138 



EXCREMENTAL ORGANISMS 



^ 



animal faeces and would seem to differentiate between animal 

 and human pollution, but as the difference is one of degree only 

 and is not specific, no definite significance can be attached to it. 

 Although a large amount of work has been done on the separa- 

 tion of the colon group of organisms, no test or combination of 

 tests has been evolved that would indicate that any one sub- 

 group is more typical than another, and it must, therefore, be 

 borne in mind that to designate any organism as being typical B. 

 coli because it possesses certain biochemical and morphological 

 characteristics is a purely arbitrary and empirical procedure. 

 Moreover, these organisms are not to be regarded as having 

 immutable properties hke chemical compounds, but to form 

 involution and mutation varieties according to the environment. 

 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 c.cm. quantities 

 of byre milk. Even after bottling and delivery to the pur- 

 chaser milk can be produced that will average less than two 

 B. coli per cubic centimetre, even during the summer months. 

 This is exemplified in Table LVII. 



Table LVII 

 BACTERIA AND B. COLI IN CERTIFIED MILK (Author) 



Month. 



Mean Bacterial 

 Count per c.cm. 



Mean B. Coli 

 per c.cm. 



May 



June 



July 



August. . . 

 September, 



5,700 

 10,900 

 5,.000 

 4,500 

 5,500 



1 



2 



0.1 



0.8 



1.4 



When milk is kept at a temperature not exceeding 45° F. 

 the B. coli do not increase {vide p. 104) and this temperature 

 may, therefore, be regarded as the critical anaboHc tempera- 

 ture. Above this point they multiply rapidly and in summer 



