40 GENERAL MICROBIOLOGY 



liquid in which the agar is dissolved, the per cent content 

 of agar, and the method of dissolving. The influence of the 

 reaction of agar solutions has been treated above. For 

 general culture use, however, ordinary agar is made +15 

 Fuller's scale (agar solidifies with difficulty above +30 

 Fuller's scale). 



One per cent agar is much more easily soluble under 

 equal conditions than a higher per cent. One and one- 

 half per cent is the amount used in ordinary agar media, 

 giving a somewhat stiffer and thus more desirable jelly. 



Agar is digested most rapidly over a free flame. If not 

 heated sufficiently, after the filtration and sterilization of 

 the agar by the intermittent method, a flocculent precip- 

 itate frequently appears in the previously clear medium. 

 This can be made to disappear in most cases by subjecting 

 to the temperature of the autoclav (120 C. 15 Ibs.). 



Agar for culture media should be entirely clear when 

 liquid, and homogeneously opaque-translucent when solid; 

 it should have a translucence sufficient to allow deep colonies 

 on plates or stab cultures to be observed readily; it should 

 not contain flocculent material, sediment, or pieces of cotton 

 or filter paper, as these hinder typical colony development 

 of microorganisms and, to the inexperienced, may some- 

 times be mistaken for colonies. 



In the first methods ever used for making agar culture 

 media, instead of filtering the hot agar through filter paper, 

 absorbent cotton, or asbestos, it was allowed to cool, dur- 

 ing which process the sediment settled to the bottom; when 

 solid the sediment was cut off. This method was not 

 desirable, as the clearness of the resultant agar would depend 

 upon the rate of cooling; the slower the cooling, the more 

 completely would sedimentation take place. 



Agar is not a food for microorganisms in general, i.e., 

 it is not affected by the digestive enzymes of most bacteria, 

 as is gelatin. However, a few bacteria are known which 

 have the power of liquefying agar, among which are B. 



