METHODS OF PURE CULTURE STUDY 523 



Interpreting results the two media should be taken into ac- 

 count as some organisms produce more easily detectable acid 

 on the peptone media, some on the peptone free media. In the 

 fonner case it may be assumed that the acid produced by the 

 organisms is obscured either by the buffer of the peptone or by 

 the alkalinity produced from it. 



If it is decided to observe the production of alkalinity as well 

 as acidity, it is recommended that two indicators be used, e.g., 

 brom crcsol purple with cresol red (see 1920 report, p. 130). 

 The use of these two indicators is decidedly recommended be- 

 cause in many cases it is instructive to observe the production 

 of alkalinity. Some organisms produce acid in one part of the 

 media and alkaU in another; when this occurs it is to be noted. 

 Denote alkah with a — sign. 



In using solid media, the production of gas can be quite readily 

 detected by the presence of bubbles and cracks in the agar. It 

 would seem theoretically that this is a less accurate way of deter- 

 mining gas production than the fermentation tube ; but wherever 

 the two methods have been compared in the case of organisms 

 growing well or better on solid media, agar slants have been 

 found to give as rehable results as fermentation tubes. 



When Uquid media are to be inoculated, prepare two media 

 similar in composition to the above mentioned but without agar, 

 sterilize in fermentation tubes and incubate at 37° or 25° accord- 

 ing to the optimum temperature of the organism in question. 

 The media may contain the same indicator or indicators as those 

 already mentioned in the case of solid media, in which case the 

 cultures should be examined at intervals, as often as seems 

 necessary to record changes in the reaction. Although there 

 is ordinarily no appreciable influence of the indicator on the 

 growth of the organism still there may be cases when the in- 

 vestigator prefers using media without an indicator; in this case 

 the culture should be tested ordinarily on the first, third and 

 seventh days, although the best days of testing will depend 

 on the rapidity of growth of the organism. To test for acid, 

 use an indicator having a range which covers the reaction of 

 the culture. If possible compare the culture with a standard 

 buffer solution. 



