466 LEON S. MEDALIA 



Procedure 



In order to test the acid production of any given organism it is 

 only necessary to cultivate the organism in 10 cc. amounts of 

 fluid culture media of a known pH, that will best favor its growth. 

 To each of the tubes of culture media is added 0.8 cc. of a 0.02 

 per cent solution of indicator — either phenol-blue or brom- 

 cresol-purple — before sterilization, or if methyl-red has to be 

 used the 0.8 cc. (of 0.02 per cent watery solution) of this indicator 

 has to be added just before the reading is done, i.e., after sterili- 

 zation, inoculation, and incubation. The cultures are then in- 

 oculated and incubated for as long a time as desired. Control 

 cultures without indicator are also made and incubated alongside 

 of those containing the indicator. The control cultures are used 

 in the comparator block to offset the color and turbidity. The 

 actual reading of the pH is done by placing the culture containing 

 the indicator say brom-cresol-purple in the front of the center 

 row of the comparator block, having two tubes of distilled water 

 behind it while on either side of it are placed the pairs of "stand- 

 ard color" tubes of brom-cresol-purple approaching in color that 

 of the culture with the same indicator; in front of each pair of 

 standard colors in the comparator block one of the control cul- 

 tures is placed to offset the color and turbidity of the medium. 

 The reading is, then, simply a matter of matching colors, and 

 takes very little time. Thus the actual acid production by any 

 given organism can be easily tested from hour to hour, if desired, 

 without disturbing the culture in any way, except in the case 

 of methyl-red where fresh cultures are necessary for each reading 

 — because of the fact that it is unstable and must be added fresh 

 each time a reading is made. 



Testing for alkali production by bacteria 



The procedure for testing the alkali production by bacteria 

 could be carried out in a similar way to that just described for 

 the acid production by Bad. coli. The only difference is that 

 the proper indicators (phenol-red; brom cresol-red; and thymol- 

 blue-alkaline range) must be substituted for those just described; 



