66 VARIATIONS IN FERMENTING POWER [CH. v 



1. In the first place, there is the question of the time 

 allowance to be made. When a particular organism only 

 produces acidity in a certain sugar at the end of an incubation 

 period lasting several days, one is in doubt whether to regard 

 the organism in question as a slow fermenter of that sugar or 

 as a non-fermenter of it which has acquired a new character 

 as the result of "training." 



2. Secondly, many conditions, as we have shown, modify 

 the normal sugar reactions and may lead to erroneous con- 

 clusions. A strain of bacteria may ferment a sugar at a 

 temperature of 22 C. and fail to do so at 37 C. ; an old 

 culture may ferment substances which a young culture is 

 unable to do, and so on. 



3. In the third place, the composition of the medium may 

 be responsible for conflicting results. It is almost impossible 

 to obtain many of the carbohydrates in a pure form, and yet 

 these are used and conclusions are based on the reactions they 

 give. Others can be obtained pure but are then too costly 

 for general use and the commercial "sugar" is substituted. 

 Different specimens of the same carbohydrate, even when 

 reasonably pure, msty give different results. This is the case 

 with the starch inulin, the fermentation of which is an import- 

 ant distinction between the pneumococcus and other members 

 of the streptococcus group. The process of sterilisation is a 

 difficult one. If subjected to too high a temperature, particu- 

 larly in the presence of alkaline material, the sugar may 

 undergo a change in composition. If the temperature is not 

 raised sufficiently sterilisation may be incomplete. If the 

 vessels holding the medium are not made of the best Jena glass, 

 there is a danger of the glass yielding a considerable amount 

 of alkali to the medium during sterilisation (W. B. M. Martin, 

 1911). If the various media are kept for any length of time 

 before use, the carbohydrates may deteriorate and lead to 

 apparently abnormal sugar reactions. Examples of this have 

 already been given. Finally, if the medium is very alkaline in 

 the first instance, or if it is rendered so by the decomposition 

 of the peptone present, the acid reaction may be masked. 



4. Even if the composition of the medium is beyond re- 



