254 MAX LEVINE 



between these characters recognized the possible existence of 256 

 varieties of B. coli. 



The grouping of Jackson (1911) which was accepted by the 

 American Pubhc Health Association and included in the standard 

 methods for 1912, is very similar to that of MacConkey, but here 

 preference is given to dulcitol over sucrose for the primary divi- 

 sion. Each of the 4 groups thus formed is subdivided further 

 on raffinose and mannitol, and then on motility, indol, reduction 

 of nitrates, and gelatin liquefaction. 



A very serious objection to the classifications of MacConkey, 

 Bergey and Deehan, and Jackson, is their extreme flexibility. 

 As the number of fermentable substances, or other characters 

 observed, increases, the number of ''varieties" increases geomet- 

 rically approaching infinity. The number of "varieties" is 

 given by the formula 2" where 'n' is the number of characters 

 studied. Thus with 8 characters there are 256 possible combi- 

 nations; this number rises to 1024 with 10 characters and to 

 65,536 when 16 characters are observed. The absurdity of 

 regarding each character as of similar and equal differential 

 value is thus evident. In the more recent studies the principle 

 of the correlation of characters has been emphasized. 



Howe (1912), from a statistical study of 630 strains of B. coli 

 isolated from human feces, concludes that dulcitol, indol pro- 

 duction, nitrate reduction, etc., are not correlated with each other 

 nor with vigor of growth, and he therefore recognizes only the 

 sucrose positive B. communior and sucrose negative B. communis. 



Rogers and his associates, (1914-1916) studied a large number 

 of coli-like forms from milk, grains, and bovine feces, and con- 

 clude that two distinct groups may be recognized on the basis 

 of the accurately determined gas ratio — the low ratio B. com- 

 munis-B, communior group and the high ratio B. aerogenes-B. 

 acidi-lactici group. There is no doubt that B. communis and B. 

 communior are low ratio strains and i^. aerogenes of the high 

 ratio group but the inclusion of B. acidi-lactici with B. aerogenes 

 does not seem justified, and I believe that further studies will 

 place it definitely with the low ratio strains. 



