to-day the cultures put out by one of the most reliable 

 firms occasionally contain species that are not wanted. Absolute 

 purity and uniformity of the cultures have not yet been reached. 



In preparing a starter from a commercial culture the dairy- 

 man must remember that he is dealing with living organisms 

 and must handle it accordingly. It must not be heated, for this 

 would kill the living organisms. It must be used, if possible, 

 while fresh, for the bacteria in the culture begin some days after 

 to lose their vigor, and eventually die. An old commercial cul- 

 ture, therefore, is useless. The culture must be treated in such 

 a way as to give the bacteria the greatest chance for developing, 

 since the growth of these organisms produces a starter. The 

 cultures are usually sealed when they leave the bacteriologist's 

 laboratory. Care should be taken, if a commercial article be 



