SIGNIFICANCE OF COLON' COUNT IN MILK. 27 
ARE BOTH THE B. COLI AND B. AEROGENES TYPES PRESENT IN MILK; 
AND IF SO, IS THEIR RELATIVE PROPORTION OF ANY VALUE ? 
Since Rogers and his associates (14, 15) have shown that organisms 
of the colon-aerogenes group found in bovine feces are principally 
of the Bacillus coli type while those occurring on grains correspond 
to the B. aerogenes type, it seems as if a differentiation of these types 
in milk might be of value in determining the source of contamina- 
tion. Raw milk ordinarily contains both the B. coli and the B. 
aerogenes type. Rogers and his associates found that nearly 50 per 
cent of the cultures isolated from milk were of the B. aerogenes type. 
In our experiments an effort has been made to determine the 
presence of the two types and their proportion to each other in milk 
produced under different conditions of cleanliness and held for 24 
and 48 hours at 60° F. (15.6° C), 70° F. (21.1° C), and 86° F. (30° C). 
The B. coli and B. aerogenes types were divided on the basis of the 
methyl-red test proposed by Clark and Lubs (3). The results of the 
work as shown in Table 15 give the total and colon counts, the 
number of B. coli and B. aerogenes types in fresh milk, and the colon 
count and the percentage of the two types in the same milk held at 
different temperatures. 
Table 15 shows that, as a rule, the colon count of fresh milk com- 
prised only organisms of the B. coli type. This statement is ba.sed 
on the results obtained when 1/10 and 1/100 of a cubic centimeter 
of milk were plated. In a few cases the colonies from the fresh milk 
were of the B. aerogenes type. After the milk was held for a time and 
further examined both types of organisms were found in most cases. 
It must be remembered that even when the figures indicate that all 
the organisms of the colon-aerogenes group were of one type it 
does not mean that none of the other type were present. This is 
true because the method of differentiation was the picking off as 
near as possible of all the colonies of the group which appeared on the 
plates, inoculating them in a dextrose-peptone broth as described by 
Clark and Lubs (3), and determining their reaction by methyl red. 
It is evident, therefore, that the dilution in plating might have been 
so high as to exclude entirely the type that was in the minority. 
From the results obtained it seems safe to conclude that both 
types are usually present in raw milk. Since milk usually contains 
both types, it might be assumed that the proportion of B. coli to 
B. aerogenes would be of value in tracing the conditions under which 
the milk was produced. 
