124 REPORT OF COMMITTEE 



cresol purple is used instead of brom thymol blue to show "neu- 

 trality" and that the curdling point (Ph =4.7) is used to separate 

 between "moderate" and "strong" acidity instead of the less 

 definite point of maximum red to methyl red. The same meth- 

 ods of expression used in recording acidity in clear media should 

 be used in recording that of milk. 



Litmus milk often gives valuable information, showing not 

 only the production of acid, but also decolorization of the litmus 

 by organisms that are able to reduce it. jNIore accurate results 

 as to acidity can be obtained by using brom cresol purple, as 

 shown by Clark and Lubs (1917b). This indicator, however, 

 does not show the reduction phenomena which are sometimes 

 of diagnostic value in litmus milk cultures; so its substitution 

 for litmus is not always to be recommended. 



Reduction of nitrates. For routine work, nitrate broth should 

 have the composition given on p. 116. Always use this broth 

 first unless the organism is known to grow poorly in it. If an 

 organism does not give good growth in this medium (as indi- 

 cated by distinct cloudiness and precipitate), a negative nitrite 

 test in this medium is meaningless. In such a case record the 

 results of the test as doubtful, putting a question mark in the 

 group- number at the point which indicates action on nitrates ; or 

 else make the test in some nitrate-containing, nitrite-free me- 

 dium in which the organism does produce good growth. For 

 instance, if (like B. communis and most pathogens) it requires 

 considerable organic matter, use 2 or even 5 grams of peptone 

 per litre; or if it grows poorly in the presence of organic matter, 

 use some nitrite-free synthetic medium in which it is known to 

 grow. The tubes should be inoculated in triplicate and incu- 

 bated at 37° for organisms of series I and III, at 25° for organ- 

 isms of series II and IV. 



To test for nitrite the following reagents are necessary: (1) 

 dissolve 8 grams sulphanilic acid in 1 litre of 5N acetic acid 

 (1 part glacial acetic acid to 2.5 parts of water) or in 1 litre of 

 dilute sulphuric acid (1 part concentrated acid to 20 parts water) ; 

 (2) dissolve 5 grams o-naphthylamine in 1 litre of 5N acetic 

 acid or of very dilute sulphuric acid (1 part concentrated acid 



