ROUTINE TESTS FOR THE IDENTIFICATION OF BACTERIA 161 



medium with bromphenol blue. If it is decided to observe the production of alka- 

 linity as well as acidity, one may employ bromthymol blue or, better, a mixture of 

 bromcresol purple with cresol red; 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 15. The Sensitive Ranges op the Three Indicators Recommended 

 FOR Use in Indicator Media 



pH: 7.0 6.0 5.5 5.0 4.0 3.0 



Br. cres. purple: 

 Br. cres. green: 

 Br. phenol blue : 



purple! <— sensitive ranges [yellow 



blue] <— sensitive range-^| yellow 



blue|< sensitive range > [yellow 



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

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

 tration can be obtained by inspection, particularly when three tubes are 

 used, one with each of the three indicators recommended above. For 

 this purpose Table 15, showing the relation of the ranges of these three 

 indicators to one another, will be found useful. 



After some experience a bacteriologist can usually devise some method for recording 

 on the chart, by a system of numerals or + signs, the strength of reaction observed 

 with each indicator employed; such a system often proves practical for comparative 

 purposes but gives no very definite information as to final H-ion concentration. 



Gas production in liquid media can be measured in percentage of gas 

 in the closed arm of the Smith or the Durham fermentation tube. The 

 Durham tube, which is most commonly used, consists of a small test 

 tube (e.g., 75 by 10 mm) inverted in a large tube (e.g., 150 by 18 mm). 

 In the case of solid media it is recorded as present or absent according to 

 whether or not bubbles or cracks are present in the agar. This test is 

 especially valuable if the organism is tested in a shake culture, but the 

 presence of gas can usually be detected in an ordinary agar slant. These 

 tests for gas production are chiefly useful if the organism produces pri- 

 marily hydrogen; if the gas is all carbon dioxide, little or none will 

 accumulate in the fermentation tube because of the great solubility and 

 rapid diffusion into the air. 



, Interpretation of results. In case an organism produces gas or con- 

 siderable increase in acidity in either broth or beef-extract peptone agar 

 in the presence of some fermentable substance and this does not occur in 

 the basal medium without the addition of the fermentable substance, it 

 may safely be concluded that cleavage of this substance has occurred. 

 Very often for routine diagnostic purposes such information is enough. 

 To understand the true action of the organism on any carbon compound, 

 however, much more investigation must be made as explained elsewhere 

 (see Chap. VIII). This is particularly necessary in the case of organisms 



