10 BULLETIN 739, U. S. DEPARTMENT OF AGRICULTURE. 



Investigators who have studied the question do not believe that 

 milk can be produced even under the best conditions without con- 

 tamination by organisms of the colon-aerogenes group. Prescott 

 (12) states: 



We may take certified milk to be a milk product which is as free from filth as it can 

 be obtained under practical conditions, and when I say practical conditions I mean 

 by using extreme care. Even milk which is obtained with ultra refinement and 

 which may have a very low bacterial count is almost certain to contain some colon 

 bacilli. 



According to the author two-thirds of 200 samples of certified milk 

 examined showed colon bacilli in 1 cubic centimeter, and of the re- 

 maining one- third fully one-half showed them to be present in 5 cubic 

 centimeters or less. Race (13) has the same opinion on the subject, 

 for he says : 



Milk even when produced under the best conditions is never quite free from B. coli, 

 but if reasonable precautions are taken this group should not be present in 25 cubic 

 centimeter quantities of byre milk. 



The results obtained in our work lead us to the same opinion, 

 namely, that milk can not be produced commercially, even under 

 the best of conditions, without contamination by organisms of the 

 colon-aerogenes group. Throughout the work, unless otherwise 

 stated, the colon count has been determined by plating methods, using 

 either litmus-lactose-asp ar agin agar, the composition of which has 

 been previously given, or Endo's medium prepared according to the 

 method of Kinyoun and Dieter (11). The value of litmus-lactose- 

 asp aragin agar as a selective medium for use in the determination of 

 organisms of the colon-aerogenes group in milk can not be too highly 

 emphasized. Sometimes the count on this medium is higher than 

 on Endo's and sometimes lower, but it is believed that in general 

 it gives a more accurate colon count than the Endo medium. Ayers 

 and Johnson (2) called attention to the value of the asparagin medium, 

 and the results obtained from its use in the present investigations 

 again confirm its value. As mentioned, however, both media have 

 been used and the highest counts recorded so as to give the highest 

 colon count. 



In order to determine whether milk could be produced without 

 contamination by organisms of the colon-aerogenes group, 16 samples 

 of milk were examined which was produced in a clean barn, from 

 clean cows, and handled in sterilized utensils. The milk had a low 

 total count, ranging from 1,130 to 8,900 per cubic centimeter. As 

 may be seen in Table 5, none of the samples contained any organ- 

 isms of the colon-aerogenes group in 1/10 cubic centimeter amounts. 

 However, when these samples were held for 24 hours at 70° F. 

 (21.1° C.) the colon count ranged from 900 to 303,000 per cubic 

 centimeter. Since these organisms were present after 24 hours' 

 incubation, some must have been in the fresh milk. 



