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



would be 45° F. (7.2° C.). In other words, if the history of the milk 

 is not known it is impossible to interpret the colon count in terms of 

 the condition under which it was produced. This situation arises 

 because of the fact that organisms of the colon-aerogenes group grow 

 rapidly in milk when it is held above 50° F. (10° C). 



When the history of the milk is not known, it is believed that colon 

 counts above 2,000 per cubic centimeter indicate growth. This 

 belief is based on the fact that only one sample of fresh milk out of 

 the 254 samples which were produced under conditions ranging from 

 clean to extremely filthy contained more than 2,000 colon per cubic 

 centimeter. If, therefore, a sample of milk of unknown history is 

 examined and found to have a colon count between 100 and 2,000 

 per cubic centimeter, it is impossible to decide whether the colon 

 count indicates original contamination or whether this small number 

 of organisms was due to the growth of the few originally introduced 

 into a very clean milk. Colon counts above 2,000, however, under 

 normal conditions can be attributed to growth. In this connection 

 the colon count is significant so far as it indicates that the milk has at 

 some period in its history been held above 50° F. (10° C). 



There is one abnormal condition of production under which 

 fresh milk may have a high colon count. As previously pointed 

 out, in one series of experiments where utensils were allowed to 

 contain milk drainings for 24 hours during extremely hot weather, 

 and then washed carelessly in lukewarm water without the use of 

 brush or washing powder, the colon count of the fresh milk averaged 

 about 1,000,000 per cubic centimeter. An attempt was made to 

 duplicate these counts under similar conditions, but without success. 

 It is believed that this extreme condition of production would rarely 

 be encountered in the commercial production of milk. High colon 

 counts due to this condition can probably be readily distinguished 

 from similarly high colon counts which are due to growth, because 

 the ratio of total count to colon count would probably be less than 

 1 to 50 in the fresh milk, while in that due to growth the ratio would 

 be much higher. 



SUMMARY. 



1. The colon count as determined at the present time includes both 

 the B. coli and B. aerogenes types of organisms. Since the B. coli 

 type is principally of fecal and the B. aerogenes type of nonfecal 

 origin, the colon count at best can not be a direct measure of ma- 

 nurial contamination. 



2. Fresh milk produced under the best conditions always contains 

 some organisms of the colon-aerogenes group, but rarely contains 

 over 2,000 per cubic centimeter even when produced under the worst 

 conditions normally encountered. 



