V49-16 MANUAL OF METHODS FOR PURE CULTURE STUDY 



The list of fermentable substances often used in such work is given 

 in Leaflet VI. 



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 investi- 

 gation grow better in liquid or solid media. These media should be 

 prepared as directed in Leaflet II. 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. 



Another important basal medium is the synthetic formula (Ayers, 

 Rupp and Johnson) given on p. II44-I4 of Leaflet II. This can be 

 used only for organisms that utilize ammonium salts as a source of 

 nitrogen; but is valuable for organisms that cause misleading changes 

 in reaction from proteins or which produce so little acid that it does 

 not become evident in a highly buffered medium. 



One should notice particularly whether or not good growth is ob- 

 tained 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 em- 

 ployed, following the suggestions given in Leaflet VI. 



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 Conn and Hucker (1920). Agar slants may be inoculated either on 

 the surface alone or partly on the surface and partly iii a stab at the 

 base. It has been found in practice that if much gas is produced it 

 may occur at the very base of the column of agar even when all the 

 growth seems to occur on the surface; but if there is reason to suspect 

 that gas production is being overlooked, shake cultures may be used 

 in addition to the agar slant. 



Demonstration of Cleavage. Utilization of the sugar (or other 

 fermentable substance) may be indicated by a chemical determination 

 showing its partial or complete disappearance, or by the demonstra- 

 tion of the end-products of fermentation. These end-products are 

 generally organic acids, sometimes accompanied with the evolution 

 of gases, e. g., free hydrogen, carbon dioxide, or occasionally methane. 

 Determinations of the amount of sugar remaining or of the nature of 

 the organic acids produced are very valuable in discriminating investi- 

 gations, but require time-consuming chemical work that is difficult 

 to employ in the routine examination of large numbers of cultures. 



