ROUTINE TESTS FOR THE DESCRIPTIVE CHART V49-I7 



These chemical methods are referred to in more detail elsewhere 

 (Leaflet VI). In many instances, however, a sufficient amount of in- 

 formation is obtained merely by demonstrating an increase in acid or 

 the presence of gas. 



For routine work in the case of organisms concerning which little 

 advance information is at hand, the use of indicators is especially 

 valuable in determining whether or not production of acid has oc- 

 curred. It must be remembered, however, that in many instances 

 more useful and significant information can be obtained by means of 

 titration. (See Leaflet VI.) 



When the indicator method is employed, the indicators may be 

 incorporated with the media in the first place or may be added subse- 

 quently when the final reaction is being determined. If they are 

 added when determining final reaction, the color obtained should be 

 compared with color standards (see Leaflet IX) in order to secure 

 accuracy. The use of indicator media is less accurate, but is a much 

 more rapid procedure; when the cultures are growing on agar, more- 

 over, it is the only satisfactory procedure. 



When using indicator media, make them up according to the directions given on 

 pp. 1I44-7, 8, of Leaflet II. The indicator most commonly added is brom cresol 

 purple; but with organisms producing considerable acid, brom cresol green or even 

 brom phenol blue may be employed. When studying a series of unknown organisms 

 it is often advisable to inoculate all onto the prescribed sugar medium with brom cresol 

 purple; later those that show acid may be reinoculated onto the same medium with 

 brom cresol green; and subsequently those positive to this indicator upon the same 

 medium with brom phenol blue. If it is decided to observe the production of alkalinity 

 as well as acidity, one may employ brom thymol blue or better a mixture of brom cresol 

 purple with cresol red, making up the medium as directed on p. 1I44-8 of Leaflet II; in 

 a solid medium this practice is often of value as it may show the production of acid 

 in one part of the tube, and of alkalinity in another. 



TABLE 1 



THE SENSITIVE IL\NGES OF THE THREE INDICATORS RECOMMENDED FOR USE IN 



INDICATOR MEDIA 



i)H: 7.0 6.0 5.5 5.0 4.0 3.0 



Br. Cres. Purple: 

 Br Cres. Green: 

 Br. Phenol Hhie: 



Purple] ^^Sensitive range->-| Yellow 



• • Blue! -^Sensitive range-^^ I Yellow 



Bluel -^-Sensitive range^^l Yellow 



With indicator media it is difficult to learn the exact reaction by 

 reference to color standards, but a good estimate as to hydrogen-ion 

 concentration can be obtained by inspection, particularly when three 

 tubes are used, one with each of the three indicators recommended 

 above. For this purpose Table 1, showing the relation of the ranges 

 of these three indicators to each other, will be found useful. 



