32 Biochemical Activity of Microorganisms [March, 



suspended in a little cold water. The medium is then sterilized in 

 the autoclave and poured with frequent shaking directly from the 

 flask into sterile Petri dishes. This gives a clear white substratum 

 in which the starch is suspended. As soon as the agar has solidi- 

 fied, the centers of the dishes are inoculated with the organisms to 

 be tested. The dishes are inverted and incubated for two to five 

 days under bell jars to prevent loss of moisture. Typical results 

 are shown in Table i and Plate i, Fig. i. 



Some of the organisms produce extracellular amylase which dif- 

 fuses from the colony, dissolves the starch suspended in the agar, 

 and renders the space about the colony clear. Such an appearance 

 is designated as a " halo." The presence of a halo, then, around the 

 edge of a bacterial or fungous colony on starch-agar indicates the 

 production, by that organism, of a readily diffusible extracellular 

 amylase. 



Some organisms, especially fungi, do not produce a halo on 

 starch-agar, but grow well and dissolve the starch from the agar 

 immediately in contact with them. In such a case the secretion of 

 extracellular amylase is apparently weak or the amylase is not so 

 diffusible as that produced by other organisms. 



Among the organisms so far tested, Streptothrix, Oospora 

 Scabies and Aspergillus niger were f ound to be the most active pro- 

 ducers of extracellular amylase. The first of these is able to pro- 

 duce abundant amylase and to dissolve large quantities of starch 

 even in the presence of an abundance of sugar. 



Inulase. Inulin is hydrolyzed by the enzyme inulase to reduc- 

 ing sugars, and as such may be utilized by microorganisms. 



TABLE 2 



Data pertaining to growth of fungi on inulin agar 



Organism Growth Spore production 



Glomerella rufomaculans Fair Fair 



Phyllosticta pirina Good Good 



Coniothyrium pirinünt Good Poor 



Aspergillus niger Good Good 



Streptothrix sp Poor Almost none 



Inulift-agar. To 1000 c.c. of the stock agar, 0.5 percent of pure 

 inulin was added, and Petri dish cultures made as in the case of the 



