ROUTINE TESTS FOR THE IDENTIFICATION OF BACTERIA 159 



of solidifying. Suitable iminoculated controls must always be run in parallel, especi- 

 ally if the optimum growth conditions for the organism necessitate prolonged exposure 

 of the gelatin to hydrolysis by mild acid, alkali, or heat. In addition, precautions 

 should always be taken to prevent evaporation of moisture which might conceivably 

 tend to obscure a slow liquefaction. This method has the advantage of rarely giving 

 positive results except in case of "true liquefaction." On the oiher hand, it may well 

 fail to detect cases of real liquefaction that have proceeded so slowly that the gelatin 

 can still set even after several week's incubation. The significance of this test can 

 be increased by using weaker than normal gelatin, 4 per cent gelatin, for example, 

 or even less. 



CLEAVAGE OF SUGARS, ALCOHOLS, AND GLUCOSIDES 



Fermentable substance to employ. Quite a wide range of pure alcohols 

 and carbohydrates is available for use in fermentation tests. In routine 

 work the choice is often limited to the more common and less expensive 

 substances, but in special research work economy is of less importance. 

 The three sugars glucose, sucrose, and lactose and the alcohols glycerol 

 and mannitol are most widely employed because they are readily avail- 

 able. Whether these compounds give valuable information depends upon 

 the group of organisms being studied. If the group, like the colon group, 

 is capable of fermenting nearly all these substances, these readily fer- 

 mented sugars and alcohols may have very Httle value in separating the 

 species one from another ; one must then employ one or more of the rarer 

 compounds if carbohydrate fermentation is considered to have any diag- 

 nostic significance in the group under study. In other words the selection 

 is based upon the group of bacteria under investigation. 



Attention must always be paid to the sterilization of heat labile fer- 

 mentable substrates (see Chap. III). 



Basal medium. The compound to be tested must be added to some 

 basal medium suited to the group of organisms under investigation. For 

 routine work it is best to employ two such basal media, namely, beef- 

 extract peptone broth and beef-extract peptone agar, selecting one or the 

 other according to whether the organisms under investigation grow better 

 in liquid or solid media. These media should be prepared as directed in 

 Chap. III. It should be noted that some commercial peptones contain 

 fermentable sugars (Vera, 1949) ; hence care must be exercised in regard 

 to the peptone selected, and controls must be run. 



One should notice particularly whether or not good growth is obtained 

 in any or all of these media after adding the fermentable substance under 

 investigation. If poor growth or none is obtained in the broth and on 

 the agar, one should vary the basal medium employed. 



If a culture is to be studied in liquid, the media should be sterilized in 

 fermentation tubes; if on solid media, agar slants should be used — see 



