434 H. J. CONN AND G. J. HUCKER 



solution of brom cresol purple can be made up and used alone 

 at the rate of 1 cc. to the liter of medium or when combined with 

 cresol red at the rate of 0.5 cc. to the liter. 



Inoculate both on the surface and in a stab at the base of the 

 slant. The tubes should be watched day by day until the char- 

 acteristic changes are complete. Changes in reaction can readily 

 be detected by the color; and gas production can be observed by 

 means of bubbles or cracks in the medium if the agar in the 

 tube is sufficiently deep. 



With the organisms studied by the writers, this method has 

 had the advantage of showing acid production after a very brief 

 incubation, sometimes in a few hours only. The acid tends to 

 be localized in one spot at first and can be recognized even though 

 it could not be detected if distributed through the entire tube of 

 medium; hence the method is very delicate. Some organisms 

 are characterized by acid production first at the top of the tube; 

 others show the effect first at the bottom. Sometimes the zone 

 of acid is formed at the top and travels slowly to the bottom, 

 followed by a zone of neutral or alkaline reaction. Such changes 

 are generally constant with a given culture. 



One of the greatest sources of error in the ordinary test for 

 acid production is the simultaneous production of acid from the 

 sugar and of alkalinity from the peptone. When both of these 

 activities occur in a tube of agar, however, the acid production 

 generally predominates over the production of alkalinity during 

 the early stages of growth, or if not, the two activities are likely 

 to be localized in separate zones of the agar. For this reason, 

 the agar slant method is a great help in overcoming this source 

 of error. 



Bronfenbrenner and Schlesinger (1918) recently recommended 

 a method of using indicator agar for detecting acid production, 

 by placing a series of drops in a single petri dish. Their method 

 has the advantage of reducing the amount of glassware used, 

 but it does not show gas production nor the characteristic locali- 

 zation of acid and alkali brought out by the agar slant. 



