CULTURAL TECHNIQUE 19 



Quantitative analysis of milk. The number of bacteria 

 present in any material is determined by making " plate cul- 

 tures " from it. A definite quantity is placed in a Petri dish, 

 and a tube of one of the liquefiable solid media is poured into 

 the Petri dish and intimately mixed with the milk. The 

 liquid medium spreads in a thin layer over the bottom of 

 the plate, which is placed on a level surface to cool, thus 

 allowing the medium to solidify. The bacterial cells dis- 

 tributed in the liquid medium are held in place when the 

 medium solidifies. The resulting growth thus appears as dis- 

 tinct spots, called " colonies," each of which is the progeny 

 of a single type of organism, although not necessarily of a 

 single cell. The counting of these colonies after a sufficient 

 time has elapsed for their growth will give the approximate 

 number of living bacterial cells present in the original sub- 

 stance at the time the plate cultures were made. This cul- 

 ture method, originally devised by Koch, has been of greatest 

 value in placing bacteriological study on a strictly scientific 

 foundation. 



It is impossible by any method yet devised to determine 

 the absolute number of bacteria present in any substance, 

 which, like milk, contains under natural conditions a variety 

 of forms differing in requirements for their growth. The col- 

 onies that appear on the plate cultures represent the organisms 

 that, under the given conditions of food, temperature, and air 

 supply, find favorable conditions for development. If another 

 food substance is used, if the reaction of the medium is varied 

 widely, or if the temperature is changed, a different class of 

 bacteria will be found on the plate cultures. Hence the plate 

 method of determining the number of bacteria in milk or any 

 substance is not an absolute one, and one which gives com- 

 parable results only under constant conditions. 



