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 t o 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-asparagin 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- 
asparagin 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 hi general 
it gives a more accurate colon count than the Endo medium. Avers 
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. 
