ACID FUCHSINE. 275 



stage from the beginning ; it alkalinises the medium so that at the 

 end of twenty-four hours the pink bouillon has lost almost all its 

 colour. 



4. — The Bacillus typhosus shows the acid production of the 

 initial stage later than any of the others ; it occurs after ten or 

 twelve hours, and remains acid for a long period, varying from 

 seven to ten days, and even longer ; then the acidity gradually 

 disappears ; usually, by the twelfth day, the decolourisation of the 

 pink medium is accomplished. 



The changes of reaction of the cultures are shown by the 

 colour assumed by the medium, and there will be a sharp distinction 

 between the Bacillus typhosus and the other bacteria mentioned, 

 so great that it is easy to distinguish it from the others. Especially 

 is this well marked in fresh cultures. If the culture has been kept 

 on laboratory media for a long time, this change is not so pro- 

 nounced as was demonstrated in one of the specimens of the 

 typhoid bacillus, which had been kept under prolonged cultivation. 

 There was a marked diminution in its acid-producing power, 

 making the difference between its culture and those of the Bacillus 

 colt communis very slight. It was found, on further study, that 

 this property was influenced by the character of the media, espe- 

 cially in the composition of the beef tea, which is by no means 

 constant. Accordingly, other means were brought into use, 

 bearing always in mmd that such would always be of constant 

 composition, and in which the differences of reaction change 

 between the Bacillus typhosus and the other intestinal micro-organ- 

 isms should be the same, notwithstanding the different ages and 

 sources of the cultures used. This medium is the peptone solu- 

 tion prepared according to Dunham's formula with the addition of 

 glycerin and acid fuchsine. The addition of the glycerin is 

 essential to bring about the re-action, as no marked change is 

 observed in simple Dunham's solution. The following results 

 were obtained with this medium : — 



I. — During the first forty-eight hours Xhe Bacillus actdi lacttci 

 and the Bacillus lactis aerogenes produce a strong acid reaction, 

 especially so with the former; but the Bacillus xoli commtmis, the 

 Bacillus proteus vulgaris and the Bacillus typhosus do not produce 

 any marked change. 



