4 BULLETIN 739, U. S. DEPARTMENT OF AGRICULTURE. 
tioned have shown that practically all the organisms found in this 
group are of the B. coli type. Among 150 cultures isolated from 
16 samples of bovine feces they found only one culture of B. 
aerogenes. Even special methods used to demonstrate the pres- 
ence of the B. aerogenes type in bovine feces failed to reveal their 
presence. 
Our results in a general way confirm those of Kogers and his asso- 
ciates, for among 1,160 cultures isolated from 20 samples of fresh 
cow feces only 4 cultures of the B. aerogenes type were found. By 
means of special enrichment methods, however, it was often possible 
to isolate B. aerogenes types from 1/100 gram of fresh feces. The 
most satisfactory method of enrichment was to inoculate varying 
small amounts of cow feces into sterile skim milk. It seems evident, 
therefore, that the colon-aerogenes group found in cow feces consists 
mainly of the B. coli type. The B. aerogenes type is probably always 
present, but in very small numbers. 
In view of the facts just presented, the contamination of milk by 
the colon-aerogenes group through cow feces would introduce prin- 
cipally organisms of the B. coli type. Occasionally the examination 
of fresh milk produced under clean conditions with sterilized utensils 
shows the presence of the colon-aerogenes group in small numbers 
which are mostly of the B. aerogenes type. Milk produced under these 
conditions would probably have an exceedingly small amount of fecal 
contamination, and taking into account the very small number of the 
B. aerogenes type in fresh cow feces compared to the number of the 
B. coli type, it is evident that the B. aerogenes found in the milk did 
not represent fecal contamination. 
The B. aerogenes type being found at times in small numbers in 
low-count fresh milk led to the suspicion that the proportion of the 
B. coli and B. aerogenes types in cow feces might change as the feces 
aged and dried. Should this change occur it might explain the pre- 
ponderance of the B. aerogenes type sometimes found in such milk. 
This seemed a very logical possibility because fecal material entering 
milk is probably not fresh but more or less dry, and gains entrance to 
the milk largely by dropping from the body of the cow. In order to 
determine whether the proportion of B. coli to B. aerogenes types 
changed in feces as it dried, a composite sample consisting of equal 
amounts of fresh feces from 4 cows was placed in a sterilized dish and 
allowed to dry at 86° F. (30° C.) to a practically constant moisture 
content, which after 14 days was 5 per cent. At regular intervals 
1/100 of a gram of the material was inoculated into sterile milk. Two 
flasks of milk were inoculated with equal amounts of feces, one was 
incubated for 24 hours at 70° F. (21.1° C.) and the other for the same 
length of time at 86° F. (30° C). After incubation, samples of milk 
