34 A CONSIDERATION OF THE EVIDENCE [ch. hi 



process of sterilisation or they may deteriorate if kept for 

 some time before being used, and failure to guard against 

 these sources of error may lead to discordant results. 



The streptococcus pyogenes normally fails to ferment both 

 saccharose and raflBnose in broth, but it produces acidity in 

 old media containing either of these sugars (Martin, 1908-9), 

 Fisher (1909) on the other hand found that diphtheria and 

 diphtheroid bacilli which gave fermentation tests readily in 

 fresh beef serum, failed to do so if the serum were old. 



This observer and Theobald Smith (1899) both state that 

 even virulent diphtheria bacilli may fail to yield toxin if the 

 medium in which they are growing contains more than a 

 trace of sugar ; while Williams (1902) found many strains, 

 which were non-pathogenic when inoculated from ordinary 

 broth, were highly toxic when inoculated from serum culture 

 or ascitic broth. 



The neutral red reaction in the case of B. coli not 

 infrequently fails but Moore and Revis (1905) claim that if 

 lactose is substituted for glucose in the broth a positive result 

 is invariably obtained with this organism. 



Glenn (1911) observes that the acidity produced in a 

 medium by the fermentation of its carbohydrate constituents 

 inhibits the production of indol and may account for the 

 failure of the indol test. 



Wood (1889) has stated that the presence of glycerine in 

 a medium will prevent the liquefaction of gelatin by organisms, 

 not by interfering with their power of fermentation but by 

 offering them a pabulum they prefer. 



Again, if the medium used in the case of fermentation 

 tests is itself markedly alkaline, the production of acid in 

 small quantities may be completely masked, since it merely 

 results in a diminution in alkalinity and this requires special 

 means of detection. Miss Peckham (1897) quotes Timpe to 

 the effect that all albuminous bodies give an alkaline reaction 

 to litmus and it is well known that alkaline products are 

 formed by the breaking down of peptone, so that the use of 

 litmus as an indicator in peptone holding material makes the 

 alkaline reaction prominent even when a considerable quantity 



