304 



H. R. BAKER 



which were much higher than need to be used in actual acid 

 determinations. 



Bad. paratyphosum A, Bact. dysenteriae, Bact. sanguinarium, 

 Bact. pullorum, and Bact. enteritidis were also tested out in the 

 same dilutions shown in the tables. Results were obtained 

 closely approximating those for Bact. typhosum. 



It has been found by experience that a 1:41,666 solution of 

 brom-thymol-blue gives the most desirable concentration for 

 colorimetric comparison. The data in the tables show that this 

 concentration can be used in the media without inhibiting the 



TABLE 4 



The influence of brom-thymol-blue upon acid production by Bacillus subtilis, using 

 media and dilutions as in table 1 



production of acid. This dilution is easily made by adding 12 cc. 

 of a 0.2 per cent alcohoUc solution of the indicator to every Uter 

 of sugar free broth before it is put into the fermentation tubes. 



Media prepared in this way are now used in this laboratory by 

 the students, and have been found successful for all their quaUta- 

 tive acid production tests. 



It has also been used by a member of the department in deter- 

 mining acid production by a certain bacterium, upon about 

 twenty carbohydrates. In this particular instance, readings of 

 the reaction of the cultures were made every day over a period 

 of four weeks of incubation. 



