ROUTINE TESTS P^OR THE DESCRIPTIVE CHART v,,,-19 



strable acid or lijas. In routine work, accordini^ly, one should record 

 as positive only those organisms that produce considerable acid or 

 gas from a given compound and as negative only those that con- 

 sistently fail to show any acid or gas, nor any increase of growth 

 when supplied with the carbon compound under investigation. All 

 others should be regarded as border-line cultures, calling for further 

 investigation as given in Leaflet VI. 



Hydrolysis of Starch 



The breaking down of starch is rather more complicated than that 

 of sugars because of the extensive hydrolysis that is necessary be- 

 fore it can be utilized by the bacteria. The first stage of this process 

 is generally known as diastatic action because of the similarity to 

 that brought about by the enzyme diastase. The final end result is 

 usually an increase in acid, so one may obtain good evidence as to the 

 utilization of starch by substituting it for sugar in the above methods 

 (pp. V49I5-I7) and determining acid produced or increase in H-ion 

 concentration. It is often desirable, however, to secure evidence as to 

 the intermediate products and as to whether the starch has been 

 entirely consumed or not; and various methods have been proposed 

 for this purpose. 



This test may be made on raw starch, dissolved by boiling, or on 

 the so-called "soluble starch." The latter is a partly hydrolyzed 

 product; but it is often used as "starch" in this test because its 

 iodine reaction is like that of true starch and different from that given 

 by typical dextrins. If soluble starch is used, its true nature must 

 be taken into account; but at the same time it must be remembered 

 that true starch is partly hydrolyzed when sterilized in culture media, 

 and even cultures growing in such a substratum are not furnished 

 with raw starch as the sole carbohydrate. When such media are 

 filtered, possibly "soluble starch" is all that remains. 



A satisfactory method has been proposed by Eckford (1927) for 

 learning the type of action on starch brought about by organisms 

 capable of making good growth in broth. The same method may be 

 adapted to organisms which prefer some other liquid medium by 

 substituting it for broth in Eckford's method. The procedure, 

 however, is not well adapted to those bacteria that fail to grow well 

 in liquid medium. The technic is as follows: 



Add 0.2% soluble starch to broth and incubate cultures a week to ten days. Ex- 

 amine on 2nd, 4th, 7th and 10th days for hydrolysis of starch, production of acid, and 

 reduction of Fehling's solution. For this test a drop is placed in a depression on a 

 porcelain plate and a larger quantity in a serological test tube. The latter is tested 

 for acid production with an indicator of the proper pll-range. To the drop on the 



