ESTIMATION OF THE BACTERIAL CONTENT OF MILK 603 



The numbers of bacteria in milk are determined in practice by two 

 distinct methods: 



(a) The numbers of organisms which will grow upon ordinary 

 laboratory media, as nutrient agar (cultural count), and: 



(6) By direct microscopic count. 



(a) Cultural Count. Method: 1 c.c. of a well-mixed sample 

 of milk is diluted J^Q, 1^5, joSoo or even lOTo with sterile water, 

 depending upon the grade of the sample, and plated on nutrient 

 agar. The number of colonies which develop after forty-eight hours' 

 incubation at 37 C. multiplied by the dilution is taken as the bac- 

 terial count of the milk. It is customary in some laboratories to make 

 a parallel count at 20 C., after four days' incubation. The numbers 

 of colonies developing on agar at the lower temperature may be much 

 greater than those incubated at body temperature. The difference 

 between the counts is usually more marked in samples of milk which 

 have been maintained for some time at a relatively low temperature, 

 and in ice-cream. In such cases bacteria whose minimal temperature 

 of growth is relatively low 4 to 12 C. may multiply with con- 

 siderable rapidity. These organisms frequently fail to develop at 

 37 C. 



The cultural count possesses advantages and disadvantages. The 

 principal advantages are: the simplicity of the method, comparative 

 accuracy of results provided uniform conditions are maintained, and 

 some differentiation of the types of organisms present in the milk. 

 The disadvantages are: the time required to obtain results milk 

 is perishable and cannot be held pending examination by this proce- 

 dure. Furthermore, by no means all the bacteria which may theoreti- 

 cally gain access to the milk will grow upon plain agar; this is par- 

 ticularly true of pathogenic microorganisms. Bacteria which remain 

 adherent in groups or chains are frequently not separated during the 

 shaking of the sample and a single colony may originate from such 

 a clump or chain. This naturally introduces an error which may be 

 very considerable if, for example, a long chain of streptococci develops 

 as a single colony. 



(b) Direct microscopic count. Milk hygienists have long recognized 

 the advantages of a direct estimation of the bacterial count of milk 

 and numerous methods have been proposed, from time to time, to 

 accomplish this object. The most practical method thus far prescribed 

 appears to be that of Prescott and Breed. 1 The theory involved is to 



1 Centralb. f. Bakt., 1911, 1, 246. 



