70 VARIATIONS IN FERMENTING POWER [CH. v 



THE VALUE OF VARIATIONS IN THE SUGAR REACTIONS 

 IN THE IDENTIFICATION OF BACTERIA. 



There are ways in which the actual variations in the sugar 

 reactions may be of value in identifying an organism. 



In the first place, the variations observed may themselves 

 be specific in character and so far from obscuring the identity 

 of an organism may in some cases actually contribute to its 

 recognition, in the same way that the morphological variations 

 of B. diphtheriae help to establish its identity. 



In the second place the variations may help to identify 

 the source of the organism by indicating the nature of its 

 recent environment. Thus, if "passage" is found to modify 

 the fermenting power of a particular species of bacteria in 

 a particular direction, then such modification when found to 

 exist in a member of that species may indicate a recent animal 

 host. Again, if growth in a certain material is known to lead to 

 certain modified "sugar reactions" on the part of a particular 

 species, then such modification, when it is found, may furnish 

 a clue to the source of the organism in question. For example, 

 it has been pointed out by several observers (Re vis, 1908) that 

 the saccharose-fermenting type of the colon bacillus is more 

 often isolated from milk than from cow-dung and MacConkey 

 has stated (quoted, ibid.) that the more prolonged the sojourn 

 in the former medium is, the more prolonged is the power to 

 ferment saccharose on the part of the organism. Revis also 

 describes a strain of bacteria which failed to ferment saccharose 

 when isolated from cow-dung, but did so after being cultured 

 in milk. Again, strains of B. coli exhibiting variation in gas 

 production are more commonly of urinary than of intestinal 

 origin. 



This question will be further considered in a later chapter 

 (vide p. 144). 



