260 BACTERIA IN MILK AND THEIR SOURCES 



Catch a fly in the cowshed, place it under the cover of the 

 Petri dish and allow it to run over the surface of the gelatine 

 or agar medium for one minute : then let it escape. 



Incubate the plate at 20 C. and 37 C. respectively. 



Count the number and note the kind of colonies which grow. 



Try the experiment with flies caught in other places. 



Ex. 131. Place a cow's hair on the surface of a sterile lactose 

 gelatine or agar plate. 



Incubate for forty -eight hours, and note the growths which take 

 place where the hair touches the medium. 



2. Influence of Temperature and Time upon the 

 numbers of Bacteria in Milk. Although milk is an 

 excellent medium for the nutrition of bacteria, their 

 development in it is dependent upon suitable tempera- 

 tures. Park found that when kept at freezing point 

 (o C.) for four to seven days the bacteria in it decreased 

 slightly in number, and even at temperatures up to 

 6 C. growth was checked and little or no increase 

 occurred. At higher temperatures, up to the optimum 

 of 25 C. (77 F.), at which division goes on most 

 rapidly, bacteria increase at an enormous rate, and milk 

 in which only a small number are found initially may 

 contain many millions in a few hours if kept in a warm 

 place. This becomes obvious when it is remembered 

 that a single bacterium which divides once every half hour 

 can produce in twelve hours 16,777,216 offspring, pro- 

 vided that the temperature is suitable and the amount 

 and nature of the medium will allow of such increase. 



The following figures indicate the effect of time and 

 temperature on the bacterial content of milk ; the 

 samples taken at the farm contained 22,000 bacteria, 

 per c.c. 



